The star formation histories of galaxies in distant clusters

We present a detailed analysis of the spectroscopic catalog of galaxies in 10 distant clusters from Dressier et al. We investigate the nature of the different spectral classes defined by Dressier et al., including star-forming, poststarburst, and passive galaxy populations, and reproduce their basic properties using our spectral synthesis model. We attempt to identify the evolutionary pathways between the various spectral classes in order to search for the progenitors of the numerous poststarburst galaxies. The comparison of the spectra of the distant galaxy populations with samples drawn from the local universe leads us to identify a significant population of dust-enshrouded starburst galaxies, showing both strong Balmer absorption and relatively modest [O II] emission, that we believe are the most likely progenitors of the poststarburst population. We present the differences between the field and cluster galaxies at z = 0.4-0.5. We then compare the spectral and morphological properties of the distant cluster galaxies, exploring the connection between the quenching of star formation inferred from the spectra and the strong evolution of the S0 population discussed by Dressier et al. We conclude that either two different timescales and/or two different physical processes are responsible for the spectral and morphological transformation.