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I'm leading an independent research
group in the
Theory Division of the
Galaxies and Cosmology
Department at the Max-Planck-Institute for Astronomy (MPIA)
in Heidelberg.
Our research focusses on various theoretical aspects of cosmology, large scale
structure, and galaxy formation. In particular, we have ongoing projects to
study the structure and formation of dark matter haloes, the formation of disk
galaxies, the galaxy occupation statistics of dark matter haloes, galaxy-galaxy
lensing, preheating of the IGM by pancake formation, and on the bias of
galaxies and dark matter haloes.
For more details please consult the research page.
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October 17, 2008: New Constraints on Star Formation Quenching
We investigate the correlation of star formation quenching with internal galaxy properties and
large scale environment (halo mass) in empirical data and theoretical models. For central galaxies,
we find that the fractions of ``red'' and ``passive'' galaxies in the observational group catalogs
are a strong function of halo mass at fixed stellar mass, and a weak function of stellar mass
at fixed halo mass. For satellite galaxies, a nearly equally strong
dependence on halo mass and stellar mass is seen. A comparison with
five different semi-analytic models shows that models with AGN
feedback can reproduce the trends of central galaxies, but not for
satellite galaxies, which are too red/passivein the models.
This satellite overquenching problem is
caused by the too-rapid stripping of the satellites' hot gas halos, which
leads to rapid strangulation of star formation.
September 13, 2008: Environmental Effects on Satellite Galaxies
Using our SDSS group catalogue, we compare the sizes, concentrations, colour gradients and
surface brightness profiles of central and satellite galaxies as fixed stellar mass. We find
that at fixed stellar mass, late type satellite galaxies have smaller radii and larger
concentrations than late type central galaxies. No such differences are found for early-type
galaxies. Late-type satellite galaxies have a lower surface brightness and redder colours than
late-type central galaxies. All these differences between satellite and central galaxies can
be explained by a simple fading model, in which the star formation in the disk decreases over
timescales of 2-3 Gyr after a galaxy becomes a satellite.
August 19, 2008: The Subhalo - Satellite Connection
We use the conditional stellar mass functions of satellite galaxies obtained from our SDSS
galaxy group catalogue together with models of the subhalo mass functions to explore the fraction
and fate of stars stripped from satellites in galaxy groups and clusters of different masses.
The majority of the stripped stars in massive halos are predicted to end up as intra-cluster
stars, and the predicted amounts of the intra-cluster component as a function halo mass are in
good agreement with direct observational constraints.
August 6, 2008: Luminosity and Stellar Mass Functions of Galaxies
Using our SDSS galaxy group catalogue, we investigate the luminosity and stellar mass functions
for different populations of galaxies and for groups themselves. We also determine the
conditional stellar mass function, which describes the stellar distribution of galaxies in
halos of a given mass. We use the observed stellar mass function of central galaxies to constrain
the stellar mass - halo mass relation for low mass halos, and obtain that M_* \propto M_{halo}^{4.9}
August 1, 2008: Galaxy-Galaxy Lensing with the CLF
We use the abundance and clustering properties of galaxies in the SDSS in order to constrain
the conditional luminosity function (CLF), which describes the link between galaxies and dark
matter haloes. The resulting CLF model is then used to predict the galaxy-galaxy lensing signal.
We show that the predicted lensing signal is in agreement with the data for a cosmology with
Omega_m=0.238 and sigma_8=0.734, but not for a cosmology with Omega_m=0.3 and sigma_8=0.9.
August 1, 2008: Galaxy-Galaxy Lensing with a Group Catalogue
We use a large galaxy group catalogue to predict the galaxy-galaxy lensing signal, which we
compare to data from the SDSS. In agreement with our
CLF study
we only find a
good match to the data if we assume a cosmology with relatively low Omega_m and sigma_8.
July 30, 2008: Satellite Kinematics II: Results
Using the method outlined in Paper I
we determine both the mean and the scatter of
the mass-luminosity relation of central galaxies in the SDSS. We find that the scatter in
halo masses for centrals of a given luminosity increases with increasing luminosity, and
discuss the implications for the amount of stochasticity in galaxy formation.
