The Effect of Environment on the Ultraviolet Color-Magnitude Relation of Early-Type Galaxies

We use GALEX near-UV (NUV) photometry of a sample of early-type galaxies selected in the SDSS (Sloan Digital Sky Survey) to study the UV color-magnitude relation (CMR). NUV-r color is an excellent tracer of even small amounts (~1% mass fraction) of recent (<~1 Gyr) star formation, and so the NUV-r CMR allows us to study the effect of environment on the recent star formation history. We analyze a volume-limited sample of 839 visually inspected early-type galaxies in the redshift range 0.05<z<0.10 brighter than M_r of -21.5 with any possible emission-line or radio-selected active galactic nuclei (AGNs) removed to avoid contamination. We find that contamination by AGN candidates and late-type interlopers highly bias any study of recent star formation in early-type galaxies and that, after removing those, our lower limit to the fraction of massive early-type galaxies showing signs of recent star formation is roughly 30%+/-3%. This suggests that residual star formation is common even among the present day early-type galaxy population. We find that the fraction of UV-bright early-type galaxies is 25% higher in low-density environments. However, the density effect is clear only in the lowest density bin. The blue galaxy fraction for the subsample of the brightest early-type galaxies, however, shows a very strong density dependence, in the sense that the blue galaxy fraction is lower in a higher density region.