XCOLD GASS: The Complete IRAM 30 m Legacy Survey of Molecular Gas for Galaxy Evolution Studies

We introduce xCOLD GASS, a legacy survey providing a census of molecular gas in the local universe. Building on the original COLD GASS survey, we present here the full sample of 532 galaxies with CO (1-0) measurements from the IRAM 30 m telescope. The sample is mass-selected in the redshift interval 0.01 < z < 0.05 from the Sloan Digital Sky Survey (SDSS) and therefore representative of the local galaxy population with M_∗ > 10⁹ M_⊙. The CO (1-0) flux measurements are complemented by observations of the CO (2-1) line with both the IRAM 30 m and APEX telescopes, H I observations from Arecibo, and photometry from SDSS, WISE, and GALEX. Combining the IRAM and APEX data, we find that the ratio of CO (2-1) to CO (1-0) luminosity for integrated measurements is r₂₁ = 0.79 ± 0.03, with no systematic variations across the sample. The CO (1-0) luminosity function is constructed and best fit with a Schechter function with parameters L_CO ^∗ = (7.77 ± 2.11) × 10⁹ K km s^-1 pc², φ^∗ = (9.84 ± 5.41) × 10^-4 Mpc^-3, and α = -1.19 ± 0.05.With the sample now complete down to stellar masses of 10⁹M_⊙, we are able to extend our study of gas scaling relations and confirm that both molecular gas fractions ( f_H2) and depletion timescale (t_dep (H₂)) vary with specific star formation rate (or offset from the star formation main sequence) much more strongly than they depend on stellar mass. Comparing the xCOLD GASS results with outputs from hydrodynamic and semianalytic models, we highlight the constraining power of cold gas scaling relations on models of galaxy formation.