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Since 2007, the first IMPRS-HD alumni have left Heidelberg.
Below, please find a all PhD researchers graduated within IMPRS-HD
in the years
2011,
2010,
2009,
2008, or
2007).
We hope they all enjoyed to stay with us and wish them good luck
and success for their future careers!
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Christoph Deil
(Germany)
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HESS & FERMI surveys of the Galactic gamma-ray source population
(thesis pdf, 80MB)
| Supervisor:
Werner Hofmann (MPIK)
| | Thesis abstract:
The High Energy Stereoscopic System (HESS) is an array of four imaging atmospheric
Cherenkov telecopes, located in Namibia, observing the gamma-ray sky at energies
> 100 GeV since 2004. For the first time HESS has surveyed the Galactic plane (approximately
in the range GLON = -110 to +70 deg, GLAT = -3 to 3 deg) and detected
a number of sources, each one a cosmic particle accelerator emitting gamma rays produced
in interactions of the cosmic rays with ambient matter and radiation fields. In
this thesis the full HESS Galactic plane survey dataset was used to derive significance
and flux maps as well as a catalog of 62 sources containing their position, extension
and spectrum. Several new methods for an improved and semi-automatic detection
and analysis of all sources were developed. The Fermi Large Area Telescope (LAT) is
a space-based gamma-ray telescope, continuously performing an all-sky survey above
100 MeV since June 2008. Based on two years of data in the energy range 100 MeV to
100 GeV the LAT collaboration has published a catalog of 1873 sources, 244 of which,
mostly of Galactic origin, are located inside the HESS survey region. In this work allsky
significance and flux maps and catalogs of 74 Fermi sources above 10 GeV and 42
sources above 100 GeV are derived and a preliminary comparison with the HESS data
is presented. This work can serve as the basis for future detailed studies of the Galactic
gamma-ray source population.
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Gisella de Rosa
(Italy)
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A comprehensive analysis of optical and near-infrared spectroscopy of z~6 quasars
(thesis pdf, 5MB)
| Supervisors:
Fabian Walter / Hans-Walter Rix (MPIA)
| Thesis abstract:
High redshift (z~6) quasars (QSOs) are unique probes of the early growth of accreting supermassive black holes
(BHs) and are fundamental tools to study the ionization state of the intergalactic medium and its chemical
enrichment. Until now, only the most luminous QSOs have been studied, often one object at the time. In this
thesis we present the most extended consistent analysis to date of optical and near-infrared spectra of z~6 QSOs.
Our NIR sample is composed of 22 spectra of QSOs at 4.5
An analysis of the wavelength shifts of the optical lines shows that the redshifts inferred from the OI+SiII complex,
visible in most of the optical spectra, are consistent with those obtained from the MgII line. This implies that zOI is
a reliable proxy for the QSO systemic redshift. We create a composite template of the z~6 QSOs that, red-ward the
Ly-alpha line, remarkably resembles the one obtained for low-redshift QSOs. Investigating the existence of possible
correlations amongst optical emission properties, we do not detect the expected anti-correlation between the EW and
the QSO continuum luminosity (Baldwin effect) for the high ionization lines (NV & SiIV+OIV]). We show that a strong
correlation holds between the line FWHM and its luminosity. Given the narrow distribution of Eddington ratios that
characterizes the z~6 QSOs population, this implies that QSOs with brighter lines are the ones that host more massive
BHs. We further confirm this statement by testing the relation between the line luminosity and MBH.
Finally we use our optical spectra to study the ionization state of the IGM at high redshift, by measuring the
evolution of the near zone (NZ) sizes and that of the Gunn Peterson (GP) effective optical depth. Our comprehensive
analysis shows that the correlation between the NZ radii and the QSO systemic redshifts is significantly reduced with
respect to previous results. This implies that changes of the IGM ionization state are difficult to constrain through
such measurements. We also outline a new method to detect the evolution of the GP effective optical depth through the
analysis of the Ly-alpha and Ly-beta flux ratios. For each QSO we build a model based on the SDSS QSO composite spectrum
and on the optical depth parametrization of lower-redshift absorbers (z<5.7). Comparing the QSOs flux ratios with the
ones obtained from the models we detect a steep increase in the evolution of the effective optical depth.
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Carolina Bergfors
(Sweden)
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Formation of stars, substellar objects and exoplanets : Observations of multiplicity
(thesis pdf, 2MB)
| Supervisors:
Wolfgang Brandner / Thomas Henning (MPIA)
| Thesis abstract:
Many nearby stars are part of a binary or multiple system. Details about their history are preserved
in their multiplicity characteristics, and observations of binary/multiple star systems provide a way
to measure fundamental physical properties of the stars as well as clues to their formation and
evolution. Moreover, planet formation and dynamics may also be affected by the presence of a second
star, or by giant planets in the same system.
In this thesis, high resolution imaging of low-mass stars, planet host stars and a multiple planet
system is presented. The results of observations and analyses include the discovery of several previously
unknown companion stars and multiplicity statistics for M dwarfs in the largest M dwarf multiplicity
survey to date. We also present near-infrared characterization of four close M dwarf systems, previously
unknown companion candidates to exoplanet host stars, and investigate how a close companion may affect
planet formation. New astrometric data is presented for three of the directly imaged planets in the
HR8799 system, and an analysis of a possible orbital configuration of planet HR 8799 d.
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Mario Gennaro
(Italy)
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Massive clusters revealed in the near infrared : Constraining the early stages of stellar evolution
(thesis pdf, 25MB)
| Supervisors:
Wolfgang Brandner / Thomas Henning (MPIA)
| Thesis abstract:
Stellar clusters can be considered as the building blocks of galaxies. Studying how clusters form and evolve is crucial
in understanding the evolution of their host galaxies. Young massive stellar clusters, in particular, play a key role in
placing constraints on the physics of star formation and evolution. The stellar population inside these extremely massive
objects spans the full range of stellar masses from very low mass brown dwarfs to the most massive stellar objects
currently known. One of the most important tools to study stellar clusters are stellar evolutionary models, i.e. tracks
and isochrones from which masses and ages can be derived. Resolved stellar populations in clusters can in turn be used to
test and calibrate these theoretical models.
In this work I first present a study of the current generation of pre-main sequence models and their thorough use for
assessing stellar properties. I then continue using stellar models to study the most massive young cluster in the Milky
Way: Westerlund 1. The last part of this work deals with the first multiple wavelength investigation of a recently
discovered star forming region: the CN15/16/17 complex. This region was observed in the framework of an extended cluster
search. I used near infrared observations to look for the missing cluster that are yet to be found in our Galaxy in order
to unveil the mysteries of star formation in the inner parts of the Milky Way.
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Alexander Karim
(Germany)
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Star Formation in the COSMOS Field : A radio view on the build-up of stellar mass over 12 billion years
(thesis pdf, 15MB)
| Supervisors:
Eva Schinnerer / Hans-Walter Rix (MPIA)
| Thesis abstract:
In this thesis I study the evolution of galaxies with a special focus on their star forming ability by
using the extensive multi-wavelength data sets available for the 2 square degree COSMOS deep
field. The deep radio continuum data from Very Large Array observations at a frequency of 1.4 GHz (a
wavelength of 20 cm) form the basis of my analysis of the cosmic star formation history unaffected by
dust obscuration.
A newly developed stacking algorithm enabled an unprecedentedly representative view on the evolution of
the average star formation rate within galaxies down to low limiting (stellar) masses since a redshift of
z~3 (i.e. ~2 billion years after the Big Bang). My findings are in good agreement with results from
different star formation diagnostics that often suffer from large dust corrections or significantly worse
statistics. A main result of this thesis is the identification of a constant characteristic mass for
star forming galaxies. It implies that galaxies with masses similar to our Milky Way have always been
the main sites of star formation. Therefore the often debated 'downsizing scenario' where the characteristic
mass decreases with cosmic time is ruled out. In the young universe (<1.5 billion years of cosmic age) I
successfully searched for the most extreme star forming environments. These provide critical constraints on
cosmic structure formation and dust enshrouded star formation at early times. A detailed case study not
only reveals large amounts of molecular gas but also a powerful hidden active galactic nucleus in one
such massive starburst. This finding demonstrates the diversity of this cosmologically important galaxy population.
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Mauricio Cisternas
(Chile)
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Galaxies and supermassive black holes evolving in a secular universe
(thesis pdf, 2MB)
| Supervisors:
Knud Jahnke / Josef Fried (MPIA)
| Thesis abstract:
Most massive galaxies host a supermassive black hole (BH), that had most
of its mass built up throughout bright periods of vigorous accretion,
during which it is referred to as an active galactic nucleus (AGN). In the
local universe it has been observed that BH mass (M_BH) follows tight
correlations with various properties of the galactic bulge in which it
resides. This led to the currently popular ``co-evolution'' picture, in
which most present-day galaxies went through at least one active phase in
the past, during which a link between galaxy and BH gets established.
However, more robust observational constraints are required on how galaxy
and BH related at earlier times, and which mechanism is responsible for
triggering these BH growth phases. This thesis studies a large sample of
AGN out to z~1 from the COSMOS survey, selected from their X-ray emission
and imaged in finest detail with the Hubble Space Telescope, allowing the
study of growing BHs together with their host galaxies. We present new
constraints on the ratio of BH mass to $total$ galaxy stellar mass (M_*)
over the last 7 Gyr for 32 type-1 AGN. We show that the M_BH-M_* ratio at
z~0.7 is consistent with the local relation between BH mass and galactic
bulge mass. For these galaxies to obey the local relation only a
redistribution of disk-to-bulge mass is needed, likely driven by passive
secular evolution.
We then tackle and answer a 30-year old question: what is the relevance of
major mergers and interactions as triggering mechanisms for AGN activity?
We visually analyze the morphologies of 140 AGN out to z~1 looking for
signatures of recent mergers, and compare them with a control sample of
over 1200 matched inactive galaxies. We find that the merger fraction of
the AGN host galaxies is statistically identical to the corresponding
inactive galaxy population, at roughly 15%. Together with the fact that
the majority of the AGN host galaxies are disk-dominated, unlikely relics
of a recent major merger, these results are the strongest evidence to date
that secular evolution rather than major merging has dominated BH fueling
at least since z~1, and that BHs and galaxies have, for the last 7 Gyr,
evolved in a secular universe.
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Roman Follert
(Germany)
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The atmospheric piston simulator for LINC-NIRVANA and Interferometric observations
of massive young stellar objects
(thesis pdf, 15MB)
| Supervisors:
Tom Herbst / Hans-Walter Rix (MPIA)
| | Thesis abstract:
The scope of this dissertation is two - fold. The first part describes the design,
construction and verification of the atmospheric piston simulator. The atmospheric
piston simulator is a device which will allow the verification of the LINC-NIRVANA
fringe tracking system under as realistic as possible conditions. The design of the
atmospheric fringe tracker is based on optical fibers for light guiding and
integrated optics components for the manipulation of the phase of the transmitted
light. This fiber based setup allows on the one hand an easy an reliable way to
introduce differential piston sequences, on the other hand integration into
LINC-NIRVANA will be trivial.
The second part of this dissertation describes the application of Interferometry in
the case of the observation of massive young stellar objects. Two objects are
presented and discussed in detail: the Kleinmann & Wright object and AFGL 2136. The
interferometric measurements as well as the data reduction are described. The spectrally
dispersed visibility curves and the differential phase data are fitted by analytical
geometrical models. Then, the interferometric data and results from a spectral energy
distribution analysis are combined and compared to Monte-Carlo radiative transfer
simulations. The results of this analysis indicate the existence of circumstellar
disks similar to those found in low mass star formation. A detailed analysis of the
mid-infrared silicate absorption features also indicates significant dust grain
evolution in AFGL 2136.
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Oliver Porth
(Germany)
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Formation of Relativistic Jets : Magnetohydrodynamics and Synchrotron Radiation
(thesis pdf, 20MB)
| Supervisor:
Christian Fendt (MPIA)
| | Thesis abstract:
In this thesis, the formation of relativistic jets is investigated by means of special
relativistic magnetohydrodynamic simulations and synchrotron radiative transfer. Our
results show that the magnetohydrodynamic jet self-collimation paradigm can also be
applied to the relativistic case. In the first part, jets launched from rotating hot
accretion disk coronae are explored, leading to well collimated, but only mildly
relativistic flows. Beyond the light-cylinder, the electric charge separation force
balances the classical trans-field Lorentz force almost entirely, resulting in a
decreased efficiency of acceleration and collimation in comparison to non-relativistic
disk winds. In the second part, we examine Poynting dominated flows of various electric
current distributions. By following the outflow for over 3000 Schwarzschild radii, highly
relativistic jets of Lorentz factor 8 and half-opening angles below 1 degree are obtained,
providing dynamical models for the parsec scale jets of active galactic nuclei. Applying
the magnetohydrodynamic structure of the quasi-stationary simulation models, we solve the
relativistically beamed synchrotron radiation transport. This yields synthetic radiation
maps and polarization patterns that can be used to confront high resolution radio and
(sub-) mm observations of nearby active galactic nuclei. Relativistic motion together with
the helical magnetic fields of the jet formation site imprint a clear signature on the
observed polarization and Faraday rotation. In particular, asymmetries in the polarization
direction across the jet can disclose the handedness of the magnetic helix and thus the
spin direction of the central engine. Finally, we show first results from fully
three-dimensional, high resolution adaptive mesh refinement simulations of jet formation
from a rotating magnetosphere and examine the jet stability. Relativistic field-line
rotation leads to an electric charge separation force that opposes the magnetic Lorentz
force, such that we obtain an increased stability of relativistic flows. Accordingly,
the non-axisymmetric modes applied to the field-line foot-points saturate quickly, with
no signs of enhanced dissipation or disruption near the jet launching site.
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Bhargav Vaidya
(India)
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Theory of disks and outflows around massive young stellar objects
(thesis pdf, 19MB)
| Supervisors:
Christian Fendt / Henrik Beuther (MPIA)
| Thesis abstract:
The inner most regions around massive young stellar objects (YSO) are associated with complex
interactions between numerous physical processes. Since the inner few Astronomical Units (AU)
are tough to resolve observationally, a theoretical approach is important to create a
qualitative picture for these regions around young high-mass stars. This thesis investigates
the interplay between important physical processes with respect to the dynamics of jets and
inner accretion disks. This thesis provides a bridge between the physical structures of the
inner and the outer disk, where the later is observationally easier to access. Above all, the
importance of the radiative force in altering the dynamics of a magnetically launched jet is
outlined in this thesis.
A thin accretion disk model with proper gas and dust opacities is applied for a luminous young
high-mass star. This study has furnished estimates of various physical quantities in the inner
few AU of the accretion disk. In particular, I have found that the mid-plane temperature around
0.1 AU could be as high as 10^5 K for a 10 Msun star. Such high temperatures in the disk destroy
most of the dust grains already at large radii from the central star. This in turn reduces the
opacity of the accreted matter thereby overcoming the central radiation-pressure from the young
massive star. In addition, such disks are stable to gravitational fragmentation inside of 100 AU
from the central star. Thus they form an ideal launching base for long-lasting outflows.
Outflows and jets are an ubiquitous phenomenon in young massive star forming regions. Observational
surveys have suggested that the outflows become wider as the star grows in luminosity (thus mass)
with time. I have performed magneto-hydrodynamical simulations of jet launching in presence of
radiative forces from the luminous star and the inner hot accretion disk. The major outcome of
this work, is that the radiative force from the central star plays a dominating role in accelerating
and de-collimating the magnetically launched jet, while the influence of the disk radiative force
is rather small. In addition, conducting an extensive parameter study, I have found that the
outflows become wider as the mass (or luminosity) of the central star increases. The degree of
collimation is also affected by the magnetic field strength and optical thickness of the line.
