The GALEX Arecibo SDSS Survey - I. Gas fraction scaling relations of massive galaxies and first data release

We introduce the GALEX Arecibo SDSS Survey (GASS), an on-going large programme that is gathering high quality H I-line spectra using the Arecibo radio telescope for an unbiased sample of 1000 galaxies with stellar masses greater than 10 10 M and redshifts 0.025 < z < 0.05, selected from the Sloan Digital Sky Survey (SDSS) spectroscopic and Galaxy Evolution Explorer (GALEX) imaging surveys. The galaxies are observed until detected or until a low gas mass fraction limit (1.5-5 per cent) is reached. This paper presents the first Data Release, consisting of 20 per cent of the final GASS sample. We use this data set to explore the main scaling relations of the H I gas fraction with galaxy structure and NUVr colour. A large fraction (60 per cent) of the galaxies in our sample are detected in H I. Even at stellar masses above 10 11 M , the detected fraction does not fall below 40 per cent. We find that the atomic gas fraction M H I /M decreases strongly with stellar mass, stellar surface mass density and NUVr colour, but is only weakly correlated with the galaxy bulge-to-disc ratio (as measured by the concentration index of the r-band light). We also find that the fraction of galaxies with significant (more than a few per cent) H I decreases sharply above a characteristic stellar surface mass density of 10 8.5 M kpc -2 . The fraction of gas-rich galaxies decreases much