xGASS: total cold gas scaling relations and molecular-to-atomic gas ratios of galaxies in the local Universe

We present the extended GALEX Arecibo SDSS Survey (xGASS), a gas fraction-limited census of the atomic hydrogen (H I) gas content of 1179 galaxies selected only by stellar mass (M_⋆ = 10⁹-10^11.5 M_⊙) and redshift (0.01 < z < 0.05). This includes new Arecibo observations of 208 galaxies, for which we release catalogues and H I spectra. In addition to extending the GASS H I scaling relations by one decade in stellar mass, we quantify total (atomic+molecular) cold gas fractions and molecular-to-atomic gas mass ratios, R_mol, for the subset of 477 galaxies observed with the IRAM 30 m telescope. We find that atomic gas fractions keep increasing with decreasing stellar mass, with no sign of a plateau down to log M_⋆/M_⊙ = 9. Total gas reservoirs remain H I-dominated across our full stellar mass range, hence total gas fraction scaling relations closely resemble atomic ones, but with a scatter that strongly correlates with R_mol, especially at fixed specific star formation rate. On average, R_mol weakly increases with stellar mass and stellar surface density μ_⋆, but individual values vary by almost two orders of magnitude at fixed M_⋆ or μ_⋆. We show that, for galaxies on the star-forming sequence, variations of R_mol are mostly driven by changes of the H I reservoirs, with a clear dependence on μ_⋆. Establishing if galaxy mass or structure plays the most important role in regulating the cold gas content of galaxies requires an accurate separation of bulge and disc components for the study of gas scaling relations.