This interplay of radiative and magnetic forces provides a physical insight to the trend in degree
of collimation suggested by observations.
Finally, a fully three-dimensional simulation is conducted to understand the manner in which the
inner accretion disk transports material onto the central massive star. The hydrodynamic flow in
the disk is simulated in the presence of radiative transfer and/or self-gravity. The transport of
angular momentum is solely due to gravitational torques. My first results indicate that a locally
isothermal disk becomes gravitationally instable and fragments in the inner parts as it is fed
with matter from the outer massive core with a steady accretion of 10^-3 Msun yr^-1. About 10% of
the mass added onto the disk is accreted onto the central star in form of clumps. On the other
hand, no fragmentation is seen in an adiabatic disk whose initial temperature profile is consistently
derived from radiative transfer calculations. This investigation complements the above semi-analytical
study of the inner disk to single out the physics of angular momentum transport in massive accretion
disks.
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Rene Andrae
(Germany)
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Alignment of disc galaxies : Testing the theory of angular-momentum acquisition
(thesis pdf, 7MB)
| Supervisors:
Knud Jahnke (MPIA)
| | Thesis abstract:
This thesis is concerned with the formation of disc galaxies, a special type of galaxies
characterised by ordered rotation. The main focus is on testing predictions of
theories that try to explain the angular-momentum acquisition of these galaxies. A
major issue concerns the proper quantification of galaxy morphologies. This is of vital
importance for automated and reproducible classification of disc galaxies in large
data samples. The most promising approach is to expand galaxy morphologies into
orthonormal basis functions based on a well motived light profile (the Sersic profifile).
In the main part of this thesis, the angular momenta of several thousand disc
galaxies from the Sloan Digital Sky Survey are analysed. It is demonstrated that
previous investigations lacked a rigorous account for the relevant error sources, which
lead to unjustified confirmations of theoretical predictions. If all relevant errors are
considered, there is no statistical evidence confirming these predictions. The methods
developed here are also relevant in a wider context, as they are applicable to other
astrophysical investigations. This thesis concludes with a detailed discussion of potential
improvements of measurements and an outline of an observational strategy
that will allow for a more decisive empirical test of the theoretical predictions.
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Ekaterina Luettjohann
(Russia)
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Chemische und mineralogische Entwicklung Protoplanetarer Scheiben
(thesis pdf, 4MB)
| Supervisor:
Hans-Peter Gail (ITA)
| Thesis abstract:
Modern measurement techniques provide a lot of information about the chemical composition of
comets and protoplanetary disks. Studies of the istotopic composition of meteorites provide further
information about the conditions of formation for these compounds. However, the history of formation
and the evolution paths of these species are often not known.
This work shows that the oldest compounds of the solar system, the calcium aluminum rich inclusions
(CAIs) may form during the collaps of a molecular cloud. After the formation they are transported
to distances several ten AU away from the region of their formation.
Furthermore, the destruction mechanisms of presolar SiC particles are studied in this work. The
calculations show that the ratio between presolar SiC grains and presolar carbon grains in meteorites
cannot be explained solely by oxidation processes in the solar nebula.
Another chapter of this work describes the chemical evolution of hydrocarbons in protoplanetary
disks. The gas phase chemistry is calculated using methods of chemical kinetics. For this purpose 45
species and 220 reactions are included in the calculation. It is shown that the destruction of carbon
grains leads to the formation of hydrocarbons in the inner parts of protoplanetary disks.
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Christine Ruhland
(Germany)
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Signposts of Hierarchical Merging
(thesis pdf, 22MB)
| Supervisors:
Eric Bell, Hans-Walter Rix (MPIA)
| Thesis abstract:
In a Lambda-CDM universe, the merging of galaxies plays a key role in their formation
and evolution. In this thesis I focus on two aspects of this picture.
The first one is the accretion of satellite galaxies onto disk galaxies, specifically
the Milky Way, leaving the galaxy mostly undisturbed in its morphology, but contributing
to the stellar populations of its faintest, but most extended component, the stellar
halo. I have developed techniques for detecting and exploring stellar streams formed
through the tidal disruption of satellites. The main results are the discovery of a
candidate stellar stream, and detailed characterization of the Sagittarius tidal stream,
a key probe of the Milky Way potential.
In the other project, I explore the evolution of early-type galaxies—thought to be the
result of merging of disk galaxies. I show that the observed evolution of their
color-magnitude relation (a probe of their stellar populations) is consistent with a
constant addition of galaxies with recently truncated star formation. On both scales
these studies are consistent with the notion that merging has played an important role
in shaping the galaxies as we see them in the present day universe and continues to
do so.
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Ioanna Arka
(Greece)
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Non-linear waves in the laboratory and in astrophysics:
Pair production in counter-propagating laser beams and strong waves in pulsar winds
(thesis pdf, 5MB)
| Supervisor:
John Kirk (MPIK)
| Thesis abstract:
In this work we are investigating non-linear electromagnetic waves in two different physical
environments: laboratories on earth and the astrophysical objects known as pulsars.
In the first part of our work the interaction of electrons and positrons with strong waves in the
form of high intensity laser beams is analyzed. The possibility of emission of energetic radiation
which can result in prolific pair production in the focus of two short, counter-propagating ultrahigh
intensity laser pulses is examined, taking into account several different possibilities for the
relative polarizations and the waveform of the beams. The conclusion is reached that in the
next generation laser facilities currently under construction mainly in Europe, like ELI and the
10PW Vulcan laser, pair production and electromagnetic pair cascades should be observed for
intensities as low as 1024Wcm−2.
In the second part of this work we focus on large amplitude, low frequency waves that are
emitted by pulsars. After a brief review of the current understanding of pulsar winds and the
problems inherent to it, we show that the interaction of a relativistic striped pulsar wind with the
the termination shock should result in reflection of electromagnetic energy in the upstream, which
can affect the outflow, creating a precursor. We then investigate the possible conversion of the
pulsar wind to a superluminal linearly polarized wave propagating upstream of the termination
shock and show that this will result in the transfer of energy from the fields to the outflow
particles in the precursor, lowering the magnetization of the outflow and opening the way for
further particle acceleration at the shock front.
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Leonard Burtscher
(Austria)
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Mid-infrared interferometry of AGN cores
(thesis pdf, 27MB)
| Supervisor:
Klaus Meisenheimer (MPIA)
| Thesis abstract:
Active Galactic Nuclei (AGNs) are among the most luminous objects in the universe and are
classified into a number of types and subtypes. Unified models of AGNs explain some of this
variety as a result of different viewing angles towards their cores instead of intrinsic
differences: from some viewing angles, our line of sight would be blocked by a dusty torus.
The spatial resolution necessary to resolve these warm dust structures at 10 micron is currently
only provided by the Very Large Telescope Interferometer (VLTI) in Chile. First, extensive
observations of the radio galaxy Centaurus A are examined that show an extended structure, which
is probably connected to the Northern nuclear radio jet, at a distance of about 40 milli arcseconds
(0.7 parsec) in front of the nucleus. Secondly, a study of the brightest so-called type 1 Seyfert
galaxy, NGC 4151, reveals for the first time a resolved nuclear emitter in such a source.
Its properties (size, temperature profile, emissivity) are similar to those of the alternative
type 2 galaxies, studied previ- ously. This is consistent with, though not necessarily confirmation
of, unified models. Since the previous studies of type 2 source and modern torus models show
a wide variety of nuclear dust structures, finally, the so-far most comprehensive study of resolved
nuclear dust emission of AGNs is set up to study 13 AGNs of various luminosities L and distances
reaching several hundred Mega Parsec. The first full analysis of this study shows in all but one
galaxy that tori can be resolved and that their size on parsec scales is not simply proportional
to L^0.5.
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Christian Angrick
(Germany)
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On the derivation of an X-ray temperature function without reference to mass and
the prediction of weak-lensing number counts from the statistics of Gaussian random
fields
(thesis pdf, 5MB)
| Supervisor:
Matthias Bartelmann (ITA)
| | Thesis abstract:
We present a novel approach for the derivation of the X-ray temperature function for galaxy
clusters, which is based on the statistics of Gaussian random fields applied to the cosmic
gravitational potential. It invokes only locally defined quantities so that no reference to
the cluster's mass is made. To relate linear and non-linear potential and to take into
account only structures that have collapsed, we include either spherical- or ellipsoidal-collapse
dynamics and compare both resulting models to temperature functions derived from a numerical
simulation. Since deviations from the theoretical prediction are found in the simulation for
high redshifts, we develop an analytic model to include the effects of mergers in our formalism.
We jointly determine the cosmological parameters Omega_m0 and sigma_8 from two different cluster
samples for different temperature definitions and find good agreement with constraints from
WMAP5. Introducing theoretically a refined detection definition based on the upcrossing criterion,
we reformulate our analytic approach for 2D and use it to predict the number density of spurious
detections caused by large-scale structure and shot noise in filtered weak-lensing convergence
maps. Agreement with a numerical simulation is found at the expected level.
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Sanjaya Paudel
(Nepal)
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Early-type dwarf galaxies: Insight from stellar population studies
(thesis pdf, 10MB)
| Supervisor:
Thorsten Lisker (ARI)
| | Thesis abstract:
We have studied the stellar population properties of a sample of early-type dwarf galaxies (dEs)
in Virgo cluster. We derived the simple stellar population (SSP) parameters age and metallicity
using the method of Lick indices.
We found that not all dEs exhibit the same stellar population properties. The dEs with disc
features are relatively younger and more metal enhanced than dEs without disc. We also found
the nuclei of dEs have smaller ages and higher metal content than the respective galactic main
bodies. Comparing the SSP parameters at the same local density, where the Ultra Compact
Dwarf galaxies (UCDs) are located, we do not find any diㄦence in the stellar population
properties of dEs nuclei and UCDs.
We confirmed that the metallicity correlate with the galaxies luminosity in all classes of earlytype
galaxies i.e. Es, dEs and dSphs and dEs luminosity-metallicity relation has the steepest
slope among all classes. We discovered a clear break in this relation in the dEs region. The
bright and metal rich dEs seem to follow a faint end extension of Es and the metal poor and old
dEs either hold a genuine class of early-type dwarf galaxies or just a bright end extension of
dSph.
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Anne Bochow
(Germany)
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A systematic study of Supernova Remnants as seen with H.E.S.S.
(thesis pdf, 2MB)
| Supervisor:
Werner Hofmann (MPIK)
| | Thesis abstract:
Supernova remnants (SNRs) are the remainders of extremely energetic explosions occurring at the
end of a star's life. With the energy released during the supernova explosion they are believed
to accelerate charged particles to energies of up to 10^15 eV. In the very-high-energy
(VHE, > 10^11 eV) gamma-ray band, SNRs represent one of the most populous classes of Galactic
sources. Due to its unprecedented sensitivity, H.E.S.S. was the first instrument to allow for
a morphological resolution of individual SNRs, proving the existence of particle acceleration and
subsequent VHE gamma-ray emission. However, to date many more SNRs are known in the radio waveband
than in VHE gamma-rays. This work presents a systematic study of the VHE gamma-ray signal of a
sample of around 200 radio SNRs. The VHE gamma-ray-signal of these SNRs is studied individually.
Besides the spatial correlation of radio SNRs and VHE gamma-ray sources, the measured flux of
VHE gamma-rays is compared to theoretical flux predictions. These predictions are based on
assumptions of the total explosion energy, the particle acceleration effciency, the density of
the surrounding medium and the distance of the SNRs. The results presented here suggest that these
parameters can vary strongly for individual SNRs. Future observations of SNRs in VHE gamma-rays
and other wavebands will help to constrain the parameter space and will allow to further discuss
acceleration mechanisms in SNRs.
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Nikolay Nikolov
(Bulgaria)
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A Photometric Study of Transiting Extrasolar Planets
(thesis pdf, 50MB)
| Supervisors:
Cristina Afonso / Thomas Henning (MPIA)
| Thesis abstract:
In this thesis I present observational methods to detect and characterize transiting
extrasolar planets. I study the most common types of astrophysical false positives,
identified by ground based photometric transit surveys. I implement and test three
robust methods for the Pan-Planets pipeline to classify false alarms, based on the
transit light curve morphology: evaluation of out-of-eclipse ellipsoidal variability;
determination of astrophysical parameters from a transit fit; and exoplanet diagnostic
(eta).
I obtain results from systematic tests of the pipeline using data from OGLE, Pan-Planets
simulations and confirmed transiting exoplanets. I confirm the first method as most
reliable in ruling out transit-like events due to stellar binaries. I further, present
results from a photometric analysis of data obtained with LAIWO at the 1-m telescope at
Wise Observatory. I study the necessary methods to obtain high quality light curves at
the millimagnitude level down to R = 16 mag. I perform a search for periodic transit
events (1 - 2%) and variable stars and identify 22 transit candidates as well as 20 periodic
variables. An analysis of the photometric colors, ellipsoidal variability and the exoplanet
diagnostic suggests that between zero and two of the light curves should be produced by
exoplanets. Furthermore, I propose a follow-up scheme for their confirmation.
Finally, I obtain high signal-to-noise ratio simultaneous multi-band optical/NIR photometry
of WASP-4, during three transits of its exoplanet, with the GROND instrument at the ESO/MPG
2.2-m telescope at La Silla Observatory. I produce and analyze its light curves and measure
the planet radius, orbital period, semimajor axis and inclination in conjunction with a study
of the transit mid-times. An analysis of WASP-4b transit ephemeris, derived from the results,
confirms the lack of transit timing variations (TTVs).
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Sofia Lianou
(Greece)
|
The interacting M81 group of galaxies
(thesis pdf, 20MB)
| Supervisor:
Eva Grebel (ARI)
| | Thesis abstract:
The topic of this Thesis is the role of the environment and interactions on the formation and
evolution of dwarf galaxies in nearby groups. The M81 group is a highly interacting group,
offering a unique opportunity to study the impact of the environment on shaping the properties
of its dwarf galaxy population. We use the Tully-Fisher relation to search for potential tidal
dwarf galaxies in the M81 group. No potential tidal dwarfs were identified. Furthermore, we derive
photometric metallicity distribution functions and examine the presence of metallicity gradients
for nine early-type dwarf galaxies of the M81 group. The comparison of their mean metallicity
properties with those of Local Group dwarfs shows that these are similar. Not all of the dwarfs
show a metallicity gradient, as is also the case of Local Group dwarfs. The fraction of luminous
asymptotic giant branch stars in each dwarf is small, while their fraction as a function of their
distance from the M81 galaxy does not show any trend. The results indicate that the studied
properties are affected by internal processes. Finally, the method of deriving photometric
metallicities is evaluated for dwarf galaxies with complex star formation histories, using
Galactic dwarf spheroidals. The resulting mean photometric metallicity properties are in good
agreement with spectroscopic measurements, while individual star differences become larger the
more complex the star formation is.
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Isabel Franco Rico
(Mexico)
|
The origin of low-mass early-type galaxies: A combined view from stellar populations and N-body simulations
(thesis pdf, 3MB)
| Supervisor:
Thorsten Lisker (ARI)
| | Thesis abstract:
The COMBO-17 survey data is used to measure stellar masses of late and early-type galaxies
in the redshift range 0 < z < 2.1. Hierachical galaxy formation predicts that early-type
galaxies leave the blue cloud and assemble at z~1. The transit from blue cloud to red sequence
is yet not fully understood. We explore one formation mechanism that might explain how late-type
galaxies can transform in low-mass early type galaxies via galaxy harassment. We aim to
understand the formation of early-type dwarf galaxies through the so-called galaxy harassment
scenario. We perform N-body simulations in which a three-component galaxy (disk + bulge + halo)
enters a galaxy cluster in an eccentric orbit. While the galaxy is falling, it gets tidally
disrupted by close encounters with other galaxies that populate the cluster. Such encounters
are sufficiently vigorous to morphologically transform the galaxy into an early-type dwarf.
