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

We use \textit{GALEX} (Galaxy Evolution Explorer) near-UV (NUV) photometry ofa sample of early-type galaxies selected in \textit{SDSS} (Sloan Digital SkySurvey) to study the UV color-magnitude relation (CMR). $NUV-r$ color is anexcellent tracer of even small amounts ($\sim 1$% mass fraction) of recent($\la 1$ Gyr) star formation and so the $NUV-r$ CMR allows us to study theeffect of environment on the recent star formation history. We analyze avolume-limited sample of 839 visually-inspected early-type galaxies in theredshift range $0.05 < z < 0.10$ brighter than $M_{r}$ of -21.5 with anypossible emission-line or radio-selected AGN removed to avoid contamination. Wefind that contamination by AGN candidates and late-type interlopers highly biasany study of recent star formation in early-type galaxies and that, afterremoving those, our lower limit to the fraction of massive early-type galaxiesshowing signs of recent star formation is roughly $30 \pm 3%$ This suggeststhat residual star formation is common even amongst the present day early-typegalaxy population. We find that the fraction of UV-bright early-type galaxies is 25% higher inlow-density environments. However, the density effect is clear only in thelowest density bin. The blue galaxy fraction for the subsample of the brightestearly-type galaxies however shows a very strong density dependence, in thesense that the blue galaxy fraction is lower in a higher density region.