Evolutionary Synthesis Models for the Formation of S0 Galaxies in Clusters

Abstract

Rich galaxy clusters in the local Universe show a large population of S0 galaxies (~ 40%) of all luminous galaxies). With increasing redshift the fraction of this S0 galaxy population is observed to strongly decrease (e.g. by a factor ~2-3 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.