Through a combined N-body and stellar population approach, we assign multiple stellar generations
to the simulated galaxy and trace their evolution. This allows us to extract observable quantities,
in particular integrated colours, in order to compare the simulation results to observed Virgo
cluster early-type dwarfs. Based on this analysis we discuss whether harassment is able to form
typical early-type dwarf galaxies.
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Maximiliano Leonardo Moyano D'Angelo
(Chile)
|
A search for transiting extrasolar planets with the LAIWO instrument
(thesis pdf, 20MB)
| Supervisor:
Thomas Henning (MPIA)
| | Thesis abstract:
In this thesis we study the necessary methods to perform a transit search
for extrasolar planets. We apply these methods to search for planets in
one of the fields of the LAIWO project: the Cygnus-Lyra field
(Laiwo VI). We describe the problems that systematic effects can
introduce for precise relative photometry at the millimagnitude level
(~3mmag). Ways to minimize and quantify this correlated noise are
also described. We test the weaknesses and strenghts of two transit
detection algorithms (TDA) namely the Box fitting algorithm (BLS) and
the TRUFAS algorithm using archive data from the OGLE project and
simulations of the first year of the Pan-Planets survey. These projects
are similar in terms of telescope size and field of view to the LAIWO survey.
We have found that the main limitations of the BLS algorithm are the
transit depth and correlated noise (Red Noise). The TRUFAS detection
efficiency correlates with the number of points in transit and the number
of transits present in the light curve, and, its detection efficiency is
low (less than ~ 50%) for these type of ground-based observations.
Finally, we create from the LAIWO data light curves which are suitable
to detect planets among the stars brighter than R = 16.5 mag. We have
found 31 eclipsing binaries and 18 light curves that have transits
consistent with a planet. Of these detections, 3 eclipsing binaries and
8 planet candidates were independently found by the KEPLER survey.
Of the 10 newly discovered transiting planets, 3 are promising to
justify follow-up confirmation studies, which are always necessary to
probe the planetary nature of a transiting companion.
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Hsiang-Hsu Wang
(Taiwan)
|
Gas evolution in disk galaxies: disk stability,
gas accretion & the impacts of spiral density waves
(thesis pdf, 4MB)
| Supervisors:
Ralf Klessen (ITA), Kees Dullemond (MPIA)
| | Thesis abstract:
This thesis studies many aspects of gas evolution in disk galaxies. A simple, effective method is developed for initializing a three-dimensional gaseous disk which is in detailed equilibrium. With this method, theoretical predictions for disk stability and swing amplification are numerically studied for three-dimensional disks. The missing link between intergalactic gas accretion and the star formation activity is found for the galaxy M83. We improve the analysis method to search for the signature of gas infall. For the first time, gas accretion with sufficient fresh gas to fuel star forming disk is kinematically confirmed. The impacts of spiral density waves on gas motions are studied numerically. Shock driven turbulence is quantified and is found to match excellently with observations. Furthermore, the evolution of shock itself has profound impacts on redistributing gaseous surface density, angular momentum and on the development of substructures.
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Min Fang
(China)
|
The disks and accretion behavior of Young Stellar Objects
(thesis pdf, 10MB)
| Supervisors:
Roy van Boekel, Thomas Henning (MPIA)
| | Thesis abstract:
Circumstellar disks play an essential role in the star formation process and are thought to be the sites
where new planetary systems come into being. Knowledge of the evolution of these objects is pivotal
to our understanding of star and planet formation. In this thesis, I present three studies aimed at characterizing the properties of newly formed stars and their circumstellar accretion disks in three qualitatively different environments: the sparse stellar association epsilon Cha, the L1630 and L1641 regions of clustered and distributed star formation in Orion, and the massive cluster Pismis 24. I combined infrared observations, taken from the ground and with the Spitzer Space Telescope, with optical imaging and spectroscopy obtained with various ground-based facilities. I characterized the disk geometry and evolutionary state using the infrared data, and from the optical data I determined the stellar properties as well as the rate at which disk material is accreted onto the central stars. Two important insights gained through my studies are: (1) the disk lifetimes for stars formed in isolation or sparse stellar associations are longer than those of stars formed in relatively dense clusters, whereas the disk lifetimes in clusters harboring very massive stars with spectral types earlier than
~O5 are shorter than those in otherwise similar clusters that lack such very massive stars; (2) the accretion rates show a steeper dependence on the stellar mass in the sub-solar mass regime than in the solar or intermediate mass regime.
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Baybars Kuelebi
(Turkey)
|
Constraints on the origins of magnetic White Dwarfs
(thesis pdf, 15MB)
| Supervisor:
Stefan Jordan (ARI)
| | Thesis abstract:
The central theme of this work is the most frequent final stage of the evolution of magnetic stars, the Magnetic White Dwarfs (MWDs). Improved statistical investigations coming from new surveys and very precise observations of unique MWDs offer the possibility to test various hypotheses on the evolution of these objects. In the first part of our work we identify hydrogen-rich MWDs (DAHs) in the Sloan Digital Sky Survey (SDSS) and investigate the population statistics of all known DAHs in the SDSS. Additionally, we investigate the evolutionary histories of a few of these objects using constraints from the observations of their binary counterparts or through their membership in open clusters. In the second part of our work, we investigate the unique MWD REJ 0317-853, by a parallax measurement with the Hubble Space Telescope and by time resolved spectro-polarimetric modeling. We show that the assumption of centered magnetic dipoles for the field geometry is not correct for more than half of the objects in our sample; this is in particular true for REJ 0317-853 which shows a very uniform field during one rotation phase. This is validated by the first observation of cyclotron absorption in the polarization spectrum of a white dwarf, which is explained with a new model for the self-consistent physical treatment of this absorption process. Furthermore, we study the possible influence of magnetism on the mass loss during the stellar evolution and on the structure of the star which is of importance to understand the nature of the massive white dwarf REJ 0317-853.
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Denija Crnojevic
(Italy)
|
Resolved stellar populations of dwarf galaxies in the Centaurus A/M83 group
(thesis pdf, 47MB)
| Supervisor:
Eva Grebel (ARI)
| | Thesis abstract:
Dwarf galaxies, fundamental ingredients of galactic structures in our Universe, are ubiquitous and surprisingly heterogeneous systems. The study of such objects in nearby groups is a powerful way of investigating their formation and evolutionary mechanisms. The Centaurus A/M83 group is denser and in a more advanced evolutionary phase than our own Local Group, thus being an appealing target for the study of dwarf galaxies. Its more than 50 dwarf members, with different morphologies and stellar contents, can still be resolved into individual stars (at an average Galactocentric distance of ~4 Mpc).
We use optical (HST/ACS) and near-infrared (VLT/ISAAC) photometric data to derive physical properties and evolutionary histories for sixteen dwarfs in this group. Specifically, the predominantly old populations of our target early-type dwarfs show metallicity contents that resemble what is found for Local Group members. However, they intriguingly contain lower intermediate-age population fractions than those found in morphologically similar dwarfs around the Milky Way, thus being more comparable to the M31 companions. We also apply our analysis techniques to the deeper photometric data available for M31 early-type dwarfs. The star formation histories derived for our star-forming late-type dwarf targets reveal that the star formation is quenched for galaxies currently found in the densest group regions. The time-dependent spatial distribution of their stellar populations points toward a stochastic star formation mode in these objects.
Our results show that the evolution of dwarf galaxies depends on their mass but that it is, at the same time, heavily influenced by the surrounding environment. The Centaurus A/M83 group, along with other nearby galaxy groups, deserves further attention in order for us to ultimately gain deeper insights into the processes that regulate the evolution of dwarf galaxies.
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Massimo Viola
(Italy)
|
On shear and flexion measurements and properties of dark matter halos
(thesis pdf, 7MB)
| Supervisor:
Matthias Bartelmann (ITA)
| | Thesis abstract:
In this work we analyse the Kaiser-Squires-Broadhurst method (KSB) to estimate gravitational
shear from surface-brightness moments of small and noisy
galaxy images. We show how KSB relies on restrictive mathematical assumptions
concerning the relation between convolved and unconvolved ellipticity, the shape
of the telescope’s PSF as well as the relation between ellipticity and shear, neither
of which hold in practise. We propose improvements to the original KSB relations
and we demonstrate that this extension lowers substantially the biases in the shear
measurements. Moreover we discuss the impossibility to weaken the assumption on
the PSF shape in the KSB framework. For this reason we develop a novel method
for weak-lensing measurements, DEIMOS, which is based on a mathematically exact
deconvolution of the moments of the apparent brightness distribution of galaxies from
the PSFs. We demonstrate the accuracy and capabilities of this new method with a set
of specialized tests and show its competitive performance on the GREAT08 challenge
data.
Moreover we present a possible application of shear measurement for studying proper-
ties of galaxy clusters based on linear filtering techniques to constrain the inner slope of
the density profile of dark-matter halos. We find that under idealised assumptions, the
inner slope is constrained to 15% if the halo concentration c is known, and to < 30%
if not. If the signals of many halos can be stacked, their density profiles should thus
be well constrained by the proposed linear filters with the advantage, in contrast with
strong lensing analysis, to be insensitive to the cluster substructures.
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Nicola Da Rio
(Italy)
|
An Optical Study of Young Stellar Clusters
(thesis pdf, 11MB)
| Supervisors:
Dimitrios Gouliermis, Thomas Henning (MPIA)
| | Thesis abstract:
In the first part of this thesis a new analysis of the Orion Nebula Cluster, one of the
most studied star-forming regions in the Galaxy, is presented. Based on multi-band
optical photometry and spectroscopy obtained with the Wide-Field Imager (WFI) at the
ESO/MPI 2.2-m telescope at La Silla Observatory, I study the systematic effects that
bias the derivation of stellar parameters of pre-main sequence (PMS) stars. I derive the
new H-R diagram of the entire region, and assign masses and ages to all the members.
The age is found to be ∼ 2 − 3 Myr, older than previously estimated. I also confirm the
presence of an age spread, and show how the previously found mass-age correlation can
be affected by the sample incompleteness and uncertainties in the evolutionary models.
In the second part of this thesis, I carry out a research on the low-mass stellar population
of the young stellar cluster LH 95 in the Large Magellanic Cloud, based on deep
optical photometry with the Advanced Camera for Surveys (ACS) onboard the Hubble
Space Telescope; the deepest ever obtained toward this galaxy, down to V ≃ 28 mag. I
isolate the PMS cluster population, and derive the first extragalactic Initial Mass Function
(IMF) down to the subsolar regime. It shows a flattening below 1 M⊙, in agreement with
the Galactic IMF once results are corrected for unresolved binarity. I study the age distribution
of LH 95, introducing a statistical method to derive average age and age-spreads
accounting simultaneously for unresolved binarity, differential extinction, variability, accretion
and crowding of PMS stars. The best-fit solution for LH 95 suggests an age of
∼ 4 Myr with a gaussian age spread of σ ∼ 1.2 Myr. Finally, I study the early-type highmass
stellar population of the cluster, through ground based spectroscopy obtained with
the Fiber-fed Extended Range Optical Spectrograph (FEROS) at the ESO/MPI 2.2-m
telescope at La Silla Observatory, and photometry from the 1-m telescope at Siding Spring
Observatory. The derived stellar masses are combined with my results on the low-mass
IMF of the cluster for the study of the most complete extragalactic IMF ever performed.
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Olga Zacharopoulou
(Greece)
|
On the origin of the unusually hard gamma-ray spectra of TeV blazars
(thesis pdf, 2MB)
| Supervisor:
Felix Aharonian (MPIK)
| | Thesis abstract:
The observed VHE spectra from blazars may be significantly modified due to interactions of $gamma$-rays with intergalactic radiation fields. To study the emission production in these kind of objects, one should reconstruct the intrinsic spectra using an Extragalactic Background Light (EBL) model. Interestingly, this correction often leads to unusually hard spectra. In this dissertation we take into account the EBL absorption by using different models to reconstruct blazars' spectra and we study the formation of broad-band spectra in the framework of a proton synchrotron scenario with non-negligible gamma-gamma absorption in the production region. This internal absorption leads to rather hard VHE spectra. Moreover, a significant fraction of the energy absorbed in the VHE band may be transferred into secondary electron-positron pairs providing an additional radiation channel that explains the observed radiation in the Optical/X-ray regimes. In order to demonstrate the potential of the model, we model two relatively distant blazars, 1ES 0229+200 (z=0.1396) and 3C 66A (z=0.444). In addition, we perform numerical simulations using relativistic MHD in time variable injection setup, in an attempt to understand better the conditions under which radiation is produced in relativistic jets.
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Tatiana Vasyunina
(Russia)
|
Very early stages of massive stars
(thesis pdf, 27MB)
| Supervisor:
Thomas Henning (MPIA)
| | Thesis abstract:
The goal of this thesis work is to investigate the initial conditions and the early phases
of the formation of high{mass stars by means of dedicated observational studies. We consider
two object classes: infrared dark clouds (IRDCs), and a subsequent stage characterized by the
presence of young embedded (proto-)stellar clusters.
First, we estimate masses and column densities for a completely new sample of southern
IRDCs, utilizing our own millimeter continuum emission maps as well as Spitzer satellite data.
The parameters we derive show that IRDCs from our sample have the potential to form not only
low- and intermediate-mass stars, but can also be the birth places of massive stars. In particular,
the comparison of our results with previously obtained data for low-mass starless cores shows a
clear trend for IRDCs to have systematically higher column densities. The interpolated IRDC
peak column densities exceed the threshold for the onset of massive star formation previously
advocated by theoretical studies. We also critically evaluate the limitations of the applied
emission and extinction methods.
Second, we investigate the physical conditions and the chemical composition of the dense gas
within the IRDCs of our sample by means of a multi{line molecular spectroscopy analysis. All
the clouds have complex HCO+ line proles, and we detect SiO emission in some clouds. These
findings mark the presence of infall and out
ow motions in at least some parts of the IRDCs,
and hence, ongoing star formation. In comparison with low-mass starless cores, the IRDCs have
broader and more intense lines. Thus, they may be characterised by more turbulent conditions.
Nevertheless, we find a tendency for the IRDCs to have molecular abundances similar to the
case of low-mass starless cores. This indicates similar chemical initial conditions for low- and
high-mass star-forming regions.
Finally, we study the stellar composition and environmental conditions in the young embedded
cluster IRAS 06058+2138. We perform an analysis of all the available data, covering
the near- and mid-infrared as well as the centimeter and millimeter wavelength range. In particular
new VLT/SINFONI integral{field spectroscopy data have played a pivotal role. We
identify three star-forming clusters at diㄦent evolutionary stages. Comparing the relative
ages of these clusters and their relative locations, we conclude that triggering could not have
led to the cluster configuration we see in the region. Thus, we assume a merely sequential
star-formation scenario for this region. With this study, we also demonstrate the importance of
a multi{wavelength approach. Combining data at diㄦent wavelengths allows us to investigate
both the more evolved stellar population, as well as very young embedded objects, and hence,
to reconstruct the star-forming history of structured cluster environments.
