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 similar to 1000 galaxies with stellar masses greater than 1010 M-circle dot 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 similar to 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 NUV- r colour. A large fraction (similar to 60 per cent) of the galaxies in our sample are detected in H i. Even at stellar masses above 1011 M-circle dot, the detected fraction does not fall below similar to 40 per cent. We find that the atomic gas fraction M-H i/M-star decreases strongly with stellar mass, stellar surface mass density and NUV- r 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 108.5 M-circle dot kpc-2. The fraction of gas-rich galaxies decreases much more smoothly with stellar mass. One of the key goals of GASS is to identify and quantify the incidence of galaxies that are transitioning between the blue, star-forming cloud and the red sequence of passively evolving galaxies. Likely transition candidates can be identified as outliers from the mean scaling relations between M-H i/M-star and other galaxy properties. We have fitted a plane to the two-dimensional relation between the H i mass fraction, stellar surface mass density and NUV- r colour. Interesting outliers from this plane include gas-rich red sequence galaxies that may be in the process of regrowing their discs, as well as blue, but gas-poor spirals.