UV-optical colors as probes of early-type galaxy evolution

S. Kaviraj, K. Schawinski, J.E.G. Devriendt, I. Ferreras, 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. SmallT. Wyder, S. Neff, D. Schiminovich, L. Bianchi, T. Heckman, A. Szalay, B. Madore, R.M. Rich

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We have studied ∼2100 early-type galaxies in the SDSS DR3 which have been detected by the GALEX Medium Imaging Survey (MIS), in the redshift range O <z <0.1.1. Combining GALEXUV photometry with corollary optical data from the SDSS, we find that, at a 95% confidence level, at least ∼30% of galaxies in this sample have UV to optical colors consistent with some recent star formation within the last Gyr. In particular, galaxies with an NUV - r color less than 5.5 are very likely to have experienced such recent star formation, taking into account the possibility of a contribution to NUV flux from the UV upturn phenomenon. We find quantitative agreement between the observations and the predictions of a semianalytical ACDM hierarchical merger model and deduce that early-type galaxies in the redshift range 0 <z <0.11 have ∼ 1 % -3 % of their stellar mass in stars less than 1 Gyr old. The average age of this recently formed population is ∼300-500 Myr. We also find that "monolithically" evolving galaxies, where recent star formation can be driven solely by recycled gas from stellar mass loss, cannot exhibit the blue colors (NUV - r <5.5) seen in a significant fraction (∼30%) of our observed sample.
Original languageEnglish
Pages (from-to)619-642
Number of pages24
JournalAstrophysical Journal, Supplement Series
Issue number2
Publication statusPublished - 1 Dec 2007


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