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Jean-Claude Waizmann (ITA)
(Germany)
|
On finding galaxy clusters with Planck and the spherical collapse model in different Dark Energy cosmologies
(thesis pdf, 20MB)
| Supervisor:
Matthias Bartelmann (ITA)
| | Thesis abstract:
One of the main objectives of the Planck mission is to perform a full-sky cluster survey based on the Sunyaev-Zel'dovich (SZ) effect, which leads to the question of how such a survey would be affected by cosmological models with a different history of structure formation than LCDM. To answer this question, I developed a fast semi-analytic approach for simulating full-sky maps of the Compton-y parameter, ready to be fed into a realistic simulation pipeline. I also implemented a filter and detection pipeline based on spherical multi-frequency matched filters, that was used to study the expected SZ cluster sample of Planck. It turned out that realistic samples will comprise ~1000 clusters at low rate of contamination, significantly lower than originally anticipated.
Driven by wrong estimates of the impact of early dark energy models on structure formation, we studied the spherical collapse model in dark energy model, finding that models with varying equation-of-state have a negligible impact on the structure formation. Yet, the different expansion history for the different models can be detected via volume effects, when counting objects in a known volume. Furthermore, it turned out that the different expansion history strongly affects the angular SZ power spectra for the various models, making them an interesting tool to distinguish and constrain alternative cosmologies.
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Andras Zsom (MPIA)
(Hungary)
|
Monte Carlo models of dust coagulation
(thesis pdf, 40MB)
| Supervisor:
Kees Dullemond (MPIA)
| | Thesis abstract:
The thesis deals with the first stage of planet formation, namely dust coagulation from micron to millimeter sizes in circumstellar disks. For the first time, we collect and compile the recent laboratory experiments on dust aggregates into a collision model that can be implemented into dust coagulation models. We put this model into a Monte Carlo code that uses representative particles to simulate dust evolution. Simulations are performed using three different disk models in a local box (0D) located at 1 AU distance from the central star. We find that the dust evolution does not follow the previously assumed growth-fragmentation cycle, but growth is halted by bouncing before the fragmentation regime is reached. We call this the bouncing barrier which is an additional obstacle during the already complex formation process of planetesimals. The absence of the growth-fragmentation cycle and the halted growth has two important consequences for planet formation. 1) It is observed that disk atmospheres are dusty throughout their lifetime. Previous models concluded that the small, continuously produced fragments can keep the disk atmospheres dusty. We however show that small fragments are not produced because bouncing prevents fragmentation. 2) As particles do not reach the fragmentation barrier, their sizes are smaller compared to the sizes reached in previous dust models. Forming planetesimals from such tiny aggregates is a challenging task. We decided to investigate point 1) in more detail. A vertical column of a disk (1D) is modeled including the sedimentation of the particles. We find that already intermediate levels of turbulence can prevent particles settling to the midplane. We also find that, due to bouncing, the particle size distribution is narrow and homogenous as a function of height in the disk. This finding has important implications for observations. If it is reasonable to assume that the turbulence is constant as a function of height, the particles observed at the disk atmospheres have the same properties as the ones at the midplane.
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Ronny Zhao-Geisler
(Germany)
|
The surprising mid-IR appearance of the Asymptotic Giant Branch stars R Aql,
R Aqr, R Hya, V Hya and W Hya: molecular and dust shell diameters and their
pulsation dependence probed with the MIDI interferometer
(thesis pdf, 20MB)
| Supervisor:
Andreas Quirrenbach (LSW)
| Thesis abstract:
Asymptotic Giant Branch (AGB) stars are the main distributors of dust into the interstellar
medium due to their high mass loss rates in combination with an effective dust condensation.
It is therefore important to understand the dust formation process and sequence in their
extended atmosphere. The wind of these stars is driven by strong stellar pulsation in
combination with radiation pressure on dust. High-resolution mid-IR interferometry is
sensitive to the structure of the stellar atmosphere, consisting of the continuum photosphere
and overlying molecular layers, as well as to the properties of the dust shell.
This work studies the location of molecular layers and newly formed dust as a function of
pulsation cycle and chemistry, as well as tries to identify molecules and dust species
which cause the diameter of the star to vary across the N-band spectral domain (8-13 microns).
Mid-IR interferometric data of the oxygen-rich AGB stars R Aql, R Aqr, R Hya and W Hya,
and the carbon rich AGB star V Hya were obtained with MIDI/VLTI between April 2007 and
September 2009, covering several pulsation cycles. The spectrally dispersed visibility data
are modeled by fitting a fully limb-darkened disk in order to analyze the molecular layers,
and by fitting a Gaussian in order to constrain the extension of the dust shell. Because
uv-coverage was sufficient for R Hya and W Hya, asymmetries could be studied with an
elliptical fully limb-darkened disk.
The angular diameters of all Oxygen-rich stars in the sample appear to be about two times
larger in the mid-IR than their photospheric diameters estimated from the near-IR. The
overall larger diameter in the mid-IR originates from a warm optically thick molecular layer
of H2O, and a detected gradual increase longword of 10 microns can be attributed to the
contribution of a spatially resolved, optically thin, close corundum (Al2O3) dust shell.
A significant contribution of SiO shortward of 10 microns cannot be ruled out for R Aqr.
The circumstellar silicate dust shells of all oxygen-rich stars are found to be very extended except
for R Aqr. For the carbon-rich star V Hya, it can only be concluded that amorphous
carbon and SiC dust is already present close to the star.
The observed angular diameters are smaller at visual minimum than at visual maximum with
peak-to-peak variations on the order of 20% to 30% except for W Hya for which the
variation is 6%. The diameter periodicity can be explained with the phase-dependent water
vapor and corundum dust presence and its temperature sensitivity. Since this variation traces only
the location of constituents which are not relevant for the wind formation, no firm conclusions
can be drawn concerning the mass loss mechanism. One can only speculate that more dust forms at
visual minimum. Cycle-to-cycle variations of the layer traced with MIDI are lower than intracycle
variations, and are on the order of 6%. R Hya does not show any deviations from circular
symmetry, while an asymmetry of the extended atmosphere of W Hya can be confirmed.
These observations of a larger sample of stars than available before confirm previous results, and
emphasize the need for dynamic stellar model atmospheres with consistently included dust
formation close to the star. It can also be concluded that interferometric observations in the
N-band are an irreplaceable tool to resolve close stellar structures and to search for
atmospheric constituents.
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Benjamin Moster
(Germany)
|
Hydrodynamic Simulations of Cosmological Galaxy Merger Trees
(thesis pdf, 6MB)
| Supervisor:
Hans-Walter Rix (MPIA)
| | Thesis abstract:
In this thesis we use cosmological merger trees and semi-analytic models of galaxy formation to
provide the initial conditions for multi-merger hydrodynamic simulations.
In this way we exploit the advantages of merger simulations (high resolution and detailed
treatment of the gas physics) and semi-analytic models (cosmological background and low
computational cost), and integrate them to create a novel tool.
This approach allows us to study the evolution of various galaxy properties with an improved
treatment of the gas components, including, for the first time, the hot gaseous halo from which gas
cools and accretes onto the central disc.
Using a suite of minor merger simulations we find that disc thickening is reduced relative to the
collisionless case through the absorption of kinetic impact energy by the gas.
In a following series of major merger simulations, we show that adding the hot gas component is
a key ingredient in order to reproduce several observed properties of elliptical galaxies, like
the abundance of fast rotators. Moreover, the presence of a gaseous halo reduces the starburst
efficiency.
We then focus on the effects of multiple concurrent mergers, which we found to be cosmologically
more common than sequences of isolated binary mergers.
For this, we investigate, whether accreted satellite stars can be distinguished kinematically from
stars formed 'in situ' in the central galaxy, and find that this is only possible for a fraction
of the disrupted satellites.
Our simulations to date indicate that the combination of a detailed treatment of gas physics, high
resolution, and a cosmological background, brings numerical simulations in better agreement with
observations.
Overall, the novel tool developed in this thesis will be very useful for pursuing a number of
questions pertaining to the transformation of galaxy internal structure by mergers and accretion.
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Kelly Foyle
(Canada)
|
Spiral Arms and their Effects on Secular Evolution and Star Formation in Disk Galaxies
(thesis pdf, 20MB)
| Supervisor:
Hans-Walter Rix (MPIA)
| | Thesis abstract:
We investigate how spiral structure affects the observational properties of disk galaxies
both in terms of dynamical secular evolution and of star formation. We derived the first
observational estimate of the torque-induced instantaneous angular momentum flow, resulting
from non-axisymmetric features in the stellar distribution for a sample of 24 galaxies.
The strongest torques were found among barred galaxies. In the inner regions, the average
torques are strong enough to redistribute angular momentum on a timescale of ~4 Gyr with
an outward angular momentum flow. In examining the role of spiral arms in star formation we
found that they do not dominate, even in grand-design spiral galaxies as there is a comparable
amount of interarm star formation. Further, we found that the arms show no enhancement in the
efficiency of star formation in terms of molecular gas. We searched algorithmically for angular
offsets between star formation tracers and found that there was no systematic spatial ordering
of these tracers, as would be predictable by a shock triggering model of spiral structure. It
seems spiral structure is most likely transient or at least more complex than the simplest
models predict. These results point to a spiral structure that plays a lesser role in shaping
a galaxy’s observable properties as was previously thought. The strength of gravitational torques
depends more strongly on bars than on spiral structure, and spiral arms are not regions of
enhanced star formation efficiency. At best they act to reorganize the interstellar medium and
concentrate the gas.
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Olaf Schulz
(Germany)
|
The design study of IceCube DeepCore : Characterization and veto studies
(thesis pdf, 15MB)
| Supervisor:
Elisa Resconi (MPIK)
| | Thesis abstract:
In this thesis the design study for the low energy extension to the IceCube neutrino
telescope, called DeepCore, is presented. IceCube and DeepCore are currently under
construction at the geographic South Pole, Antarctica. With DeepCore the energy
threshold of IceCube is significantly lowered, opening new possibilities to, e.g.,
improve low-energy point source searches, to measure standard vacuum neutrino oscillations
and to increase the sensitivity to solar dark matter annihilation signals. In parallel
to the DeepCore design study a, software based, active atmospheric muon veto technique
has been developed, which for the first time, allows IceCube to measure neutrinos with
a full sky (4 Pi) acceptance, although at a significantly reduced effective volume.
Furthermore, in the course of the active atmospheric muon veto development, a new
opportunity has been discovered to also reduce the measured rate of atmospheric muon
neutrinos, a previously irreducible background to any search for extra-terrestrial
neutrinos.
This effect has been analytically studied, independent of any detector design.
As of today, the first data-taking period with full atmospheric muon veto capabilities
lies ahead. This thesis concludes with a first look on early data taken with DeepCore
and a first estimation of a sensitivity to a southern hemisphere point source.
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Christian Schwab
(Germany)
|
A calibration system for a Rayleigh laser guide star constellation & a planet around a giant star
(thesis pdf, 4MB)
| Supervisor:
Andreas Quirrenbach
| | Thesis abstract:
My thesis consists of two parts. In Part I, I describe my contributions to the laser guide star
adaptive optics facility at the Large Binocular Telescope, the Advanced Rayleigh guided Ground
layer adaptive Optics System , ARGOS. I investigate the effect of scattered light from propagating
the laser light across the aperture, and find that contamination is not a concern for the science
instruments. I present a study of reflective laser launch telescopes based on commercial optics,
and compare this to the refractive design chosen as baseline for ARGOS. The discussed options
present an interesting alternative with only small additional light loss at substantially reduced
cost and procurement risk. I develop a calibration scheme for the full adaptive optics system,
based on artificial light sources that illuminate the deformable mirror, imitating the laser guide
star beacons. This enables the interaction between deformable mirror and wavefront sensor to be
calibrated at any time, greatly enhancing the possibilities and time available for engineering on
the installed system. The light source has to be placed in the prime fcous, as ARGOS uses the
adaptive secondary mirrors of LBT. The optical design of the calibration light source is not
trivial as the guide star constellation comprises of three beacons forming an equilateral triangle
on a circle with a radius of 2 arcminutes. The images of the beacons that have to be reproduced
suffer from strong aberrations caused by the large off-axis distance. To match the wavefront of the
beacons' images to the desired precision of 50 nanometres rms, I designed a custom objective,
incorporating a computer generated hologram to shape the wavefronts of three optical fibres forming
the light sources. The elliptical front surface of the objective is used in reflection to generate
a central, diffraction limited spot serving as an alignment aid and truth sensor for the measurements.
A thorough tolerance analysis including the assembly and the alignment at the telescope ensures that
the design specifications can be met during operation. The second part of the thesis concerns the
search for planets around a sample of nearby giant stars with the Doppler technique carried out at
Lick Observatory's CAT telescope. I analyse new data from the previous three years, which together
with the existing data from our survey form a database covering 11 years. The radial velocity
measurements reveal the presence of a planetary companion around one of our target stars, which was
previously unknown. This is the lightest planet found around a giant star to date with a minimum
mass of 1.92 Jupiter masses in a 785 day orbit.
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|
Peter Melchior
(Germany)
|
Shapelets for gravitational lensing and galaxy morphology studies
(thesis pdf, 3MB)
| Supervisor:
Matthias Bartelmann
| | Thesis abstract:
The presented work is concerned with the morphological description of stars and galaxies
in the framework of the shapelet method. This method constitutes a linear expansion in
the orthonormal set of Gauss-Hermite polynomials. Its main advantages – linearity,
compactness, invariance under Fourier transformation, and the relation to the moments
of the brightness distribution – are extensively discussed. The practical treatment of
the image decomposition and of the deconvolution from the point spread function are
further elaborated. Moreover, three fields of application are presented together with
new investigations on the applicability and validity of the method: weak gravitational
lensing, morphological class discovery, and realistic simulation of extragalactic
observations.
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Giovanni Natale
(Italy)
|
Dust emission and star formation in the Stephan's Quintet compact group of galaxies
(thesis pdf, 5MB)
| Supervisor:
Richard Tuffs (MPIK)
| Thesis abstract:
Understanding the intergalactic medium (IGM) gas cooling processes, which are necessary
to fuel star formation in galaxies, and the effect of galaxy-galaxy and galaxy-IGM
interactions, which modify stellar and gas distributions of galaxies in groups and clusters,
is vital to construct realistic models of galaxy formation and evolution.
The Stephan's Quintet (SQ) compact group of galaxies is a natural laboratory for studying
these phenomena because the galaxies of this group are heavily interacting between each other
and with the group IGM. Furthermore its vicinity allows to study the details of the interaction
phenomena, which are believed to be much more common in the early universe, and its compactness
on the sky permits studies of diffuse components associated with the group IGM.
In this thesis we present an analysis of a comprehensive set of MIR/FIR observations of
Stephan's Quintet, taken with the Spitzer Space Observatory.
The emission seen at these wavelengths is produced by dust particles and can be used to
trace star formation events, AGN activity and also hot gas cooling, in the case dust emission
is powered by collisions between plasma particles and dust.
Applying a novel fitting technique to the Spitzer FIR maps, we have been able to separate the
different sources of dust emission in this group and perform their photometry at FIR as well
as MIR wavelengths. Our study has revealed for the first time the presence of a luminous and
extended component of infrared dust emission, not connected with the mainbodies of the group
galaxies, and roughly coincident with the X-ray halo of the group. We fitted the inferred
dust emission spectral energy distribution of this extended source and the other main infrared
emission components of SQ, including the intergalactic shock, to elucidate the mechanisms powering
the dust and PAH emission, taking into account dust collisional heating and heating through UV
and optical photons.
Combining the fraction of dust luminosity powered by UV photons, as derived from the SED fitting,
with the UV luminosity directly observed on the GALEX FUV map of SQ, we estimated the star
formation rate (SFR) for each dust emitting source, thus providing a complete picture of star
formation in SQ embracing obscured and unobscured components. The total SFR of SQ is
$7.5~{rm M_odot/yr}$, similar to the value expected for non interacting galaxies of the same mass
of SQ galaxies. However the star formation sites are found mainly at the periphery of the galaxies or
in the intergalactic medium, at variance with the usual pattern of star formation in field
galaxies which is typically distributed in the central regions or main bodies of galaxies.
