COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies - I. Relations between H2, H i, stellar content and structural properties

Abstract

We are conducting COLD GASS, a legacy survey for molecular gas in nearby\ngalaxies. Using the IRAM 30m telescope, we measure the CO(1-0) line in a sample\nof ~350 nearby (D=100-200 Mpc), massive galaxies (log(M*/Msun)>10.0). The\nsample is selected purely according to stellar mass, and therefore provides an\nunbiased view of molecular gas in these systems. By combining the IRAM data\nwith SDSS photometry and spectroscopy, GALEX imaging and high-quality Arecibo\nHI data, we investigate the partition of condensed baryons between stars,\natomic gas and molecular gas in 0.1-10L* galaxies. In this paper, we present CO\nluminosities and molecular hydrogen masses for the first 222 galaxies. The\noverall CO detection rate is 54%, but our survey also uncovers the existence of\nsharp thresholds in galaxy structural parameters such as stellar mass surface\ndensity and concentration index, below which all galaxies have a measurable\ncold gas component but above which the detection rate of the CO line drops\nsuddenly. The mean molecular gas fraction MH2/M* of the CO detections is\n0.066+/-0.039, and this fraction does not depend on stellar mass, but is a\nstrong function of NUV-r colour. Through stacking, we set a firm upper limit of\nMH2/M*=0.0016+/-0.0005 for red galaxies with NUV-r>5.0. The average\nmolecular-to-atomic hydrogen ratio in present-day galaxies is 0.3, with\nsignificant scatter from one galaxy to the next. The existence of strong\ndetection thresholds in both the HI and CO lines suggests that "quenching"\nprocesses have occurred in these systems. Intriguingly, atomic gas strongly\ndominates in the minority of galaxies with significant cold gas that lie above\nthese thresholds. This suggests that some re-accretion of gas may still be\npossible following the quenching event.\n