The effect of environment on the ultraviolet color-magnitude relation of early-type galaxies

K. Schawinski, S. Kaviraj, S. Khochfar, S.-J. Yoon, S.K. Yi, Y.-W. Lee, J.-M. Deharveng, A. Boselli, J. Donas, B. Milliard, T. Barlow, T. Conrow, K. Forster, P.G. Friedman, D.C. Martin, P. Morrissey, M. Seibert, T. Small, T. Wyder, S. NeffD. Schiminovich, L. Blanchi, T. Heckman, A. Szalay, B. Madore, R.M. Rich

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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.
Original languageEnglish
Pages (from-to)512-523
Number of pages12
JournalAstrophysical Journal, Supplement Series
Volume173
Issue number2
DOIs
Publication statusPublished - 1 Dec 2007

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