Despite the unusual location of star formation sites, we have found that, for the brightest
sources in SQ, the SFR per unit physical area is similar to that characteristic of disk galaxy
star formation regions when compared to the corresponding gas column density on a Kennicutt-Schmidt
diagram. We also show that even though the detected extended component of dust emission trace the
distributed group star formation, available sources of dust in the group halo can provide enough
to produce up to $L_{IR}approx10^{42}~{rm erg/s}$ powered by collisional heating. This amount,
several times higher than the X-ray halo luminosity, could provide an important cooling mechanism
for the IGM hot gas.
At the end of the thesis we present a theoretical model of a high velocity shock, similar to the
one occuring in SQ IGM, taking into account dust cooling and dust destruction. This model shows
that, although the efficiency of dust cooling drops quickly because of dust removal by sputtering,
the gas cooling time is reduced by a factor of 2-3, compared to the case where only radiative cooling
is considered.
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Yolanda Sestayo de la Cerra
(Spain)
|
Search for high energy neutrinos from our galaxy with IceCube
(thesis pdf, 5MB)
| Supervisor:
Elisa Resconi / Konrad Bernloehr (MPIK)
| | Thesis abstract:
High energy neutrino telescopes are expected to play a major role in the discovery
of the first unambiguous sources of cosmic-rays. With completion in 2011, the IceCube
neutrino detector constitutes the most sensitive instrument to sources of high energy
neutrinos. Its performance and discovery potential are usually given in the energy
range above a few TeV, in order to achieve the best signal to noise for sources
following an E^-2 spectrum without an energy cutoff up to at least 1 PeV. However,
given the present understanding and multiwavelength picture of our galaxy, we can
expect that galactic sources of high energy neutrinos show significant deviations
from the E^-2, no cutoff approximation. The common data analysis are therefore not
optimal for such galactic scenarios, requiring exposure times of the order of
several years, even a decade, to reach a level of sensitivity at which a possible
detection starts to be plausible. The main goal of this thesis is to improve the
discovery potential of IceCube to galactic sources of high energy neutrinos, aiming
to a better understanding of the high energy processes taking place in our galaxy.
In order to fulfill this goal, I follow two lines of action: (1) to increase the
detection capabilities of IceCube for neutrinos in the energy range between
100 GeV < E < 1 TeV; and (2) to develop a search method which is able to reduce the
minimum detectable flux per point source. The improvement of the IceCube performance
at energies below 1 TeV is achieved with the use of the combined detector configuration
IceCube 22 strings plus AMANDA (Antarctic Muon And Neutrino Detector Array). The data
processing scheme is designed in order to keep as many good low energy events as
possible. As a result, this analysis achieved the best sensitivity for sources with
neutrino spectra steeper than E^-2 and/or an energy cutoff below 1 PeV. The second
goal of this thesis is motivated in order to search efficiently for high energy neutrinos
from the Cygnus star forming region of the Galaxy. In order to extend the search beyond
a single point source, I developed a method based on two-point analysis to detect,
within an extended region, event patterns which might go undetected in conventional
point source analysis. The results obtained with this method indicate that the minimum
detectable flux per point source is reduced by 26%-59% with respect to standard point
source analysis, provided there is more than one point source within the region under
study. This method was applied on the Cygnus region of the Galaxy using the data
sample obtained with the combined detector IceCube 22 strings plus AMANDA, yielding
a significance of 2.3-sigma.
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Julian Merten
(Germany)
|
Gravitational lensing : an advanced method to recover the mass distribution of galaxy clusters
(thesis pdf, 20MB)
| Supervisor:
Matthias Bartelmann (ITA)
| | Thesis abstract:
This work shall be on clusters of galaxies. Those most massive, gravitationally bound objects
in the observable Universe represent the high-mass tail of the mass function, rendering them
as objects of interest for cosmology. Not only that they allow for the derivation of several
cosmological parameters, but they are also ideal cosmic laboratories. Direct comparisons
between numerical simulations and observations are particularly appealing in the case of
clusters, as we will show. Maybe the most promising method to derive cluster properties from
observations is gravitational lensing. Light rays of distant background sources are bent on
the way to the observer, due to the high mass concentrations of clusters, and thereby carry
important information about the deflector. In this work we develop an advanced, nonparametric
method to recover the mass distribution of galaxy clusters by combining weak and strong
gravitational lensing. The underlying numerical algorithm makes use of modern concepts of
high-performance computing and is fully parallelised.
We proof the capabilities of our method, compared to established methods, while reconstructing
simulated clusters of galaxies and capitalising realistic lensing scenarios. We close our work
with the reconstruction of two well-known, strongly lensing clusters, namely
MS2137.3-2353 and CL0024+1654.
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|
Christoph Federrath
(Germany)
|
The formation of molecular clouds and stars by turbulent compression and collapse
(thesis pdf, 13MB)
| Supervisor:
Ralf Klessen (ITA)
| | Thesis abstract:
The goal of my PhD thesis was to improve our understanding of the role of interstellar turbulence
in star formation. In particular, the mechanism of turbulence energy injection, the turbulence
forcing, is investigated with hydrodynamical simulations. In a systematic comparison, I study the
two limiting cases of turbulence forcing: solenoidal (divergence-free) forcing and compressive
(curl-free) forcing. I show that these two cases yield significantly different gas density and
velocity statistics. The fractal structure of the gas and the turbulent density probability
distribution function (PDF) are explored in detail. I find that compressive forcing yields a
three times higher standard deviation of the roughly Gaussian density PDF. I discuss the impact
of this result on analytic models of star formation. A detailed comparison with observational data
reveals that different observed regions show evidence of different mixtures of compressive and
solenoidal forcing, with more compressive forcing occurring primarily in swept-up shells. To
follow the gravitational collapse of dense gas in numerical simulations, I implemented accreting
sink particles in the adaptive mesh refinement code FLASH. Using sink particles, I show that
compressive forcing yields star formation rates more than one order of magnitude higher than
solenoidal forcing, consistent with analytic models.
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Aday Robaina
(Spain)
|
Eight Giga years of galaxy mergers
(thesis pdf, 3MB)
| Supervisors:
Hans-Walter Rix / Eric Bell (MPIA)
| Thesis abstract:
Galaxy interactions are expected to play a crucial role in the build-up of stellar mass in
any cold dark matter cosmology. Of particular interest are the mergers between systems of
a comparable mass, as they are predicted to be one of the main modes of galaxy growth and
have a crucial impact in the shaping of galaxy morphologies and dynamics.
In this thesis I
study two key aspects of the role that mergers play in galaxy evolution: a) What is the
contribution of major galaxy interactions to the star formation history of the Universe at
z < 1?, and b) How important are galaxy interactions for the build-up of the massive end
of the red sequence?
To answer the first question I use photometric redshifts, stellar
masses and UV star formation rates from COMBO-17, 24 microns star formation rates from Spitzer
and galaxy morphologies from two deep Hubble Space Telescope cosmological survey fields to
study the enhancement in star formation activity as a function of galaxy separation. I
apply robust statistical tools to find galaxies in close pairs, augmented with morphologically-selected
very close pairs (unresolved in the ground-based photometry) and merger remnants from the
Hubble Space Telescope imaging, finding that, on average, major galaxy interactions between
galaxies more massive than 10^10 M_sun at 0.4 < z < 0.8 enhance the star formation activity by a
factor of less than 2. I carry out detailed modeling of the methodology using a mock galaxy
catalog from the Millenium Simulation, finding that in the regime applicable to this work
the recovered enhancement in SF rate is accurate to better than 10%, smaller than the other
sources of uncertainty.
Accounting for the fraction of merging and interacting systems, I integrate the enhanced star
formation to demonstrate that less than 10% of star formation activity is directly triggered
by those interactions. To answer the second question I look for close pairs of galaxies on a
sample drawn from the COSMOS and COMBO-17 galaxy surveys to find that the fraction of
M_* > 5x10^10M_sun galaxies in close pairs (a proxy for the fraction of objects involved in
an interaction) were more common 7 Gyrs ago by a factor ~2. By converting this merger fraction
to a merger rate I estimate that 70% of the very massive galaxies (M_*>10^11 M_sun)
have undergone a merger since z=1.2. This merger rate is sufficient to explain the observed
number density evolution of such massive galaxies in the last 7 Gyrs.
Merging plays, therefore, a dominant role in the formation of massive galaxies in the Universe.
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Rosalind Skelton
(South Africa)
|
The Effect of Mergers on Galaxy Formation and Evolution
(thesis pdf, 4MB)
| Supervisor:
Eric Bell (MPIA)
| | Thesis abstract:
This thesis explores the effect of galaxy mergers on the evolution of galaxies
over the last 8 billion years using the merger trees from a semi-analytic model
(SAM) of galaxy formation. The SAM produces reasonable agreement with the distribution
of mass, luminosity and colour at low redshifts, as well as the observed merger
fractions. I revisit two apparent contradictions between the standard hierarchical model
of galaxy formation and observations of early-type galaxies, using the galaxy merger
trees as the basis for further modelling. The observed colour--magnitude relation from
the Sloan Digital Sky Survey has a change in slope and smaller scatter at the bright
end. A simple toy model shows that dry mergers produce similar characteristics. Contrary
to previous claims, the small scatter in the observed CMR thus cannot be used to constrain
the amount of dry merging. I incorporate stellar population synthesis modelling into
this framework to explore the evolution of early-type galaxies since z=1. There is strong
evolution in colour and magnitude if no mergers occur after this time. Dry mergers and the
recent addition of younger populations onto the red sequence reduce the evolution, mimicking
that of an ancient passively-evolving population. Early-type galaxies can therefore appear to
have evolved passively even though significant merging activity continues to recent times.
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Cassandra Fallscheer
(United States)
|
Massive Star Formation : the Role of Disks
(thesis pdf)
| Supervisor:
Henrik Beuther (MPIA)
| Thesis abstract:
In this thesis, I study three different evolutionary stages of the massive star formation
process looking for supporting evidence for an accretion-based formation scenario of massive
stars. The first source studied, the Infrared Dark Cloud IRDC 18223-3, is at one of the
earliest observable phases of massive star formation. This source is characterized by a
cone-shaped molecular outflow component which is used to establish the outflow orientation.
A velocity gradient traced by the molecule N2H+ but more convincingly by CH3OH is indicative of
a rotating object oriented orthogonally to the outflow direction. This object is on the order
of 28,000 AU in size and does not exhibit Keplerian rotation, but may host a disk within.
Modeling this velocity gradient shows that a single rotating and infalling entity is capable of
reproducing the observations.
Moving to a High Mass Protostellar Object, IRAS 18151-1208, a well-defined outflow orientation
is observed as well as an elongation in the 1.3 millimeter dust continuum that is perpendicular to
the outflow. This elongation is modeled using a Monte Carlo 3D radiative transfer code. Comparing
the modeling results to those of low mass protostars it is deduced that a scaled up version of
low-mass star formation provides a plausible description of the observations in this high mass
case. In the scaled up version, the density and flaring exponents as well as the relative scale
height at one third of the outer radius remain the same as in the low-mass model. The disk mass,
outer radius, and central star's mass and luminosity all increase.
The third source studied in this thesis, the hot molecular core IRAS 18507+0121, exhibits the rich
chemistry characterizing the hot core phase of massive star formation. The outflow orientation is
confirmed and each chemical species is looked at for indication of rotation. Somewhat surprisingly,
clear signatures of rotation are not detected and several possible explanations for this are
discussed such as insufficient spatial resolution. However, along the lines of what has been observed
in IRAS 18151-1208, a slight elongation in the dust continuum perpendicular to the outflow
orientation is detected.
Several approaches are explored as a means of studying whether the observable differences in the
massive star formation regions are a result of evolution. Taken individually, no indicator is
sufficient to definitively determine an age sequence for the three sources. However, taken
collectively, the trends seen in these case studies can be attributed to an evolutionary sequence.
The results of this thesis are consistent with an accretion based formation mechanism of massive
stars and I conclude that the structural changes of the observed disk-like structures from
large-scale to more compact may be the result of evolution.
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|
Bagmeet Behera
(India)
|
Effects of EBL extinction on the VHE spectra of blazars
(thesis pdf, 15MB)
| Supervisor:
Stefan Wagner (LSW)
| Thesis abstract:
Active Galactic Nuclei (AGN) are the most powerful steady sources of electromagnetic
radiation in the cosmos. Blazars are AGN with their jets pointing straight at us, giving
us the opportunity to probe deep into the core which generates this tremendous luminosity.
The Very High Energy (VHE; E>100 GeV) photons from blazars while travelling through the
intergalactic medium, interact with the low energy photons (in the UV-IR range) constituting
the Extragalactic Background Light (EBL), by producing e-e+ pairs, and get absorbed. More
than 25 nearby blazars (z<0.3) have been detected in VHE, but due to EBL extinction it is
extremely difficult to detect distant sources. This phenomena gives us the means to measure
the EBL level which cannot be directly measured. In this thesis the effects of EBL absorption
on the spectra of blazars is studied. Two methods to discover distant VHE blazars were
explored. The observations with HESS led to the detection of the blazar PKS 1510-089 at
z=0.36, at a 4.8 sigma level. A cross check analysis with a more advanced analysis tool confirmed
this detection at a 8.5 sigma level. The GeV spectrum obtained by analyzing Fermi GST data,
was adopted as the intrinsic source spectrum. Models for the gamma-ray spectrum including the
EBL absorption was fitted to the gamma-ray data and it was shown that all the 4 current EBL models
considered here, fit the data well. It is concluded that this is due to the sensitivity limits
of the current VHE instruments. Stronger constraints on the EBL would only be possible for sources
with much harder spectrum. The outlook for future experiments is discussed.
EBL extinction causes bias in the sample of detected blazars. MonteCarlo simulations were used
to generate parent samples for the VHE blazars. These were compared to the true VHE blazar sample
and constraints on the parent sample properties were drawn. The lack of a spectral softening with
z in the true VHE sample, is found consistent with a parent sample that clearly shows this softening,
making it unnecessary to assume any special dependence of the intrinsic spectral index on z. The
dependence of the evolution of the EBL on the cosmological model considered, was explored. The
uncertainty in cosmology was found to be negligible compared to the uncertainty on the EBL. The
dependence of the extinction on the error in the EBL models as well as the effect of neglecting
the EBL evolution with z was illustrated.
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Rolf Kuiper
(Germany)
|
Modeling the formation of massive stars
(thesis pdf)
| Supervisors:
Hubert Klahr / Thomas Henning (MPIA)
| | Thesis abstract:
I investigate the radiation pressure problem in the formation of massive stars using a newly developed frequency
dependent radiation transport module for three-dimensional hydrodynamics simulations. The nature of the
radiative impact depending on the morphology of the stellar environment is examined in one-, two-, and three-
dimensional monolithic collapse calculations of massive pre-stellar cores. Contrary to previous research, a
highly superior frequency dependent stellar feedback is considered, the vicinity of the forming star is resolved
down to 1.27 AU, and the evolution is computed for a factor of ten longer. For the first time a broad survey of the
parameter space is possible. The simulations demonstrate the need of including the dust condensation front to
compute the radiative feedback correctly. Earlier calculations, which ignore these physics, lead to an artificial
truncation of the accretion phase. The most fundamental result is that the formation of a massive accretion
disk in slowly rotating cores bypasses the radiative flux through the optically thin atmosphere, enabling steady
accretion. A revealed close-by gravitational instability in the disk drives a sufficiently high accretion rate to
overcome the residual stellar radiation feedback. This mechanisms allow the star to grow far beyond any limit
found in earlier calculations.
