The effect of environment on the ultraviolet color-magnitude relation of early-type galaxies
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Author
Schawinski, K.
Kaviraj, S.
Khochfar, S.
Yoon, S.-J.
Yi, S.K.
Lee, Y.-W.
Deharveng, J.-M.
Boselli, A.
Donas, J.
Milliard, B.
Barlow, T.
Conrow, T.
Forster, K.
Friedman, P.G.
Martin, D.C.
Morrissey, P.
Seibert, M.
Small, T.
Wyder, T.
Neff, S.
Schiminovich, D.
Blanchi, L.
Heckman, T.
Szalay, A.
Madore, B.
Rich, R.M.
Attention
2299/11926
Abstract
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 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.