2009MNRAS.397.1243Z

We use a sample of 800 galaxies with Hi mass measurements from the HyperLeda catalogue and optical photometry from the fourth data release of the Sloan Digital Sky Survey (SDSS) to calibrate a new photometric estimator of the Hi-to-stellar-mass ratio for nearby galaxies. Our estimator, which is motivated by the Kennicutt-Schmidt star formation law, is log₁₀(G_Hi/S) = -1.73238(g - r) + 0.215182µ_i- 4.08451, where µ_i is the i-band surface brightness and g - r is the optical colour estimated from the g- and r-band Petrosian apparent magnitudes. This estimator has a scatter of σ = 0.31 dex in log (G_Hi/S), compared to σ ∼ 0.4 dex for previous estimators that were based on colour alone. We investigate whether the residuals in our estimate of log (G_Hi/S) depend in a systematic way on a variety of different galaxy properties. We find no effect as a function of stellar mass or 4000 Å break strength, but there is a systematic effect as a function of the concentration index of the light. We then apply our estimator to a sample of 10⁵ emission-line galaxies in the SDSS Data Release 4 (DR4) and derive an estimate of the Hi mass function, which is in excellent agreement with recent results from Hi blind surveys. Finally, we re-examine the well-known relation between gas-phase metallicity and stellar mass, and ask whether there is a dependence on Hi-to-stellar-mass ratio, as predicted by chemical evolution models. We do find that gas-poor galaxies are more metal rich at fixed stellar mass. We compare our results with the semi-analytic models of De Lucia & Blaizot, which include supernova feedback, as well as the cosmological infall of gas.