COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies: I. Relations between H2, HI, stellar content and structural properties (1103.1642v1)

Abstract: We are conducting COLD GASS, a legacy survey for molecular gas in nearby galaxies. Using the IRAM 30m telescope, we measure the CO(1-0) line in a sample of ~350 nearby (D=100-200 Mpc), massive galaxies (log(M*/Msun)>10.0). The sample is selected purely according to stellar mass, and therefore provides an unbiased view of molecular gas in these systems. By combining the IRAM data with SDSS photometry and spectroscopy, GALEX imaging and high-quality Arecibo HI data, we investigate the partition of condensed baryons between stars, atomic gas and molecular gas in 0.1-10L* galaxies. In this paper, we present CO luminosities and molecular hydrogen masses for the first 222 galaxies. The overall CO detection rate is 54%, but our survey also uncovers the existence of sharp thresholds in galaxy structural parameters such as stellar mass surface density and concentration index, below which all galaxies have a measurable cold gas component but above which the detection rate of the CO line drops suddenly. The mean molecular gas fraction MH2/M* of the CO detections is 0.066+/-0.039, and this fraction does not depend on stellar mass, but is a strong function of NUV-r colour. Through stacking, we set a firm upper limit of MH2/M*=0.0016+/-0.0005 for red galaxies with NUV-r>5.0. The average molecular-to-atomic hydrogen ratio in present-day galaxies is 0.3, with significant scatter from one galaxy to the next. The existence of strong detection thresholds in both the HI and CO lines suggests that "quenching" processes have occurred in these systems. Intriguingly, atomic gas strongly dominates in the minority of galaxies with significant cold gas that lie above these thresholds. This suggests that some re-accretion of gas may still be possible following the quenching event.

• The paper reveals that molecular gas fractions correlate with NUV–r color rather than stellar mass, suggesting that galaxy colors serve as effective proxies for star formation activity.
• It establishes that CO detections drop sharply above specific stellar surface density thresholds, implying quenching processes at work in galaxy evolution.
• The study integrates IRAM, SDSS, GALEX, and Arecibo data to extend scaling relations to cold gas phases, thereby refining empirical models of galaxy evolution.

An Analysis of Molecular Gas in Massive Galaxies from the COLD GASS Survey

The paper, "COLD GASS, an IRAM Legacy Survey of Molecular Gas in Massive Galaxies: I. Relations between H$_2$, HI, Stellar Content and Structural Properties" by A. Saintonge et al., explores the comprehensive investigation of cold gas contents in a range of massive galaxies. The COLD GASS survey, utilizing the IRAM 30m telescope, is designed to systematically measure the CO(1-0) line across a sample of approximately 350 galaxies, selected based on stellar mass. By synthesizing these new observations with data from the SDSS, GALEX, and Arecibo HI pool, this paper seeks to illuminate the distribution and dynamics of cold baryonic matter—both atomic and molecular gas—as well as stars within these galaxies.

Key Findings

1. Detection Rates and Correlations: CO detections were accomplished in 54% of the sample. Analysis indicates that the molecular gas fraction ($M_{H2}/M_{\ast}$) does not scale with stellar mass but shows substantial correlation with NUV-$r$ color, suggesting color as a proxy for specific star formation rates. Conversely, sharp thresholds in structural parameters such as stellar surface density were noted, beyond which the CO detection rate plummets, implicating potential quenching processes in galaxy evolution.
2. Molecular-to-Atomic Hydrogen Ratios: The averaged molecular-to-atomic hydrogen ratio is found to be 0.3, highlighting that atomic gas is strongly prevalent over molecular gas in galaxies where significant cold gas content is present beyond the identified detection thresholds. Crucially, atomic gas could even dominate galaxies undergoing or having undergone quenching.
3. Scaling Relations: The paper extends discussions of scaling laws predominantly observed in stellar components to incorporate cold gas contents. While established laws like Tully-Fisher relate to the stellar attributes, few such correlations exist for the cold gas phases. The results of COLD GASS allow for formulation of these empirical relationships.
4. Implications for Galaxy Evolution Models: By evidencing cold gas partitioning, the implications cut across both theoretical models regarding galaxy formation—especially those parameterizing the interstellar media—and practical applications like assessing star formation potentials.

Implications and Future Directions

The outcomes of the COLD GASS survey reveal complex dynamics of cold gas in galaxies, indicating a significant step forward in empirical astronomy that couples the molecular medium with intrinsic galactic parameters. The detection thresholds in stellar mass surface density and other critical structural aspects might suggest a pivotal period in galaxy evolution where processes lead to the cessation of star formation and gas re-accretion. This offers fertile ground for refining galaxy evolution models that simulate cold gas behaviors alongside stellar growth and distribution.

Future research leveraging the COLD GASS datasets could explore the role of environment, morphological modifiers, and active galactic nuclei in modulating cold gas fractions. Furthermore, continued analysis in tandem with high-resolution radio and CO maps will enrich our comprehension of localized star-forming activities and the mechanisms affecting the cold gas caches indispensable to galactic life cycles.

In essence, the paper by Saintonge et al. provides a methodological and insightful addition to our understanding of galaxy evolution through its detailed assessment of molecular gases, presenting new pathways to paper the cold interstellar medium's role relative to large-scale cosmological principles.