July 30, 2008: Satellite Kinematics I: Methodology
We present a new method that uses satellite kinematics to constrain
both the mean and the scatter of the mass-luminosity relation of
central galaxies.
July 29, 2008: The Rise and Fall of Galaxy Acticity in Dark Matter Haloes
Using a large SDSS galaxy group catalogue, we study the dependence of
galaxy activity on stellar mass, halo mass, and group hierarchy
(centrals vs. satellites). We split our galaxy sample in star-forming
galaxies, galaxies with optical AGN activity and radio sources. Among
others, we find a smooth transition in halo mass as the activity of central
galaxies changes from star formation to optical AGN activity to radio emission.
May 12, 2008: Halo Properties as Function of Halo Mass and Cosmology
Using a large suite of numerical simulations, we study the concentrations
of dark matter haloes as function of halo mass and cosmology.
Our simulations cover 5 order of magnitude in halo mass, and three
different cosmological models. We present a new model to compute
the average concentration as function of halo mass which accuretely
fits our simulation results. We also compare our simulation results
to constraints from various observational techniques.
May 1, 2008: Satellite Ecology: The Dearth of Environment Dependence
We study the dependence of the colors and concentrations of satellite
galaxies on their stellar mass, their halo mass, and their halo-centric
distance. We find that the properties of satellite galaxies are
almost entirely determined by their stellar mass, with no significant
environment dependence. This has important implications for the
physical processes responsible for transforming satellite galaxies.
March 11, 2008: Reconstructing the Cosmic Density Field
We present a new method to reconstruct the cosmic density field from
the distribution of dark matter haloes extracted from a large galaxy
group catalogue. We demonstrate that our method works remarkably well
using haloes (groups) with masses down to 10^12 Msun, even in redshift
space. We discuss a number of potentially powerful applications.
January 11, 2008: Constraining Satellite Strangulation Processes
Using semi-analytical models for galaxy formation, and data from
the SDSSS we constrain the efficiency with which satellite galaxies
are stripped of hot gas, and tidally disrupted by the potential
of their host halo.
January 9, 2008: The Dark Matter Halo of the Milky Way
Using a set of halo stars extracted from the SDSS as kinematic
tracers, we derive new constraints on the mass of the Milky Way's
dark matter halo. Our results indicate a halo virial mass of
M$_{\rm vir}=1.1\pm 0.2 \times 10^{12}$M$_\odot$. This implies that
nearly 40% of the baryons within the virial radius of the Milky Way's
dark matter halo reside in the stellar components of our Galaxy.
December 10, 2007: The Population of Dark Matter Subhaloes
Using N-Body simulations we study the mass loss rates of dark matter
subhaloes, and interpret the mass function of subhaloes at redshift
zero in terms of the evolution of the mass function of systems
accreted by the main halo progenitor.
November 29, 2007: Mass and Color Dependence of Halo Bias
Using the cross-correlation between galaxy groups and galaxies in the SDSS
we determine how the bias of dark matter halos depends on halo mass and on
the color of its galaxy population. We find that the mass dependence is in
excellent agreement with predictions for a LCDM cosmology, and that red groups
are more strongly clustered than blue groups of the same mass. We discuss how
this relates to the assembly bias of dark matter haloes.
October 29, 2007: Halo Occupation Statistics
Using a large SDSS galaxy group catalogue we study the occupation statistics
or dark matter haloes. In particular, we determine the conditional luminosity
function, halo occupation numbers, gap statistics in both luminosity and stellar
mass, and satellites fractions, all for both red and blue galaxies.
October 18, 2007: The Impact of Satellite Quenching
Using a large SDSS galaxy group catalogue we study the importance of satellite
for the build-up of the red sequence of present day galaxies. We do this by
comparing the properties of central and satellite galaxies that are matched
in stellar mass.
