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

We present a study of the spectroscopic and photometric properties of galaxies in six nearby clusters. We perform a partial correlation analysis on our data set to investigate whether the correlation between star formation rates in galaxies and their environment is merely another aspect of correlations of morphology, stellar mass, or mean stellar age with environment, or whether star formation rates vary independently of these other correlations. We find a residual correlation of ongoing star formation with environment, indicating that even galaxies with similar morphologies, stellar masses, and mean stellar ages have lower star formation rates in denser environments. Thus, the current star formation gradient in clusters is not just another aspect of the morphology-density, stellar mass-density, or mean stellar age-density relations. Furthermore, the star formation gradient cannot be solely the result of initial conditions, but must partly be due to subsequent evolution through a mechanism (or mechanisms) sensitive to environment. Our results constitute a true "smoking gun" pointing to the effect of environment on the later evolution of galaxies.