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 , 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 onto 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 halos have stellar populations on average \sim 1Â 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 halos more massive than M _ { H } \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 3 \times 10 ^ { 13 } M _ { \odot } .
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 } \sim 6 \times 10 ^ { 12 } M _ { \odot } halos have stellar populations consistent with the central galaxies found in the lowest mass halos of our sample ( i.e .
M _ { H } \sim 10 ^ { 12 } M _ { \odot } ) .
This result is indicative of galaxies in lower mass halos being accreted into larger halos .