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

Creators: Kaviraj, S.; Schawinski, K.; Devriendt, J. E. G.; Ferreras, I.; Khochfar, S.; Yoon, S.-J.; Yi, S. K.; Deharveng, J.-M.; Boselli, A.; Barlow, T.; Conrow, T.; Forster, K.; Friedman, P. G.; Martin, D. C.; Morrissey, P.; Neff, S.; Schiminovich, D.; Seibert, M.; Small, T.; Wyder, T.; Bianchi, L.; Donas, J.; Heckman, T.; Lee, Y.-W.; Madore, B.; Milliard, B.; Rich, R. M.; Szalay, A.

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 ΛCDM 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.

Additional Information

© 2007 The American Astronomical Society. Received 2006 January 4; accepted 2006 September 11. We are grateful to the anonymous referee for various clarifications. We warmly thank Chris Wolf for providing high-resolution COMBO-17 images, which formed an integral part of our morphological classification process. We also thank Mariangela Bernardi for her generous help in the initial stages of this project, for providing the DR2 versions of her SDSS early-type catalog prior to publication, and many interesting discussions. We are grateful to Jeremy Blaizot for his extensive help with the GALICS model and to Andre´s Jorda´n, Joseph Silk, Roger Davies, and Andrew Benson for many useful comments regarding this work. S. K. acknowledges PPARC graduate D Phil scholarship PPA/S/S/ 2002/03532. This work was supported by grant R01-2006-000- 10716-0 from the Basic Research Program of the Korea Science & Engineering Foundation and Yonsei University Research Fund (2005) to S. K. Y.

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