UV-optical colors as probes of early-type galaxy evolution
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Author
Kaviraj, S.
Schawinski, K.
Devriendt, J.E.G.
Ferreras, I.
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.
Bianchi, L.
Heckman, T.
Szalay, A.
Madore, B.
Rich, R.M.
Attention
2299/11922
Abstract
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.