UV-optical colours as probes of early-type galaxy evolution
ArXiv astro-ph/0601029 (2006)
Authors:
S Kaviraj, K Schawinski, JEG Devriendt, I Ferreras, S Khochfar, S-J Yoon, SK Yi, J-M Deharveng, A Boselli, T Barlow, T Conrow, K Forster, P Friedman, DC Martin, P Morrissey, S Neff, D Schiminovich, M Seibert, T Small, T Wyder, L Bianchi, J Donas, T Heckman, Y-W Lee, B Madore, B Milliard, RM Rich, A Szalay
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 percent confidence level, at least ~30 percent of
galaxies in this sample have UV to optical colours consistent with some recent
star formation within the last Gyr. In particular, galaxies with a NUV - r
colour 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 semi-analytical LCDM hierarchical merger
model and deduce that early-type galaxies in the redshift range 0 < z < 0.11
have ~1 to 3 percent of their stellar mass in stars less than 1 Gyr old. The
average age of this recently formed population is ~300 to 500 Myrs. 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
colours (NUV - r < 5.5) seen in a significant fraction (~30 percent) of our
observed sample.
