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

TLDR

Understanding how galaxy mass or structure regulates cold gas content requires accurate bulge–disk separation in gas scaling studies. The study presents xGASS, a gas‑fraction limited census of HI in 1,179 galaxies, and extends GASS HI scaling relations while quantifying total cold gas fractions and molecular‑to‑atomic gas ratios for 477 IRAM‑observed galaxies. The survey combines new Arecibo HI observations of 208 galaxies with existing data to produce a gas‑fraction limited census of 1,179 galaxies, and uses IRAM 30 m observations of 477 galaxies to measure total cold gas fractions and molecular‑to‑atomic ratios. Atomic gas fractions rise steadily with decreasing stellar mass down to $\log M_\star=9$, with no plateau, while total gas reservoirs remain HI.

Abstract

We present the extended GALEX Arecibo SDSS Survey (xGASS), a gas fraction-limited census of the atomic (HI) gas content of 1179 galaxies selected only by stellar mass ($M_\star =10^{9}-10^{11.5} M_\odot$) and redshift ($0.01<z<0.05$). This includes new Arecibo observations of 208 galaxies, for which we release catalogs and HI spectra. In addition to extending the GASS HI 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 $\log M_\star/M_\odot = 9$. Total gas reservoirs remain HI-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 $\mu_\star$, but individual values vary by almost two orders of magnitude at fixed $M_\star$ or $\mu_\star$. We show that, for galaxies on the star-forming sequence, variations of $R_{mol}$ are mostly driven by changes of the HI reservoirs, with a clear dependence on $\mu_\star$. 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 disk components for the study of gas scaling relations.