COLD GASS, an IRAM legacy survey of molecular gas in massive galaxies - II. The non-universality of the molecular gas depletion time-scale

We study the relation between molecular gas and star formation in a volume-limited sample of 222 galaxies from the COLD GASS survey, with measurements of the CO(1-0) line from the IRAM 30-m telescope. The galaxies are at redshifts 0.025 < z < 0.05 and have stellar masses in the range 10.0 < logM*/M-circle dot < 11.5. The IRAM measurements are complemented by deep Arecibo HI observations and homogeneous Sloan Digital Sky Survey and GALEX photometry. A reference sample that includes both ultraviolet (UV) and far-infrared data is used to calibrate our estimates of star formation rates from the seven optical/UV bands. The mean molecular gas depletion time-scale [tdep(H-2)] for all the galaxies in our sample is 1 Gyr; however, t(dep)(H-2) increases by a factor of 6 from a value of similar to 0.5 Gyr for galaxies with stellar masses of similar to 10(10)M(circle dot) to similar to 3 Gyr for galaxies with masses of a few x 10(11)M(circle dot). In contrast, the atomic gas depletion time-scale remains constant at a value of around 3 Gyr. This implies that in high-mass galaxies, molecular and atomic gas depletion time-scales are comparable, but in low-mass galaxies, the molecular gas is being consumed much more quickly than the atomic gas. The strongest dependences of tdep(H-2) are on the stellar mass of the galaxy [parametrized as log tdep(H-2) = (0.36 +/- 0.07)(logM(*)-10.70) + (9.03 +/- 0.99)], and on the specific star formation rate (sSFR). A single t(dep)(H-2) versus sSFR relation is able to fit both 'normal' star-forming galaxies in our COLD GASS sample and more extreme starburst galaxies (luminous infrared galaxies and ultraluminous infrared galaxies), which have t(dep)(H-2) < 10(8) yr. Normal galaxies at z = 1-2 are displaced with respect to the local galaxy population in the t(dep)(H-2) versus sSFR plane and have molecular gas depletion times that are a factor of 3-5 times longer at a given value of sSFR due to their significantly larger gas fractions.