Disentangling luminosity, morphology, star formation, stellar mass, and environment in galaxy evolution

Abstract

We present a study of the photometric and spectroscopic properties of galaxies in a sample of six nearby, rich galaxy clusters. We examine the variations of fundamental galaxy properties, such as luminosity, morphology, and star formation rates with environment, providing new constraints on the mechanisms that drive the evolution of galaxies. This study also introduces a new maximum likelihood algorithm to recover the true distribution function of galaxies from an incomplete sample. This algorithm is ideally suited for modern-day surveys that gather a large amount of information about each object. The R-band luminosity function (GLF) shows no variation among clusters or between the field and clusters, with the exception of an enhancement of the luminous tip of the GLF in clusters. However, the GLF of quiescent galaxies steepens significantly between the field and clusters and is not universal in clusters either, suggesting that star formation properties may be more strongly correlated than the luminosity function with environment. The U-band GLF in clusters is slightly steeper than the R-band GLF, indicating that cluster galaxies are bluer at fainter magnitudes and that the GLF is thus weakly sensitive to star formation, dust, or metallicity effects. To constrain the mechanisms that shape the morphologies of cluster galaxies, we have calculated separate R-band luminosity functions for galaxy bulges and disks. Their distribution as a function of morphology and environment indicates that intermediate- and early-type galaxies can be generated from late-type galaxies by increasing the luminosity of the bulge, but not by fading the disks alone, favoring galaxy-galaxy interactions or mergers as the primary morphological transformation mechanism. Finally, we find a residual correlation of star formation with environment even after accounting for environmental variations of morphology, stellar mass, and stellar age. Thus, the star formation gradient in clusters is not just another aspect of the morphology-density relation, and cannot be solely the result of initial conditions, but must partly be due to subsequent evolution through a mechanism (or mechanisms) sensitive to environment. These results thus constitute a true "smoking gun" pointing to the effect of environment on the later evolution of galaxies.