Emission line galaxies at redshifts from z = 0.1 to 1.4
Introduction
The emission line survey of CADIS, performed with a tunable Fabry-Perot in the atmospheric windows at 700, 810, and 920 nm, is appropriate to find emission line galaxies (ELGs hereafter) down to very low continuum brightnesses and at a variety of redshifts inbetween z=0.1 and 1.4.Generally, the principal line observed in the Fabry-Perot windows, is accompanied by secondary lines falling into the veto filters. Thus, it is possible to decide which line is seen in the FP window and to to estimate line ratios at medium high redshifts.
The line luminosities of the most prominent lines, H-alpha, [OIII], and [OII], can be used to derive line-luminosity functions and to estimate the star forming rates at different cosmic epochs.
Making use of the line ratios the metallicities at medium redshifts can be derived and low metallicity galaxies can be found.
Data base and analysis
Up to now, the data
of 3 fields (1h, 9h, 23h) are completely reduced. For the analysis of
the ELGs a specific emission-line classification routine is applied, making
use of both the multi-color survey and the emission-line survey data.
It determines, which line is seen in the specific Fabry-Perot window,
the ELG redshift within an accuracy of 0.002 and the
line fluxes of the principal and the secondary lines.
Examples for a H-alpha galaxy at z=0.25, a [OIII] galaxy at z=0.4
and a [OII] galaxy at z=1.18 show the average
quality of SED and emission line fits.
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Star forming rates
Luminosity functions
For each redshift
window about 25 ELGs per field are found. On basis of the ELG samples,
luminosity functions can be established for each of the redshift windows
mentioned above, as well for the blue magnitudes as for the line luminosities.
We have derived the LFs for the redshift windows at z=0.25,0.4,0.64.0.88, and 1.20.
The figure shows the luminosity functions for the blue magnitude MB and
for the H-alpha line for the galaxies at
redshift 0.25 with a Schechter functions overplotted as derived from the
MC survey for late type galaxies at redshift at about 0.3.
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Results and discussion
- The blue
magnitude luminosity functions for the different ELG samples agree well
with those derived from the multicolor survey.
This means that indeed the majority of field galaxies is of late
type showing line emission.
- The LFs derived from the blue magnitudes and from the line luminosities for each ELG sample have the same shapes.
- The line ratios L[OIII]/L(H-alpha) and L[OII]/(L-Halpha) are 0.9 and 1.0, respectively, within z=0.25 and z=0.4.
- The SFR for z=0.25 (derived from ELGs seen in H-alpha) is about 3 times higher than that for the local universe. Our data points inserted in the so called Madau plot show a strong increase of the SFR with look back time (by a factor of 30 between z=0 and z=1.2), in rough agreement with the estimates based on other data samples.
- A comparison of the SFR derived from line luminosities on the one hand and of the UV continuum on the other hand shows that the conversion factor to applied to the latter is in good agreement with that one estimated from model spectra according to Bruzual and Charlot.
- The LFs derived from the blue magnitudes and from the line luminosities for each ELG sample have the same shapes.
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Low Metallicity Galaxies at medium redshift
Introduction
In the local universe, metallicity is well correlated with the absolute luminosity (stellar mass) of galaxies in that more luminous galaxies tend to be more metal rich than less luminous galaxies. At medium redshift, all previous attempts to study the evolution of metallicity with cosmic time were based on continuum selected samples, the emission lines of which were identified by spectroscopy. Thus, for instance, the CFRS sample used by Carollo and Lilly (2001), originally selected using I <22.2, contains only bright galaxies (M_B<-20) at redshift z>0.6, the continuum of which is dominated by an evolved stellar population. Consequently, these authors can hardly find any metallicity difference with respect to local samples.Candidates for faint low metallicity emission line galaxies
The deep emission line survey of CADIS is much better suited for a study of metallicity evolution, since galaxies are selected by their emission line fluxes detected via narrow-band (Fabry-Perot) imaging, irrespective of their continuum brightness. The emission line survey of CADIS can find galaxies selected by their [OIII]a emission in the atmospheric windows A and B with redshifts z=0.400+-0.012 and z=0.636+-0.010, respectively.The figure shows the galaxy 23h-1937 at z=0.639. The lower pannel shows the photometry in all 14 optical CADIS filters fitted by a continuum-model (left) and the Fabry-Perot measurements in window B with a [OIII] doublet profile fitted to the observed flux data (right). Thus, the [OIII]5007 line is seen by the Fabry-Perot in window B and we detect also the [OII]3727 line in the veto filter V611. The [OIII]5007/[OII]3727 ratio measured in this way allows us to select primary candidates for metal poor galaxies, since ionization decreases with metallicity, as showed by Stasinska and Leitherer (1996). Some galaxies with a [OIII]5007/[OII]3727 ratio greater than one have been observed with FORS2 at VLT and DOLORES at Telescopio Nazionale Galileo. Spectroscopic follow up of the galaxy 23h-1937 using FORS2 at VLT is shown in the upper pannel of the figure.
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Oxygen abundances
In the local universe, the oxygen abundance can sometimes be measured directly using the temperature-sensitive line [OIII]4363. But this line is often too weak to be measured even in the local universe let alone at z >> 0. Therefore, at higher redshift, empirical abundance indicators that use the strong forbidden lines have to be used, like the R23 ratio, first suggested by Pagel et al. (1979):R23=([OIII]5007+ [OIII]4959)/[OII]3727
McGaugh (1991) refined the R23-method to take into account the ionization factor U. The calibration between R23 and oxygen abundance was done based on a set of photoionisation models. The next figure shows the oxygen abundance 12+log(O/H) as a function of the line ratio R23. The calibration between R23 and O/H (solid lines) using the models from McGaugh (1991) shows the effect of varying the ionization parameter in terms of the observable line ratio [OIII]/[OII]. The upper horizontal dotted line shows the solar oxygen abundance. Open squares show measured oxygen abundances for galaxies observed with VLT, filled squares for GALILEO. Vertical lines show the values allowed by the measured R23 values for objects which coudn't be put on the lower or upper branch for sure. The upper and lower limits of the oxygen abundances for every object are showed by small horizontal lines. One can see that CADIS can find galaxies with metallicities down to 1/10 Z(solar).
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References
Carollo C. M. and Lilly S. J. 2001 ApJ 548,153McGaugh, Stacy 1991 ApJ 380, 140
Pagel B. E. J., Edmunds M. G., Blackwell D. E., Chun M. S. and Smith G. 1979 MNRAS 189, 95
Stasinska G. and Leitherer C. 1996 ApJS 107,661
last updated: July 4, 2002