D. Schiminovich, B. Catinella, G. Kauffmann, S. Fabello, J. Wang, C. Hummels, J. Lemonias, S. M. Moran, R. Wu, R. Giovanelli, M. Haynes, T. Heckman, A. Basu-Zych, M. R. Blanton, J. Brinchmann, T. Budavári, T. Gonçalves, B. D. Johnson, R. C Kennicutt, B. F. Madore, C. D. Martin, M. R. Rich, L. Tacconi, D. A. Thilker, V. Wild, T. K. Wyder

Abstract
We use measurements of the H_I content, stellar mass and star formation rates in ∼190 massive galaxies with M_★ > 10¹⁰ M_☉, obtained from the Galex Arecibo SDSS survey described in Paper I (Catinella et al. 2010) to explore the global scaling relations associated with the bin-averaged ratio of the star formation rate over the H_I mass (i.e., ΣSFR/ΣM_HI), which we call the H_I-based star formation efficiency (SFE). Unlike the mean specific star formation rate, which decreases with stellar mass and stellar mass surface density, the star formation efficiency remains relatively constant across the sample with a value close to SFE = 10^-9.5 yr^-1 (or an equivalent gas consumption timescale of ∼ 3×10⁹ yr). Specifically, we find little variation in SFE with stellar mass, stellar mass surface density, NUV -r color and concentration (R₉₀/R₅₀).We interpret these results as an indication that external processes or feedback mechanisms that control the gas supply are important for regulating star formation in massive galaxies. An investigation into the detailed distribution of SFEs reveals that approximately 5% of the sample shows high efficiencies with SFE > 10^-9 yr^-1, and we suggest that this is very likely due to a deficiency of cold gas rather than an excess star formation rate. Conversely, we also find a similar fraction of galaxies that appear to be gas-rich for their given specific star-formation rate, although these galaxies show both a higher than average gas fraction and lower than average specific star formation rate. Both of these populations are plausible candidates for “transition” galaxies, showing potential for a change (either decrease or increase) in their specific star formation rate in the near future. We also find that 36 ± 5 % of the total H_I mass density and 47 ± 5 % of the total SFR density is found in galaxies with M_★ > 10¹⁰ M_☉.

Keywords
galaxies:evolution - galaxies: fundamental parameters - ultraviolet: galaxies - radio lines:galaxies

Monthly Notices of the Royal Astronomical Society
Volume 408, Page 919
2010 March

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