xCOLD GASS: the complete IRAM-30m legacy survey of molecular gas for galaxy evolution studies

Abstract: We introduce xCOLD GASS, a legacy survey providing a census of molecular gas in the local Universe. Building upon the original COLD GASS survey, we present here the full sample of 532 galaxies with CO(1-0) measurements from the IRAM-30m telescope. The sample is mass-selected in the redshift interval $0.01<z\<0.05$ from SDSS, and therefore representative of the local galaxy population with M$_{\ast}\>10^9$M$_{\odot}$. The CO(1-0) flux measurements are complemented by observations of the CO(2-1) line with both the IRAM-30m and APEX telescopes, HI observations from Arecibo, and photometry from SDSS, WISE and GALEX. Combining the IRAM and APEX data, we find that the CO(2-1) to CO(1-0) luminosity ratio for integrated measurements is $r_{21}=0.79\pm0.03$, with no systematic variations across the sample. The CO(1-0) luminosity function is constructed and best fit with a Schechter function with parameters {$L_{\mathrm{CO}}^* = (7.77\pm2.11) \times 10^9\,\mathrm{K\,km\,s^{-1}\, pc^{2}}$, $\phi^{*} = (9.84\pm5.41) \times 10^{-4} \, \mathrm{Mpc^{-3}}$ and $\alpha = -1.19\pm0.05$}. With the sample now complete down to stellar masses of $10^9$M$_{\odot}$, we are able to extend our study of gas scaling relations and confirm that both molecular gas fraction and depletion timescale 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 semi-analytic models, we highlight the constraining power of cold gas scaling relations on models of galaxy formation.

• The paper presents a comprehensive molecular gas census by analyzing CO(1-0) and CO(2-1) emissions in 532 local galaxies.
• It integrates HI data and multi-wavelength photometry to robustly correlate molecular gas fractions and depletion times with star formation rates.
• The findings refine cold gas scaling relations, offering critical benchmarks for calibrating galaxy evolution models and feedback processes.

Overview of xCOLD GASS: An IRAM 30m Legacy Survey

The xCOLD GASS survey conducted a comprehensive investigation of molecular gas in the local Universe, building on the foundational efforts of the original COLD GASS project. This initiative targeted a sample of 532 galaxies within a specific redshift range to provide robust measurements of molecular gas content using the IRAM-30m telescope. By focusing on CO(1-0) and CO(2-1) emissions, the survey aims to elucidate the role of molecular gas in galaxy evolution, particularly how molecular gas fractions and depletion timescales relate to diverse galactic properties.

The inclusion of a broad mass-selected sample, spanning redshifts $0.01$10^9 M_\odot$, allowed for comprehensive statistical characterization of scaling relations across a wide range of galaxy types. The survey leveraged HI data and multi-wavelength photometry to bolster these findings, drawing insights from SDSS, WISE, and GALEX data sources.

One of the key quantitative discoveries presented in the survey is the CO(2-1)/CO(1-0) luminosity ratio, $r_{21} = 0.79 \pm 0.03$, which consistently appeared across the sample without systematic variation. This uniformity allows the xCOLD GASS data to further refine the CO(1-0) luminosity function, employing a Schechter function fit with derived parameters illustrating the distribution of molecular gas luminosities across the galaxy population.

The analysis confirms that molecular gas fractions and depletion timescales exhibit stronger correlations with specific star formation rates (sSFR) and deviations from the main sequence than with mere stellar mass. Thus, variations in sSFR play a crucial role in dictating the molecular gas dynamics within galaxies.

When juxtaposed with outputs from hydrodynamic and semi-analytic models, the xCOLD GASS dataset highlights the critical role of accurate cold gas scaling relations in galaxy formation theories. These comparisons also underscore the necessity of constraining feedback processes and molecular gas physics to produce coherent models of galaxy evolution.

Practically, the implications of this extensive dataset extend to refining existing models of galactic growth and star formation processes. xCOLD GASS offers a benchmark around which simulations can be calibrated, particularly in modeling the physical processes that regulate the conversion of atomic to molecular gas and the subsequent star formation efficiencies.

Theoretically, xCOLD GASS advocates for a multi-scale approach to understanding galactic gas dynamics—one that bridges the macroscopic environment traced by atomic gas with the microscopic conditions within molecular clouds. As future developments in telescope capabilities and simulation methodologies progress, the comprehensive framework established by xCOLD GASS will surely serve as a pivotal reference point for ongoing and subsequent research initiatives in galaxy evolution studies.