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Thomas Peters
(Germany)
|
Ionization Feedback in Massive Star Formation
(thesis pdf)
| Supervisor:
Ralf Klessen (ITA)
| | Thesis abstract:
Understanding the origin of high-mass stars is central to modern astrophysics. We shed
light on this problem using novel radiation-hydrodynamic simulations that consistently
follow the gravitational collapse of a massive molecular cloud, the subsequent build-
up and fragmentation of the accretion disk surrounding the nascent star, and, for
the first time, the interaction between its intense UV radiation field and the infalling
material. We show that ionization feedback can neither stop protostellar mass growth
nor suppress fragmentation. We present a consistent picture of the formation and
evolution of H ii regions that explains the observed morphology, time variability, and
ages of ultracompact H ii regions, solving the long-standing lifetime problem.
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Anton Vasyunin
(Russia)
|
Chemistry in the ISM and disks on the verge of planet formation
(thesis pdf)
| Supervisors:
Dmitry Semenov / Thomas Henning (MPIA)
| Thesis abstract:
The general purpose of the thesis work is to improve astrochemical
models in the era of ALMA. This era is characterized by the
active study of protoplanetary disks and the search for
extraterrestrial life.
First, we study how uncertainties in the rate coefficients of chemical
reactions affect the abundances and column densities of key
molecules in protoplanetary disks. We isolate a group of key
species which have column densities that are not very
sensitive to the rate uncertainties, making them good potential
tracers of physical conditions in disks. We identify
about a hundred reactions with the most problematic rate
coefficients, which need to be determined more accurately in
order to improve the reliability of modern astrochemical
models.
Second, we build a realistic astrochemical model using a
Monte Carlo approach to all chemical processes, which is the
first time this has been done. This allows us to
properly take into account the stochastic nature of grain
surface chemical reactions, which are of essential importance
for the formation of organic molecules -- i.e., the precursors
of life as we know it. The recent modified rate approach (MRE) of Garrod et al. (2008)
is shown to be the most accurate fast approach of
accounting for stochastic effects in astrochemical modeling.
Finally, we apply our model to the study of the chemical
composition of an evolving protoplanetary disk with grain
growth, in order to reveal chemical tracers of this
process. For the first time, a state-of-the-art astrochemical
model is coupled with a detailed model of grain growth
and sedimentation.
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Jamie O'Sullivan
(Ireland)
|
Molecular Cooling and Emissions in Large Scale Simulations of Protostellar Jets
(thesis pdf)
| Supervisor:
Max Camenzind (LSW)
| | Thesis abstract:
The origin of infrared molecular emission associated with Class 0 and Class I
protostellar outflows (such as HH211 and HH46/47) is still not fully resolved.
One successful model for describing such phenomena is the jet-driven outflow
model. It proposes that the emission occurs as a high velocity collimated jet
outflow shocks, excites and entrains the molecular ambient matter. Although
this scenario does achieve significant success in describing the dynamics and
morphology of the outflow, the exact nature of the type of shock causing the
emission in such a case - J-type or C-type - is still unclear.
Physical conditions in the gas, such the ionisation fraction and magnetic
field, are crucial parameters determining the type of shock that will form.
However, the immediate region around the class 0 sources producing molecular
outflows usually consists of dense, high-extinction gas within a molecular
core, impeding observational data regarding these details. Therefore,
numerical modelling can play an important role in explaining the observed
outflows.
We have developed and tested a module, implemented within the PLUTO
astrophysical code, to simulate the non-equilibrium molecular chemistry and
cooling in a jet outflow which is interacting with its surrounding molecular
core gas. Using large scale adaptive mesh magnetohydrodynamical simulations,
we predict observationally significant amounts of infrared emissions from
J-shock excited molecular gas. We find that the emission can be caused either
by direct shocking (''prompt entrainment'') or entrainment and ablation of the
ambient gas. We find that the nature of this emission is strongly dependent on
absolute and relative densities of the jet and ambient medium, and on the
presence of moderate magnetic fields (30 - 120 \muG) in the core. Comparing our
results with observations, we confirm that the magnitudes for the emission
strength agree with those observed in several sources. Furthermore we
demonstrate how the appearance of the emission in different sources depends
on the parameters explored here.
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|
Javier Rodon
(Argentina)
|
The fragmentation of massive star-forming Regions
(thesis pdf)
| Supervisor:
Henrik Beuther (MPIA)
| Thesis abstract:
Since its discovery by E. Salpeter in 1955, the high-mass end of the Initial Mass Function
(IMF) has been continuously tested, and its slope has not changed from the value -2.35 originally
calculated by Salpeter, the "Salpeter value". Furthermore, it is found that this value is
universal. It not only describes the mass distribution of stellar masses in the Milky Way but also
in other galaxies.
Stars form individually or in systems within molecular clouds, from local condensations of sizes
on the order of ~0.01 pc, the so-called "dense cores". In the case of low-mass star-forming
regions, it is found that the Core Mass Function (CMF) resembles the Salpeter IMF.
However, in the case of massive star-forming (MSF) regions, the answer is not that clear.
The first CMF for a MSF region was derived in 2004 by H. Beuther and P. Schilke for the MSF region
IRAS19410+2336. They found that this CMF also resembled the Salpeter IMF. Since then, a few more
CMFs for MSF regions have been derived, always with exponents comparable to Salpeter. This suggested
that the CMF and the IMF are related in a one-to-one or nearly one-to-one relationship, and that
the fragmentation processes within a molecular cloud would set the shape of the IMF at an early
evolutionary stage.
Attempting to test that scenario, in this thesis I present and analyze high angular resolution
interferometric observations of several MSF regions at millimeter wavelengths, describing their
protostellar content and deriving their CMF whenever is possible. We confirm the result of
Beuther & Schilke (2004) and obtain a CMF with a power-law slope similar to the Salpeter IMF,
however for other MSF regions we obtain a CMF with a power-law slope flatter than Salpeter.
This difference suggests that the IMF might not be set at the moment of the fragmentation of
the cloud, but insted would be a result of the evolution of the cloud, starting with a flatter
mass distribution that becomes steeper at later evolutionary stages. This result is not conclusive
yet, and we suggest a series of observations that would be needed to fully test it.
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Giovanna Pedaletti
(Italy)
|
Very high energy emission from passive supermassive black holes
(thesis pdf)
| Supervisor:
Stefan Wagner (LSW)
| | Thesis abstract:
The H.E.S.S. experiment, an array of four Imaging Cherenkov Telescopes,
widened the horizon of Very High Energy (VHE) astronomy. Its unprecedented
sensitivity is well suited for the study of new classes of expected VHE
emitters, such as passive galactic nuclei that are the main focus of the
work presented in this thesis. Acceleration of particles up to Ultra High
Energies is expected in the magnetosphere of supermassive black holes
(SMBH). The radiation losses of these accelerated particles are expected
to reach the VHE regime in which H.E.S.S. operates. Predicted fluxes
exceed the sensitivity of the array. However, strong photon fields in the
surrounding of the acceleration region might absorb the produced
radiation. Therefore observations focus on those galactic nuclei that are
underluminous at lower photon energies. This work presents data collected
by the H.E.S.S. telescopes on the test candidate NGC 1399 and their
interpretation. While no detection has been achieved, important
constraints can be derived from the obtained upper limits on the maximum
energy attainable by the accelerated particles and on the magnetic field
strength in the acceleration region. A limit on the magnetic field of B<74
Gauss is given. The limit is model dependent and a scaling of the result
with the assumptions is given. This is the tightest empirical constraint
to date. Because of the lack of signal from the test candidate, a stacking
analysis has been performed on similar sources in three cluster fields.
A search for signal from classes of active galactic nuclei has also been
made in the same three fields. None of the analyzed samples revealed a
significant signal.
Also presented are the expectations for the next generation of Cherenkov
Telescopes and an outlook on the relativistic effects expected on the VHE
emission close to SMBH.
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|
Jonathan Downing
(Canada)
|
Compact binary populations in Globular Clusters and prospects for gravitational wave detection
(thesis pdf, 16 MB)
| Supervisor:
Rainer Spurzem (ARI)
| Thesis abstract:
The inspiral and merger of compact binary stars will be major detection
events for interferometric gravitational wave observatories. These
observatories operate most effectively by comparing their output to template
waveforms. In order to make these templates the physical parameters of the
source population must be understood. Compact binaries in the galactic field
have been investigated using population synthesis models but in dense stellar
environments interactions can alter the binary population and may enhance the
merger rate.
I study compact binaries in star clusters using a Monte Carlo model for the
dynamics. I find that the black hole population interacts strongly, leading
to an enhancement in both the number of black hole binaries and the black hole
binary merger rate. Due to the high interaction rate the majority of black
hole binaries are ejected and thus the mergers occur in the galactic field. I
find a promising rate of $1-100$ detections per year for the next generation
of ground-based gravitational wave detectors and two possible sources for
space-based detectors, both highly eccentric. I conclude that star clusters
must be taken into account in order to predict accurate event rates for
gravitational wave detectors.
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Marcello Cacciato
(Italy)
|
Galaxy-Dark Matter Connection : from Astrophysics to Cosmology
(thesis pdf, 16 MB)
| Supervisor:
Frank van den Bosch (MPIA)
| | Thesis abstract:
Galaxy-galaxy (g-g) lensing represents an ideal technique to constrain the dark matter
distribution on galaxy scales. The required accuracy in the signal can be achieved only
by stacking many foreground galaxies and averaging the ellipticity of the resulting
background galaxies. Unfortunately, the stacking procedure complicates any astrophysical
interpretation. In order to extract information from the composite g-g lensing signal, a
reliable model of the way galaxies populate dark matter haloes is required. We use a
realistic description of the halo occupation statistics based on the conditional luminosity
function. It provides a statistical prescription for the number of galaxies with a given
luminosity living in dark matter haloes of a given mass. Being “a priori” constrained by
the luminosity dependence of the galaxy clustering, it can be used for predicting the g-g
lensing signal without any additional tuning. Our model allows a thorough understanding of
the different terms contributing to the signal. We carefully explore the effect of the
assumptions entering the model. Our theoretical predictions are in very good agreement with
SDSS data. Furthermore, we use the sensitivity of this technique to the underlying cosmological
model to study the feasibility of a joint analysis of galaxy clustering and g-g lensing as a
novel technique to constrain the values of cosmological parameters such as Omega_m and sigma_8 .
We show that uncertainties and systematics in the model do not significantly affect the model
predictions. We conclude that a combined analysis of galaxy clustering and g-g lensing can
be used as a novel, complementary and competitive technique to constrain cosmological parameters.
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Surhud More
(India)
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Galaxy-Dark Matter Connection : Insights from Satellite Kinematics
(thesis pdf, 2MB)
| Supervisor:
Frank van den Bosch (MPIA)
| | Thesis abstract:
A thorough knowledge of the connection between the mass of dark matter haloes and the properties
of their central galaxies is crucial to understand the physics of galaxy formation. The kinematics
of satellite galaxies is an excellent technique to measure the dark matter halo masses. However, the
kinematics can be measured with high signal-to-noise only by stacking the signal around central galaxies
with similar properties, which results in various systematic biases and complicates the interpretation of
the signal. This thesis presents an analytical framework that accounts for systematic biases and
selection effects and aids in the interpretation of the kinematics of satellite galaxies. A new method
is established to obtain the average scaling relations between halo mass and central galaxy properties,
and the scatter in these relations simultaneously. After a thorough testing of this method using a
realistic mock galaxy catalogue, it is applied to the Sloan Digital Sky Survey to extract the halo
mass-luminosity and halo mass-stellar mass relationship of central galaxies and their scatter.
Comparisons with other probes of these scaling relations, such as galaxy-galaxy lensing, show good
agreement which implies that these scaling relations are well established and supported by various
astrophysical probes. Physical insights about these scaling relations, in particular their scatter,
gained by the analysis of a semi-analytical model for galaxy formation are also presented. Finally,
the inferred scaling relations crucially depend on the transparency of the Universe. By performing a test
of the “Etherington relation” between the distances measured by standard rulers and by standard candles,
a quantitative measure of the cosmic transparency, which is relatively free from astrophysical
assumptions, is obtained.
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Thomas Greif
(Germany)
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The Formation of the First Galaxies
(thesis pdf, 16 MB)
| Supervisor:
Ralf Klessen (ITA)
| | Thesis abstract:
The primary concern of this thesis is to understand the formation and properties of the first galaxies,
as well as the influence of the first stars in terms of radiative, mechanical and chemical feedback.
In particular, we elucidate the role of turbulence, ionizing radiation by massive Population III stars,
mechanical feedback by highly energetic supernovae, and chemical enrichment. In light of the next
generation of ground- and space based telescopes, we derive their observational signature in terms of
recombination radiation, bremsstrahlung and 21 cm emission. We find that the cumulative 21 cm signal of the
first H II regions will likely be observable by the planned SKA, while the recombination radiation from
the first starbursts might be observable by JWST. These probes are essential to test the theoretical framework
of the first stars and galaxies and shed some light on this elusive period of cosmic history.
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Maria Victoria Rodriguez-Ledesma
(Argentina)
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Rotation Studies of Young Very Low Mass Stars and Brown Dwarfs
(thesis pdf, 2 MB)
| Supervisor:
Rheinhard Mundt (MPIA)
| Thesis abstract:
Rotation studies over as large a range of ages and masses as possible are important for
constraining the angular momentum evolution of young stellar objects (YSO). Of particular
interest are the very low mass (VLM) stars and brown dwarfs (BDs), because of the
relatively small number of known rotational periods (Prot) in that mass range. This thesis
for the first time provides important information on rotational properties of a large
sample of young VLM stars and BDs.
The extensive rotational period study of YSOs in the 1 Myr old Orion Nebula Cluster (ONC)
presented here is based on a deep photometric monitoring campaign. As a result, 487 periodic
variables (PVs) were found (377 of which are new) with estimated masses between 0.5-0.015
M_odot, 124 of which are BD candidates. This is by far the most extensive and complete rotational
period data set for young VLM stars and BDs to date. The dependence of the period distribution
on mass and level of rotational modulation was investigated and a comparison with higher mass
PVs in the ONC and with the PVs in the approx. 2 Myr old open cluster NGC 2264 was carried out.
Substellar objects were found to rotate on average faster than the VLM stars, a trend which was
already seen for higher mass stars. In addition, the results presented here suggest a dependence
of the Prot on position within the ONC, with the objects located near the cluster center showing
on average larger Prot. This result can be explained by a possible age spread in the ONC with
a somewhat younger central region.
An interesting correlation between Prot and modulation amplitude was found in which PVs with
larger amplitudes rotate on average slower than those with smaller amplitudes, which can in
principle be explained by different magnetic field topologies.
In addition, 732 periodic variables in the ONC with known JHK colours were analysed in order
to investigate whether rotation periods correlate with the presence of circumstellar accretion
disks in particular at very small masses. A highly significant rotation-disk correlation was
found among the low and very low mass periodic variables (0.075 < M < 0.4 M_odot), in which objects
with NIR excess tend to rotate slower than objects without NIR excess. Interestingly, no
correlation was found in the substellar regime. Possible reasons for the absence of such a
correlation are discussed in detail.
