The Star Formation Histories of Galaxies in Distant Clusters

We present a detailed analysis of the spectroscopic catalog of galaxies in 10distant clusters from Dressler et al. (1999, D99). We investigate the nature ofthe different spectral classes defined by D99 including star forming,post-starburst and passive galaxy populations, and reproduce their basicproperties using our spectral synthesis model. We attempt to identify theevolutionary pathways between the various spectral classes in order to searchfor the progenitors of the numerous post-starburst galaxies. The comparison ofthe spectra of the distant galaxy populations with samples drawn from the localUniverse leads us to identify a significant population of dust-enshroudedstarburst galaxies, showing both strong Balmer absorption and relatively modest[OII] emission, that we believe are the most likely progenitors of thepost-starburst population. We present the differences between the field andcluster galaxies at z=0.4-0.5. We then compare the spectral and themorphological properties of the distant cluster galaxies, exploring theconnection between the quenching of star formation inferred from the spectraand the strong evolution of the S0 population discussed by Dressler et al.(1997). We conclude that either two different timescales and/or two differentphysical processes are responsible for the spectral and the morphologicaltransformation.Comment: 15 pages, LaTeX, 10 figures, uses emulateapj.sty, ApJ in pres