Browsing by Author "Bicker, J."

The composite stellar populations of galaxies comprise stars of a wide range of metallicities. Subsolar metallicities become increasingly important, both in the local universe when going from early towards later galaxy types as well as for dwarf galaxies and for all types of galaxies towards higher redshifts. We present a new generation of chemically consistent evolutionary synthesis models for galaxies of various spectral types from E through Sd. The models follow the chemical enrichment of the ISM and take into account the increasing initial metallicity of successive stellar generations using recently published metallicity dependent stellar evolutionary isochrones, spectra and yields. Our first set of closed-box 1-zone models does not include any spatial resolution or dynamics. For a Salpeter initial mass function ( IMF) the star formation rate (SFR) and its time evolution are shown to successfully parameterise spectral galaxy types E,..., Sd. We show how the stellar metallicity distribution in various galaxy types build up with time to yield after similar to12 Gyr agreement with stellar metallicity distributions observed in our and other local galaxies. The models give integrated galaxy spectra over a wide wavelength range (90.9 Angstrom-160 mum), which for ages of similar to12 Gyr are in good agreement not only with observed broad band colours but also with template spectra for the respective galaxy types. Using filter functions for Johnson-Cousins U, B, V, R-C, I-C, as well as for HST broad band filters in the optical and Bessel & Brett's NIR J, H, K filter system, we calculate the luminosity and colour evolution of model galaxies over a Hubble time. Including a standard cosmological model (H-0 = 65, Omega 0 = 0.1) and the attenuation by intergalactic hydrogen we present evolutionary and cosmological corrections as well as apparent luminosities in various filters over the redshift range from z similar to 5 to the present for our galaxy types and compare to earlier models using single (= solar) metallicity input physics only. We also resent a first comparison of our cc models to HDF data. A more detailed comparison with Hubble Deep Field ( HDF) and other deep field data and an analysis and interpretation of high redshift galaxies in terms of ages, metallicities, star formation histories and, galaxy types will be the subject of a forthcoming paper.

To account for the range of stellar metallicities in local galaxies and for the increasing importance of low metallicities at higher redshift we present chemically consistent models for the spectral and chemical evolution of galaxies over cosmological timescales. We discuss advantages, limitations and future prospects of our approach.

Rich galaxy clusters in the local Universe show a large population of S0 galaxies (similar to40% of all luminous galaxies). With increasing redshift the fraction of this S0 galaxy population is observed to strongly decrease (e.g. by a factor similar to2-3 to z=0.5) in favor of the spiral galaxy fraction while the number of bright ellipticals does not seem to change. The infalling field galaxy population that successively builds up the cluster also is spiral rich and S0 poor. It has hence been suspected that galaxy transformation processes, either due to galaxy-galaxy or to galaxy-ICM interactions, are responsible for this change. Complementing dynamical and morphological studies, we use evolutionary synthesis models describing various possible effects of those interactions on the star formation rates of the infalling spirals. We study the effects of starbursts of various strengths as well as of the truncation of star formation on the color and luminosity evolution of model galaxies of various spectral types. Comparison with observed properties of the local S0 galaxy population is used to constrain possible S0 formation mechanisms. We find that star formation truncation in spiral galaxies earlier than Sd-type, if occurring not too long ago, as well as starbursts more than 3 Gyr ago and followed by complete star formation extinction in spirals-again earlier than Sd-may well account for the observed average S0 luminosities and colors. Late-type galaxies (Sd), even after a strong burst, remain either too blue or too faint. Our results are in agreement with studies of spectral features of cluster S0s but allow for stronger constraints.

We present new calibrations of the widely used H-alpha, [OII], and UV luminosity vs. star formation rate (SFR) relations. Using our evolutionary, synthesis code GALEV we compute the different calibrations for 5 metallicities, from 1/50 solar up to 2.5 solar. We find significant changes in the calibrations for lower metallicities compared to the standard calibrations using solar input physics.

We present evolutionary synthesis models for galaxies of spectral types Sa through Sd with starbursts of various strengths triggered at various redshifts and study their photometric evolution before, during, and after their bursts in a cosmological context. We find that bursts at high redshift, even very strong ones, only cause a small blueing of their intrinsically blue young parent galaxies. At lower redshift, in contrast, even small bursts cause a significant blueing of their intrinsically redder galaxies. While the burst phase is generally short, typically a few hundred Myr in normal-mass galaxies, the postburst stage with its red colors and, in particular the very red ones for early bursts at high redshift, lasts much longer, on the order of several Gyr. We find that, even without any dust, which in the postburst stage is not expected to play an important role anyway, models easily reach the colors of EROs in the redshift range z similar to 2 through z similar to 0.5 after starbursts at redshifts between 2 and 4. We therefore propose a third alternative for the ERO galaxies beyond the two established ones of passive galaxies vs. dusty starbursts: the dust-free post-(strong-) starbursts. A very first comparison of our models to HDF data with photometric redshifts shows that almost all of the outliers that could not be described with our chemically consistent models for undisturbed normal galaxy types E through Sd can now be explained very well. Galaxies in the redshift range from z similar to 2.5 to z similar to 0.5 that are redder, and in some cases much redder, than our reddest undisturbed model for a high-metallicity classical elliptical are well described by post-starburst models after starbursts at redshifts between 2 and 4. Galaxies bluer than our bluest low metallicity Sd model, most of which have redshifts lower than 1, are well explained by ongoing starbursts.