Young stellar populations in early-type galaxies in the Sloan Digital Sky Survey

We use a purely data-driven rectified factor analysis to identify early-type galaxies with recent star formation in DR4 of the SDSS Spectroscopic Catalogue. We compare the spectra and environment of these galaxies with `normal' early-types, and a sample of independently selected E+A galaxies. We calculate the projected local galaxy surface density (Sigma_5 and Sigma_10) for each galaxy in our sample, and find that the dependence, on projected local density, of the properties of E+As is not significantly different from that of early-types with young stellar populations, dropping off rapidly towards denser environments, and flattening off at densities < 0.1-0.3 Mpc^-2. The dearth of E+A galaxies in dense environments confirms that E+As are most likely the products of galaxy-galaxy merging or interactions, rather than star-forming galaxies whose star formation has been quenched by processes unique to dense environments. We see a tentative peak in the number of E+A galaxies at Sigma_10 \~ 0.1-0.3 Mpc^-2, which may represent the local galaxy density at which the rate of galaxy-galaxy merging or interaction rate peaks. Analysis of the spectra of our early-types with young stellar populations suggests that they have a stellar component dominated by F stars, ~ 1-4 Gyr old, together with a mature, metal-rich population characteristic of `typical' early-types. The young stars represent > 10% of the stellar mass in these galaxies. This, together with the similarity of the environments in which this `E+F' population and the E+A galaxy sample are found, suggests that E+F galaxies used to be E+A galaxies, but have evolved by a further ~ one to a few Gyr. Our factor analysis is sensitive enough to identify this hidden population. (Abridged)