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

Abstract

We use \\textit{GALEX} (Galaxy Evolution Explorer) near-UV (NUV) photometry of\na sample of early-type galaxies selected in \\textit{SDSS} (Sloan Digital Sky\nSurvey) to study the UV color-magnitude relation (CMR). $NUV-r$ color is an\nexcellent tracer of even small amounts ($\\sim 1$% mass fraction) of recent\n($\\la 1$ Gyr) star formation and so the $NUV-r$ CMR allows us to study the\neffect of environment on the recent star formation history. We analyze a\nvolume-limited sample of 839 visually-inspected early-type galaxies in the\nredshift range $0.05 < z < 0.10$ brighter than $M_{r}$ of -21.5 with any\npossible emission-line or radio-selected AGN removed to avoid contamination. We\nfind that contamination by AGN candidates and late-type interlopers highly bias\nany study of recent star formation in early-type galaxies and that, after\nremoving those, our lower limit to the fraction of massive early-type galaxies\nshowing signs of recent star formation is roughly $30 \\pm 3%$ This suggests\nthat residual star formation is common even amongst the present day early-type\ngalaxy population.\n We find that the fraction of UV-bright early-type galaxies is 25% higher in\nlow-density environments. However, the density effect is clear only in the\nlowest density bin. The blue galaxy fraction for the subsample of the brightest\nearly-type galaxies however shows a very strong density dependence, in the\nsense that the blue galaxy fraction is lower in a higher density region.\n