xGASS: an Arecibo legacy HI survey
The extended GALEX Arecibo SDSS Survey (xGASS) is a gas fraction-limited census of the atomic hydrogen (HI) gas content of ~1200 galaxies in the local Universe, selected only by stellar mass (M*=109 − 1011.5 M⊙) and redshift (0.01< z <0.05). Our sample is extracted from the overlap of the SDSS spectroscopic and GALEX imaging surveys, and builds on the Arecibo Legacy Fast ALFA (ALFALFA) HI-blind survey, which provides the most gas-rich systems. xGASS is the combination of two large targeted surveys at Arecibo (GASS and GASS-low), carried out in 2008-2012 and 2012-2015, respectively. Together, these surveys required ∼1300 hours of Arecibo telescope time.
Our original approach of carrying out gas fraction-limited observations down to M(HI)/M*~2% allowed us to obtain stringent upper limits, which are essential to interpret variations of gas content as a function of galaxy properties. This makes xGASS unique among all the HI surveys carried out to date, and particularly suited to investigate the HI reservoirs across both star-forming and passive galaxies.
Together with its twin project xCOLD GASS, an IRAM-30m CO survey that provided estimates of the molecular hydrogen content of ~500 xGASS systems, these are the deepest observations of cold gas in galaxies in the local Universe currently available.