October 16, 2007: The Shapes of Dark Matter Haloes
Using a large SDSS galaxy group catalogue we study the three-dimensional and
projected shapes of dark matter haloes. We find that the halo shape has a strong
dependence on the halo mass. While the haloes of low-mass groups are nearly spherical,
those of massive groups tend to be prolate.
October 15, 2007: The Assembly of Massive Galaxies
We investigate the incidence of major mergers creating massive galaxies in
present-day groups and clusters. This results in, among others, the largest
sample of dry mergers currently available.
August 2, 2007: Disk Galaxy Scaling Relations
We present the global structural parameters for a large sample of 1300 spiral
galaxies and explore the joint distribution of luminosity, rotation velocity,
and disk size.
July 31, 2007: New SDSS Galaxy Group Catalogue
We present a new galaxy group catalogue constructed from the SDSS data release 4
June 4, 2007: Spatial and Kinematic Alignments of Dark Matter Haloes
Using a large numerical simulation we investigate various spatial and kinematic
alignments between dark matter haloes probing scales out to 6 virial radii.
April 11, 2007: The Origin of the Dichotomy of Early-Type Galaxies
Using a semi-analytical model for galaxy formation, combined with a large N-body
simulation we investigate the origin of the dichotomy of early-type galaxies.
April 11, 2007: The Disky Fraction of Early-Type Galaxies
Using a sample of 847 early-type galaxies selected from the SDSS DR4
we study the disky fraction as function of luminosity and halo mass.
April 05, 2007: Three Types of Galaxy Alignment
Using a large galaxy group catalogue constructed from the SDSS DR4 we investigate three
types of galaxy alignments, and discuss their implications for weak lensing measurements.
March 15, 2007: Stochasticity & Non-Linearity in Galaxy Bias
We investigate the luminosity and color dependence of galaxy clustering in the SDSS
using measurements of the two-point auto and cross correlation functions.
We also use these to test for the importance of non-linearity and stochasticity.
January 8, 2007: Satellite Alignment
Using large N-body simulations which we populate with galaxies following a semi-analytical
model for galaxy formation, we study the alignment between the orientation of central
galaxies and the distribution of satellite galaxies.
October 25, 2006: Halo Occupation Statistics
We use the Conditional Luminosity Function and data from the 2dFGRS to constrain the average
relation between light and mass in the Universe.
August 9, 2006: The Properties of Dark Matter Haloes
Using a suite of N-body simulations, we study the concentrations, spin parameters and shapes
of dark matter haloes covering a wide range in halo masses. We also study their environment
dependence and discuss the implications for interpreting LSB rotation curves.
July 26, 2006: Probing Haloes and Sub-Haloes with Weak Lensing
We demonstrate how galaxy groups can be used to split the population of galaxies in centrals
and satellites. Galaxy-galaxy lensing measurements of these two separate
populations is shown to
yield accurate constraints on the masses and density profiles of dark matter haloes and sub-haloes.
June 21, 2006: New Insights into Star Formation Truncation
Using a galaxy group catalogue extracted from the SDSS we analyze the fraction of blue galaxies
as function of halo mass and luminosity. A comparison with semi-analytical models for galaxy formation
provides valuable new insights into the physics of star formation truncation
May 3, 2006: A Natural Explanation for the Downsizing Phenomenon
We show that the phenomenon of `downsizing' is not `anti-hierachical', but in fact
has its natural roots in the bottom-up clustering process of dark-matter haloes
For a brief summary you can also check our poster on this topic.
April 28, 2006: A Revised Model for the Formation of Disk Galaxies
We present a new model for disk formation. Contrary to common ideas, we suggest that
disks formation involves low spin parameters and halo expansion, rather than contraction
March 16, 2006: Wilkinson Microwave Anisotropy Probe confirms our results
Cosmology has lower matter density and lower power spectrum normalization than previously assumed.
January 3, 2006: Holmberg Effect No, Galaxy Alignment Yes
Using galaxy groups selected from the SDSS we show that the distribution of satellite galaxies is aligned
with the major axis of the central galaxy; this is contrary to the old Holmberg effect
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