The role of environment on the formation of early-type galaxies

The effect of environment on galaxy formation poses one of the best constraints on the interplay between mass assembly and star formation in galaxies. We present here a detailed study of the stellar populations of a volume-limited sample of early-type galaxies from the Sloan Digital Sky Survey (SDSS), across a range of environments - defined as the mass of the host dark matter halo, according to the groups catalogue of Yang et al. The stellar populations are explored through the SDSS spectra, via projection on to a set of two spectral vectors determined from principal component analysis. This method has been found to highlight differences not seen when using standard, model-dependent comparisons of photo-spectroscopic data. We find the velocity dispersion of the galaxy to be the main driver behind the different star formation histories of early-type galaxies. However, environmental effects are seen to play a role (although minor). Our principal components allow us to distinguish between the effects of environment as a change in average age (mapping the time lapse of assembly) or the presence of recent star formation (reflecting environment-related interactions). Galaxies populating the lowest mass haloes have stellar populations on average similar to 1 Gyr younger than the rest of the sample. The fraction of galaxies with small amounts of recent star formation is also seen to be truncated when occupying haloes more massive than M-H greater than or similar to 3 x 1013 M-circle dot. The sample is split into satellite and central galaxies for a further analysis of environment. Small but measurable differences are found between these two subsamples. For an unbiased comparison, we have to restrict this analysis to a range of halo masses over which a significant number of central and satellite galaxies can be found. Over this mass range, satellites are younger than central galaxies of the same stellar mass. The younger satellite galaxies in M-H similar to 6 x 1012 M-circle dot haloes have stellar populations consistent with the central galaxies found in the lowest mass haloes of our sample (i.e. M-H similar to 1012 M-circle dot). This result is indicative of galaxies in lower mass haloes being accreted into larger haloes.