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

Abstract

We use GALEX (Galaxy Evolution Explorer) near-UV (NUV) photometry of a sample of earlytype galaxies selected in 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 Mr of -21.5 with any possible emission-line or radio-selected AGN 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 23 +/- 2% This suggests that residual star formation is common even amongst the present day early-type galaxy population. We find that the fraction of UV-bright early-type galaxies is 50% 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.