Abstract
We investigate in detail 13 early-type field galaxies with 0.2<z<0.7 drawn from the FORS Deep Field. Since the majority (9 galaxies) is at z~0.4, we compare the field galaxies to 22 members of three rich clusters with z=0.37 to explore possible variations caused by environmental effects. We exploit VLT/FORS spectra (R~1200) and HST/ACS imaging to determine internal kinematics, structures and stellar population parameters. From the Faber-Jackson and Fundamental Plane scaling relations we deduce a modest luminosity evolution in the B-band of 0.3-0.5mag for both samples. We compare measured Lick absorption line strengths (Hdelta, Hgamma, Hbeta, Mg_b, & Fe5335) with evolutionary stellar population models to derive light-averaged ages, metallicities and the element abundance ratios Mg/Fe. We find that all these three stellar parameters of the distant galaxies obey a scaling with velocity dispersion (mass) which is very well consistent with the one of local nearby galaxies. In particular, the distribution of Mg/Fe ratios of local galaxies is matched by the distant ones, and their derived mean offset in age corresponds to the average lookback time. This indicates that there was little chemical enrichment and no significant star formation within the last ~5Gyr. The calculated luminosity evolution of a simple stellar population model for the derived galaxy ages and lookback times is in most cases very consistent with the mild brightening measured by the scaling relations.
Highlights
It has become feasible to study physical parameters of galaxies out to redshifts z ≈ 1
Half-light radii and total luminosities were determined on Hubble Space Telescope (HST)/WFPC2 images using the method described by Saglia et al (1997)
We have investigated the kinematic, photometric, structural and stellar population parameters of a sample of field galaxies with mean redshift of 0.4
Summary
It has become feasible to study physical parameters of galaxies out to redshifts z ≈ 1. For field early-type galaxies, Kuntschner et al (2002) find that they are younger (by 2−3 Gyr) but more metal-rich (by ≈0.2 dex) than cluster galaxies exhibiting similar supersolar Mg/Fe ratios In their recent study of field and cluster galaxies compiling high-quality data from several sources, Thomas et al (2005) show that mean ages, metallicities, and α/Fe ratios correlate with galaxy mass (σ). Eisenstein et al (2003) examined 22 000 bright early-type galaxies from the SDSS averaging the spectra within bins of luminosity, environment, and redshift (0 < z < 0.5) to produce high-S /N mean spectra They confirmed that these L∗ galaxies have mainly old quiescent stellar populations with high metallicities and an excess of α- (Mg-) elements with respect to the solar value independent of environment or redshift. This yields a distance luminosity of 104.75 for the Coma cluster which is used as a local reference sample
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