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Dominik Schleicher
(Germany)
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The early Universe:
probing primordial magnetic fields, Dark Matter models and the first supermassive black holes
(thesis pdf, 8 MB)
| Supervisor:
Ralf Klessen (ITA)
| | Thesis abstract:
The goal of this work is to better understand the universe between recombination and
reionization and to outline new possibilities to explore it in more detail. This concerns
the stellar population, the physics of the early universe, and the formation of the first
supermassive black holes. With the reionization optical depth from WMAP 5, I derive
upper limits for the strength of primordial magnetic fields and dark matter annihilation / decay,
as well as constraints for stellar population models. Further constraints can be found from
the gamma-ray and neutrino background, which rule out s-wave annihilation of light dark
matter. It was shown that future 21 cm observations will constrain primordial magnetic fields
even further. To improve our understanding of the origin of the first supermassive black
holes and their high metallicity, I explore how they can be observed with ALMA and JWST between
redshift 5 and 15. For this purpose, I estimated and classified the available observables,
and I provide several independent estimates for the expected number of high-redshift black
holes. In spite of large model uncertainties, one can expect to find at least a few sources
in a solid angle similar to the Hubble-Deep-Field.
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Marik Barnabe-Heider
(Canada)
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Performance and stability tests of bare high purity germanium
detectors in liquid argon for the GERDA experiment
(thesis pdf, 30 MB)
| Supervisor:
Stefan Schönert (MPIK)
| | Thesis abstract:
GERDA will search for neutrinoless double beta decay of 76Ge by using a novel
approach of bare germanium detectors in liquid argon (LAr). Enriched germanium
detectors from the previous Heidelberg-Moscow and IGEX experiments have been
reprocessed and will be deployed in GERDA Phase-I. At the center of this thesis
project is the study of the performance of bare germanium detectors in cryogenic
liquids. Identical detector performance as in vacuum cryostats (2.2 keV FWHM at 1.3
MeV) was achieved in cryogenic liquids with a new low-mass detector assembly and
contacts. One major result is the discovery of a radiation induced leakage current
(LC) increase when operating bare detectors with standard passivation layers in LAr.
Charge collection and build-up on the passivation layer were identified as the
origin of the LC increase. It was found that diodes without passivation do not
exhibit this feature. Three month-long stable operation in LAr at 5 pA LC
under periodic gamma irradiation demonstrated the suitability of the modified
detector design. Based on these results, all Phase-I detectors were reprocessed
without passivation layer and subsequently successfully characterized in LAr in the
GERDA underground Detector Laboratory. The mass loss during the reprocessing was
300 g out of 17.9 kg and the exposure above ground 5 days. This results
in a negligible cosmogenic background increase of 5 x 10^-4
cts/(keV kg y) at 76Ge Q for 60Co and 68Ge.
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Matteo Bocchi
(Italy)
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Magnetohydrodynamic instabilities of astrophysical jets
(thesis pdf, 6 MB)
| Supervisor:
Max Camenzind (LSW)
| | Thesis abstract:
The remarkable stability of astrophysical jets is not yet fully understood and requires further investigation.
In order to study the effects of an antiparallel magnetic field topology on the linear stage and nonlinear
evolution of the Kelvin Helmholtz (KH) instability, we performed direct numerical simulations
to solve the ideal magnetohydrodynamic equations in a variety of initial configurations. Single shear
layers presented growth rates of the instability higher than in the uniform (parallel) case, and a typical
oscillation wave vector Ka ≃ 0.4. Vortical motions were observed for Alfv´en Mach numbers Ma > 2.
The presence of tearing type magnetic islands, driven by the KH instability, reduced the magnetic field
enhancement around the perimeter of the KH vortices proper of the KH instability and, subsequently,
reduced the value of the magnetic saturation energy as compared to the uniform field case. The extended
domain simulations showed an inverse cascade to bigger scales, more turbulent than in the uniform case.
The lower magnetic amplification, due to the islands, moved the threshold for three-dimensional (3D)
reorganization to a laminar flow from Ma . 50 (uniform) to Ma . 20 (antiparallel). Two-dimensional
(2D) spatial slab-jet simulations showed episodic disruption and revival of the flow due to a magnetic
field amplification process, previously believed to be present only in subsonic flows. This result, retrieved
also in 3D simulations, is the same for uniform and antiparallel magnetic fields.
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Claudia Mignone
(Italy)
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Model-independent reconstruction of the expansion rate of the universe through
combination of different cosmological probes
(thesis pdf, 2 MB)
| Supervisor:
Matthias Bartelmann (ITA)
| | Thesis abstract:
This work proposes a method to constrain the cosmic expansion rate and the linear growth
factor for structure formation from different cosmological measurements, without reference
to a specific Friedmann model and its parameters. First, a model-independent reconstruction
technique to estimate the expansion rate from luminosity distance data has been developed:
it converts the integral relation between the expansion function and the luminosity distance
into a Volterra integral equation, which is known to have a unique solution in terms of a
Neumann series. Expanding observables such as the luminosity distances to type-Ia supernovae
into a series of orthonormal functions, the integral equation can be solved and the cosmic
expansion rate recovered within the limits allowed by the accuracy of the data. The performance
of the method is demonstrated through application to synthetic data sets of increasing
complexity, including a toy model with a sudden transition in the expansion rate. With the
additional assumption of local Newtonian dynamics, the growth rate for linear structure
formation can be calculated from the estimate of the expansion rate, in the redshift interval
over which supernovae are available, and employed in the analysis of cosmic shear data:
combined to a traditional, Lambda-CDM analysis of the same data set, this approach allows
to tighten the constraints on the matter density parameter, Omega_m, and the normalisation
of the power spectrum, sigma_8. Furthermore, the method to reconstruct the expansion rate can
be applied to angular-diameter distance data from baryon acoustic oscillation experiments; an
optimisation of the orthonormal function set employed in the algorithm has also been performed,
by means of a principal component analysis.
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Dusan Budjas
(Slovakia)
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Germanium detector studies in the framework of the GERDA experiment
(thesis pdf, 26 MB)
| Supervisor:
Wolfgang Hampel (MPIK)
| | Thesis abstract:
The GERmanium Detector Array (GERDA) is an ultra-low background experiment under
construction at Laboratori Nazionali del Gran Sasso. GERDA will search for 76Ge
neutrinoless double beta decay with an aim for 100-fold reduction in background
compared to predecessor experiments. This ambition necessitates innovative design
approaches, strict selection of low-radioactivity materials, and novel techniques
for active background suppression. The core feature of GERDA is its array of
Germanium detectors for ionizing radiation, which are enriched in 76Ge. Germanium
detectors are the central theme of this dissertation. The first part describes
the implementation, testing, and optimisation of Monte Carlo simulations of Germanium
spectrometers, intensively involved in the selection of low-radioactivity materials.
The simulations are essential for evaluations of the gamma ray measurements. The
second part concerns the development and validation of an active background suppression
technique based on germanium detector signal shape analysis. This was performed for
the first time using a BEGe-type detector, which features a small read-out electrode.
As a result of this work, BEGe is now one of the two detector technologies included
in research and development for the second phase of the GERDA experiment.
A suppression of major GERDA backgrounds is demonstrated, with (0.93 ± 0.08)% survival
probability for events from 60Co, (21 ± 3)% for 226Ra, and (40 ± 2)% for 228Th.
The acceptance of 228Th double escape events, which are analogous to double beta
decay, was kept at (89 ± 1)%.
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Marie-Helene Nicol
(Canada)
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Investigation of the stellar population in galaxies since z=2 by NIR photometry
(thesis pdf, 8 MB)
| Supervisor:
Klaus Meisenheimer (MPIA)
| | Thesis abstract:
This study investigates galaxy evolution in the last 10 Gyr of the age of the Universe.
To access the high redshift Universe deep near infrared observations have been carried
out in the frame of the COMBO-17+4 (Classifying Objects by Medium-Band Observations)
survey. Photometric redshifts, rest-frame SEDs and masses have been obtained for 14286
galaxies down to an observed magnitude H=21.7 mag in 0.2 square degree of the A901-field.
The red evolved and the blue star-forming galaxy populations distributions have been
followed through cosmic time. An evolving colour-magnitude relation as a function of
lookback time has been derived in order to separate the red galaxies from the blue
ones. Our results show that the galaxy bimodality persist out to z=1.6.The luminosity
and the mass function for the red and the blue galaxy populations has been investigated.
Our results show that the characteristic magnitude M* for both galaxy population have
considerably dimmed between 2>z>0.2. The massive end of the mass function is dominated
by the red galaxies at z<1, whereas above z=1 red evolved and blue star-forming galaxy
populations contribute in similar numbers.
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Marta Zub
(Poland)
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Galactic Aspects of Gravitational Microlensing:
High Magnification Events and Stellar Limb-darkening of a Source Star in the Galactic bulge
(thesis pdf, 4 MB)
| Supervisor:
Joachim Wambsganss (ARI)
| | Thesis abstract:
Gravitational microlensing provides a powerful tool to search for extrasolar planets
of stars at distances of order of several kpc. The suspicion of a planetary signal
in the two high magnification events OGLE 2006-BLG-245 and MOA 2006-BLG-099 led us to
perform a detailed modelling and analysis of those two events. Based on the comparison
of single-lens and binary-lens models, we demonstrate that the observed light curve
deviations are not caused by a planetary companion. Our modelling and analysis of four
other high magnification events illustrate the possibility to study detection efficiencies
of microlensing data sets to planetary companions. We also present a detailed study of
the single-lens OGLE 2004-BLG-482 microlensing event, used to measure the brightness profile
of the background lensed star located in the Galactic bulge. We performed data reduction and
analysis of well sampled observations of this event obtained by the PLANET, OGLE and MicroFUN
collaborations in the I, R and clear filters. We also used a high resolution spectrum obtained
with VLT/UVES close to the peak of the light curve to determine the fundamental parameters
of the source star, that we find to be a cool red M3 giant with Teff = 3667±150K,
log g = 2.1±1.0. We then performed a detailed microlensing modelling of the light curve
to measure linear limb-darkening coefficients and to provide new diagnostics of such measurements
through microlensing. We compare our results to model-atmosphere predictions based on
limb-darkening coefficients for the corresponding stellar parameters. Our limb-darkening
measurements agree very well with predictions of the model atmosphere, for both linear
limb-darkening laws and alternative limb-darkening profiles based on a principal component
analysis of ATLAS stellar atmosphere models.
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Timo Anguita
(Chile)
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A combined observational and theoretical study of gravitationally lensed quasars
(thesis pdf, 4 MB)
| Supervisor:
Joachim Wambsganss (ARI)
| | Thesis abstract:
In this thesis we study gravitational lensing of quasars, from strong lensing to microlensing.
We investigate one strong lens candidate recently discovered in the COSMOS field:
COSMOS 5921+0638.
Our analysis of the nature of the lens reveals that the system is composed of an early
type foreground galaxy at redshift z_l=0.551+/-0.001 lensing a background low luminosity
AGN and its host galaxy at a candidate redshift of z_s=3.14+/-0.05. We show that flux
anomalies observed in the lensed images are likely due to substructure or microlensing by
stars in the lensing galaxy. Extending the analysis of the optical emission of lensed
quasars, we have used integral field spectroscopy to study four gravitationally lensed
quasar systems: HE 0230-2130, RX J0911+0551, H 1413+117 and B 1359+154, as well as objects
in their line-of-sight. The first three systems show anomalous flux ratios consistent
with microlensing by stars in their lensing galaxies. In the final part of this work, we
probe the structure of the accretion disk of the lensed quasar Q 2237+0305 by the
analysis of a high magnification microlensing event seen in the so-called "image C" of
the system in the year 1999.
Using multi-band observations and microlensing simulations, we measure an accretion
disk size of Gaussian width sigma_g'=4.6^{+3.4}_{-3.4} x 10^15 sqrt{M/0.1M_sun} cm and
a ratio sigma_r'/sigma_g'=1.3^{+0.5}_{-0.2}, without the use of any prior, and of
Gaussian width sigma_g'=1.3^{+0.2}_{-0.7} x 10^15 sqrt{M/0.1M_sun} cm and a ratio
sigma_r'/sigma_g'=1.5^{+0.9}_{-0.3} with a prior on the relative velocity between source
and microlenses.
Both results are in agreement with the predictions of a standard Shakura-Sunyaev disk
model. The use of multi-band observations revealed that the magnification event seen in
image C of Q2237+0305 was produced by a caustic crossing with a confidence greater than 74%.
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Andreas Ernst
(Germany)
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Dissolution of Star Clusters in the Galaxy and its Center
(thesis pdf, 35 MB)
| Supervisor:
Andreas Just (ARI)
| | Thesis abstract:
This thesis is concerned with investigations on the dissolution of star clusters in the tidal field of the Galaxy and in particular its center. At first the escape process of stars from star clusters is studied in the framework of chaos-theoretical considerations. Already in the linear tidal approximation it is possible to compute the basins of escape and the chaotic saddle for the system. After the stars have left the star cluster they form tidal arms (or tails) due to the differential rotation of the Galaxy. For star clusters on circular orbits the theoretical framework for the investigation of the properties of tidal arms is discussed. The theory is applied for a star cluster model in the Galactic center. For this purpose a new N-body program called "nbody6gc" has been developed. The algorithm is described in detail and the results of N-body simulations are discussed. At certain positions, well-defined clumps develop in the tidal arms due to the epicyclic motion of the stars. The positions of the clumps are calculated with the analytical theory. Furthermore, a classification of the cluster stars according to radius and specific Jacobi energy is introduced in order to explain the dissolution times and a few results on the "paradox of youth" are formulated.
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Martin Hennemann
(Germany)
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Multiwavelength observations of massive star-forming regions selected in the far-infrared
(thesis pdf, 60 MB)
| Supervisor:
Dietrich Lemke (MPIA)
| | Thesis abstract:
The lack of observations of the earliest stages in high-mass star formation
motivated the selection of massive star-forming regions using the 170 µm
ISOPHOT Serendipity Survey. The evaluation of comprehensive follow-up observations,
covering near-infrared to (sub-)millimetre wavelengths, identifies massive clumps
and characterises the star-forming content in detail. The clumps comprise a large
fraction of cold dusty material at temperatures between 12 and 22 K, and several
have masses of 100 solar masses or more. Star formation has initiated in every clump,
as they harbour embedded sources detected in the mid-infrared that represent low- to
intermediate-mass young stellar objects. One case study uses millimetre interferometry
and discovers two compact cores of about 15 solar masses embedded in a massive
clump. They are driving energetic outflows and may accrete at high rates, and thus
represent examples for the first stages of forming intermediate- to high-mass stars.
The importance of high spatial resolution in the infrared for the study of high-mass
star formation drives dedicated observing programmes with the Herschel Space
Observatory. Furthermore, an active part in the preparation of the JWST MIRI instrument
was taken because in particular its unprecedented imaging capabilities will allow to
constrain crucial properties of the investigated sources.
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Pak Hin Tam
(Hong Kong)
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Gamma-ray burst studies using the H.E.S.S. Cherenkov array
(thesis pdf)
| Supervisor:
Stefan Wagner (LSW)
| | Thesis abstract:
Gamma-ray bursts (GRBs) are the most intense and unpredictable gamma-ray events
from the Universe. Without prior signal, an enormous amount of energy is released
for seconds, energizing particles and generating the observed 10e5-10e6 eV
gamma-ray photons. The emitting regions can produce Very-High-Energy (VHE)
gamma-ray photons of energy about 10e11 eV during and after the burst. These
VHE gamma-rays may be attenuated in the source or by the extragalactic background
light (EBL). The H.E.S.S. array of four imaging atmospheric Cherenkov telescopes
(IACT) is sensitive to VHE gamma-rays. H.E.S.S. observations of 34 GRBs were
carried out during 2003-2008, with the shortest response time being six minutes.
No evidence of VHE gamma-rays was found. Flux upper limits derived for GRB030329
and GRB060505 are compared and are found consistent with the synchrotron
self-Compton flux calculated in the context of relativistic blast-wave model.
