UV‐Optical Colors as Probes of Early‐Type Galaxy Evolution

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 0 < z < 0:11. Combining GALEX UV 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 CDM 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.