UV Star Formation Rates in the Local Universe

S. Salim, M. R. Rich, S. Charlot, J. Brinchmann, B. D. Johnson, D. Schiminovich, M. Seibert, R. Mallery, T. Heckman, K. Forster, P. G. Friedman, C. D. Martin, P. Morrissey, J. E. Neff, T. Small, T. K. Wyder, L. Bianchi, J. Donas, Y. Lee, B. F. Madore, B. Milliard, A. S. Szalay, B. Y. Welsh, S. K. Yi

Abstract
We measure star formation rates (SFRs) of  50,000 optically-selected galaxies in the local universe (z = 1), spanning a range from gas-rich dwarfs to massive ellipticals. We obtain dust-corrected SFRs by fitting the GALEX (ultraviolet) and SDSS (optical) photometry to a library of population synthesis models that include dust attenuation. For star-forming galaxies, our UV-based SFRs compare remarkably well with those derived from SDSS-measured emission lines (primarily Hα). Systematic deviations from perfect agreement between these two methods is shown to be due to difference in the dust attenuation estimates. In contrast to measurements based on Hα, UV provides reliable SFRs for galaxies with weak or no Hα emission, and where Hα is contaminated with an emission from an AGN (1/2 of the sample). We use full-SED SFRs to calibrate a simple prescription that uses GALEX far-UV magnitude and the UV slope to produce good dust-corrected SFRs for normal star-forming galaxies. The specific SFR (SFR normalized by stellar mass) is considered as a function of stellar mass for (1) star-forming galaxies with no AGN, (2) those hosting an AGN, and for (3) galaxies without Hα emission (the latter two groups forming the bulk of the optical re sequence). We find that the three have distinct star formation histories, with AGN lyin intermediate between the star-forming and the quiescent galaxies. Normal star forming galaxies (without an AGN) lie on a relatively narrow linear sequence. Remarkably, galaxies hosting a strong AGN appear to represent the massive continuation of this sequence. On the other hand, weak AGN, while also massive, have lower SFR, sometimes extending to the realm of quiescent galaxies. We propose an evolutionary sequence for massive galaxies that smoothly connects normal star-forming galaxies to quiescent (red sequence) galaxies via strong and weak AGN. We confirm that some galaxies with no Hα emission show signs of star formation in the UV. We derive a UV-based cosmic star formation density at z = 0.1 with significantly smaller total error than previous measurements.

The Astrophysical Journal Supplement Series
Volume 173, Page 267
December 2007

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