paper

The effect of environment on the UV colour-magnitude relation of early-type galaxies

arXiv:astro-ph/0601036 · doi:10.1086/516631

Abstract

We use \textit{GALEX} (Galaxy Evolution Explorer) near-UV (NUV) photometry of a sample of early-type galaxies selected in \textit{SDSS} (Sloan Digital Sky Survey) to study the UV color-magnitude relation (CMR). color is an excellent tracer of even small amounts (% mass fraction) of recent ($\la 1$ Gyr) star formation and so the 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 brighter than 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 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 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.

arXiv commentAccepted for publication in ApJ, paper with high-resolution figures can be downloaded at: http://www-astro.physics.ox.ac.uk/~kevins/PAPERS/uv_environment.ps

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