Absorption by EBL was taken into consideration. Accidentally, GRB060602B was observed
with H.E.S.S. throughout the GRB duration, during which no signal of VHE
gamma-rays was found. The distance scale and origin of GRB060602B remain unclear
and different possibilities and implications are presented. Future prospects of
VHE gamma-ray observations of GRBs are discussed
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Svitlana Zhukovska
(Ukraine)
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Dust formation by stars and evolution in the interstellar medium
| Supervisor:
Hans-Peter Gail (ITA)
| | Thesis abstract:
The main goal of this thesis is the study of the origin and evolution of interstellar dust in the Milky Way. We develop a model for the chemical evolution of the galactic disk as a basis for our new model of dust evolution, which considers for the first time the individual evolutions of stardust and of dust condensed in molecular clouds of the Galactic disk. We include dust production by AGB stars in detail, using the results of synthetic AGB models combined with models of dust condensation in stellar outflows, and estimate the efficiency of dust condensation in supernovae by matching model results for the Solar neighbourhood with observed abundances of presolar dust grains of supernova origin. Our results indicate that supernovae produce mainly carbon dust, with only small amounts of silicates, iron and silicon carbonate. We show that the interstellar dust population is dominated by dust grown in the interstellar medium across the Galactic history; moreover, dust formed in AGB stars and supernovae is a dominant source of dust only at metallicities lower than the minimal value for efficient dust growth in molecular clouds.
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Giulia Vannoni
(Italy)
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Diffusive Shock Acceleration in Radiation Dominated Environments
(thesis pdf)
| Supervisors:
Felix Aharonian /
Werner Hofmann (MPIK)
| | Thesis abstract:
In this work I describe a numerical method developed, for the first time, for
the study of Diffusive Shock Acceleration in astrophysical environments where the
radiation pressure dominates over the magnetic pressure. This work is motivated by
the overwhelming evidence of the acceleration of particles to high energy in
astrophysical objects, traced by the non–thermal radiation they emit due to
interactions with the gas, radiation fields and magnetic fields. The main objective
of this work is to create a generic framework to study self–consistently the
interaction of acceleration at shocks and radiative energy losses and the effect
such an interplay has on the particle spectrum and on the radiation they emit,
in the case when energy losses determine the maximum achievable energy. I apply
the developed method to electrons accelerated in three different types of sources:
a Supernova Remnant in the Galactic Centre region, a microquasar, and a galaxy
cluster. In all three cases the energy losses due to the interaction of electrons
with radiation dominate over synchrotron cooling. I demonstrate that there is a strong
impact due to the changing features of the inverse Compton scattering from the Thomson
to the Klein-Nishina regime, on both the spectrum of accelerated electrons and their
broadband emission. I also consider proton acceleration in galaxy clusters, where the
particles lose energy during acceleration due to the interaction
with the Cosmic Microwave Background radiation. The secondary products from pair
production and photomeson processes interact with the same photon field and the
background magnetic field, producing broadband electromagnetic radiation from
radio to gamma-rays.
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Steve Boudreault
(Canada)
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The formation of brown dwarfs as revealed by the mass function of IC 2391
(thesis pdf)
| Supervisor:
Coryn Bailer-Jones (MPIA)
| | Thesis abstract:
This PhD thesis deals with the formation and evolution of brown dwarfs. Here, we
present the stellar and substellar mass function of the open cluster IC2391, plus its
radial dependence, and use this to put constraints on the formation mechanism of brown
dwarfs. Our multiband optical and infrared photometric survey with spectroscopic follow
up covers 11 square degrees, making it the largest survey of this cluster to date. We
observe that there is no variation in the mass function over the stellar/substellar
boundary at all three cluster radius intervals analyzed. From this lack of discontinuity,
we conclude that the stellar embryo ejection mechanism cannot be the unique brown dwarf
formation path if this formation mechanism produces a higher velocity dispersion for brown
dwarfs than the stars obtain. Alternatively, the ejection mechanism could be a dominant
brown dwarf formation path only if it produces the same velocity dispersion for brown
dwarfs as exists for stars in the cluster. In addition, we observe a radial variation
in the mass function over the range 0.15 to 0.5 M_sun. We conclude that this is a signature
of mass segregation via dynamical evolution. Analysis of mass functions of other open clusters
from different ages and environment, at each side of the stellar/substellar boundary, also
indicates that dynamical evolution influences the shape of the mass function.
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Oscar Esquivel
(Mexico)
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Aspects of wave mechanics of gravitating systems
(thesis pdf)
| Supervisor:
Burkhard Fuchs (ARI)
| | Thesis abstract:
The Jeans instability of a galactic disk embedded in a dynamically responsive
dark-matter halo is investigated in this work. On small scales the instability
is suppressed, if the Toomre stability index Q_T is higher than a certain
threshold, but on large scales the Jeans instability sets invariably in.
However, using a simple self-consistent disk-halo model it is demonstrated that this
occurs on scales which are much larger than the system so that this is indeed
only a nominal effect. Also, a rigorous calculation of the dynamical friction
(DF) force exerted on a Plummer and a Hernquist sphere moving through an infinite
homogenous system of field stars is presented. By using a wave-mechanical
treatment, we recover Chandrasekhar's drag force law with a modified Coulomb
logarithm that depends on the exact shape of the perturber.
We then extend this mode analysis to anisotropic velocity distributions of the
field stars. We present easy-to-use handy formulae of the DF force exterted on
a point-mass satellite for the cases when the velocity ellipsoid is either
oblate or prolate for different values of the effective velocity dispersion
sigma_eff determining the anisotropy of the host system.
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Michael Zatloukal
(Germany)
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A search for distant galaxy clusters
(thesis pdf)
| Supervisor:
Hermann-Josef Röser (MPIA)
| | Thesis abstract:
The evolution of galaxy clusters and their members above z >= 1 is largely
unclear at present due to the small number of known distant clusters.
To probe this important period and witness the development of key cluster
characteristics observed today, the HIROCS survey is establishing a
statistically signicant 0.5 <= z <= 1.5 cluster sample.
For this thesis, the HIROCS cluster selection function was studied
in detail using a mock sky based on semi-analytical galaxy evolution
simulations. Public and proprietary data were combined for the COSMOS
field. A cluster search was performed, yielding a 0.5 <= z <= 1.59 catalog
of clusters, including the irst larger, uniformly selected sample of
distant (z > 1.2) clusters published. Comparing the color evolution of
X-ray bright and -dark clusters, the latter were found to contain fewer
passive galaxies than their X-ray counterparts. Spectroscopic follow-up
observations of four z~0.85 candidates, three of them X-ray dark,
were conducted to probe cluster detection biases related to this.
Using the distant cluster sample, it was shown that z >= 1.2 clusters
predominantly contain blue galaxies with active star formation and a
significant fraction of interacting members, but that their early type
galaxies are old and formed the bulk of their stars at zf >= 3, possibly
zf >= 5, in excellent agreement with predictions from simulations.
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Irina Golombek
(Germany)
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Non-thermal emission of galaxy clusters
(thesis pdf)
| Supervisor:
Matthias Bartelmann (ITA)
| | Thesis abstract:
We investigate the numerical modelling of relativistic protons, so-called
cosmic rays, in a magnetised plasma. For the first time we combine two different
components of a cosmological simulation code that so far have only been tested
and employed independently of each other. By means of magnetohydrodynamic shock
tube calculations in a gas that contains a constant fraction of relativistic
particles we check for the correct physical behaviour of the combined numerical
models. For this purpose we derive an analytical expression for the magnetosonic
shock and rarefaction waves in the MHD Riemann problem and solve for the
resulting system of equations using an iterative scheme. Comparing the
theoretical and numerical solutions of a number of shock tube calculations
we assess the physical correctness of the simulation.
After successful testing we simulate the structure formation of three galaxy
clusters including the consistent modelling of magnetic fields and cosmic rays.
By means of analytical models we compute the X-ray and radio emission of the
simulated clusters and reveal the temporal correlation between both quantities.
Through a comparison with the strong lensing cross sections we demonstrate that
the observable radio emission of galaxy clusters is directly connected to the
occurrence of substructure in their dark matter halos and is thus triggered by
strong merger shocks.
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Dominikus Heinzeller
(Germany)
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Mass and energy balance in accretion disks
(thesis pdf)
| Supervisor:
Wolfgang Duschl (ITA; Kiel University)
| | Thesis abstract:
We study the role of convection in black hole accretion flows.
We investigate the influence of convection on the energy transport as
well as the effect of convective turbulence on the disk’s viscosity.
The results reveal that convection supports the radiative energy transport
efficiently in massless disks, while it can turn into a negative feedback
if self-gravity becomes important. Convective turbulence adds significantly
to the total viscosity, but cannot account for it on its own. In the second
part, we study the spectral energy distribution of super-Eddington
accretion flows onto a black hole, based on 2D RHD simulation data. We
model the continuum emission as well as the iron K line emission and
absorption features with a ray-tracing radiative transfer code. We find
that mild relativistic beaming effects become important, leading to
super-Eddington luminosities for face-on seen disks. We confirm the
diagnostic power of the iron K lines on the accretion process in the
inner disk region, finding a strong correlation between the central
black hole mass and the ratio of the K-alpha to the K-beta lines.
We also detect a trend of line broadening for edge-on seen disks.
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Markus Janson
(Sweden)
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Direct imaging search for substellar companions to nearby stars
(thesis pdf)
| Supervisors:
Wolfgang Brandner / Thomas Henning (MPIA)
| | Thesis abstract:
Ever since the first detections of extrasolar planet candidates in the early
1990s, our knowledge of such objects has drastically increased, and exoplanet
science today constitutes a major branch of astrophysics, with a few hundred
individual detections. Our physical understanding is however limited by the
fact that the planets are generally only detected indirectly, with just a few
constraints on its orbital and physical parameters. Direct imaging of
exoplanets, where the planet can be spatially resolved from the star, opens up
for a much broader understanding of these objects, with the opportunity to
study their spectral characteristics. Alternatively, eclipse spectroscopy,
where the planetary signal can be temporally resolved in systems where the orbital
plane of the planet happens to align with the line of sight of an observer, can
be used for the same purpose. In this thesis, we study various approaches for direct
imaging and spectroscopy of exoplanets from the ground, using combinations of
adaptive optics and differential methods, in particular spectral and angular
differential imaging. The possibility of using an external occulter for the purpose
of decreasing the star-planet contrast is studied. We also investigate the
possibility to calibrate theoretical mass-luminosity relationships in order to improve detection
predictions and the interpretations of null-detection surveys. Scientific
results include an improved age range of the AB Dor system
thanks to the spatial distinguishing of AB Dor B
as a close binary, and the best constrained upper limit to the H-band brightness of
any known exoplanet ever achieved, from a deep imaging search for Eps Eri b.
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Xuepeng Chen
(China)
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High angular resolution observations of binary protostars
(thesis pdf)
| Supervisors:
Ralf Launhardt / Thomas Henning (MPIA)
| | Thesis abstract:
In this thesis I present a systematic effort to reveal the physical processes
that lead to the formation of binary stars. We have observed, at high angular
resolution, thirteen isolated low-mass protostellar cores, using the Owens Valley
Radio Observatory millimeter array, the Australia Telescope Compact Array, and the
IRAM Plateau de Bure Interferometer array. The observations were mainly carried out
in the N2H+(1-0) line and at 3mm dust continuum. The results were complemented by
infrared data from the Spitzer Space Telescope and the ESO Very Large Telescope.
We find that binarity/multiplicity is frequent in the protostellar phase, though it
is too early to derive a separation distribution. The circumstellar mass ratio
distribution of binary protostars appears to be flat like that of more evolved long-period
main-sequence binary stars, and more than 75% of protobinary systems have circumstellar
mass ratios below 0.5. The specific angular momenta of protostellar cores are intermediate
between those of prestellar cores and the orbital angular momenta of wide pre-main
sequence binary systems. There appears to be no significant decrease of angular momentum
between the onset of the protostellar collapse and the emergence of a binary star, which
suggests that most of the angular momentum contained in the collapse region is transformed
into orbital angular momentum of the resulting stellar binary system. We find that during
core fragmentation the angular momentum is not evenly, in value and direction, divided
between sub-cores. Furthermore, most cores with binary protostars have ratios of rotational
to potential gravitational energy of beta_rot > 1%. This is consistent with theoretical
simulations and suggests that the initial amount of rotational energy in a molecular cloud
core is playing an important role in the protostellar fragmentation process.
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Alumni - 2007
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Vernesa Smolcic
(Croatia)
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The faint radio population in the VLA-COSMOS survey:
Star forming galaxies and active galactic nuclei
(thesis pdf)
| Supervisors:
Eva Schinnerer / Hans-Walter Rix (MPIA)
| | Thesis abstract:
The composition of the faint (sub-milli-Jansky) radio population,
that has been a matter of strong debate in the past, is explored by
performing observations at 1.4 GHz (20 cm) radio continuum of the
2 sqdeg COSMOS field providing a large statistically significant sample, and by
developing a method that uses a minimal number of parameters to efficiently
discriminate between the two main populations in extragalactic radio surveys:
active galactic nuclei (AGN) and star forming galaxies. This method
bears the potential to be successfully applied to similar samples selected
at other wavelengths. One of the main findings is that star forming galaxies do
not dominate the submillijansky radio population, as often assumed, but form
only about 30-40% of it, while the remainder is composed of AGN and quasars.
Using this well defined sample of radio-selected star forming galaxies at
1.4 GHz, the cosmic star formation history is derived using radio data, for
the first time constraining the dust-unbiased cosmic evolution of star formation
rate in the most intensively star forming galaxies (>100Msun/yr) since
~5 Gyr after the Big Bang with high precision. In addition, the radio
derived cosmic star formation history confirms the validity of the large dust
corrections applied at other wavelengths.
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Slavomir Hnatic
(Slovakia)
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Very High Energy Radiation from Binary System PSR B1259-63/SS2883
(thesis ps.gz)
| Supervisor:
Felix Aharonian (MPIK)
| | Thesis abstract:
The inverse Compton (IC) scattering of ultrarelativistic electrons accelerated
at the pulsar wind termination shock is believed to be responsible for TeV
gamma-ray signal reported from the binary system PSR B1259-63/SS2883. While
this process can explain the energy spectrum of the observed TeV emission,
the gamma-ray fluxes detected by HESS at dif\/ferent epochs do not agree
with the published theoretical predictions of the TeV lightcurve. In this
work, evolution of the energy spectra of relativistic electrons under different
assumptions about the acceleration and energy-loss rates of electrons, is
studied. Consequently, it is demonstrated that the observed TeV lightcurve can
be explained (i) by adiabatic losses which dominate over the entire trajectory
of the pulsar with a signif\/icant increase towards the periastron, or (ii) by
sub-TeV cutoffs in the energy spectra of electrons due to the enhanced rate of
Compton losses close to the periastron. The calculated spectral and temporal
characteristics of the TeV radiation provide conclusive tests to distinguish
between these two working hypotheses. The Compton deceleration of the
electron-positron pulsar wind contributes to the decrease of the nonthermal power
released in the accelerated electrons after the wind termination, and thus to
the reduction of the IC and synchrotron components of radiation near the
periastron. Although this effect alone cannot explain the observed TeV and
X-ray lightcurves, the Comptonization of the cold ultrarelativistic wind leads
to the formation of gamma-radiation with a line-type energy spectrum. While
the HESS data already constrain the Lorentz factor of the wind, Gamma < 1e6,
future observations of this object with GLAST should allow a deep probe of the
wind Lorentz factor in the range between 1e4 and 1e6.
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