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

TLDR

The study investigates the relationship between molecular gas and star formation in 222 galaxies from the COLD GASS survey using CO(1–0) measurements from the IRAM 30‑m telescope. The authors analyze 222 galaxies (0.025 < z < 0.05, 10.0 < log M★ < 11.5) with CO(1–0) data from IRAM, supplemented by Arecibo HI, SDSS, and GALEX photometry, calibrate star‑formation rates using a UV–far‑IR reference sample, and fit a single Mgas–sSFR relation that also describes starburst galaxies. The mean molecular gas depletion time is 1 Gyr, rising sixfold from ~0.5 Gyr at 10^10 M⊙ to ~3 Gyr at a few ×10^11 M⊙, while atomic depletion remains ~3 Gyr; thus high‑mass galaxies have comparable molecular and atomic depletion times, low‑mass galaxies consume molecular gas more rapidly, and depletion time depends strongly on stellar mass and sSFR, with high‑z normal galaxies showing 3–5× longer depletion times at a given sSFR due to larger gas fractions.

Abstract

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 < log M★/M⊙ < 11.5. The IRAM measurements are complemented by deep Arecibo H i 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 [] for all the galaxies in our sample is 1 Gyr; however, increases by a factor of 6 from a value of ∼0.5 Gyr for galaxies with stellar masses of ∼1010 M⊙ to ∼3 Gyr for galaxies with masses of a few ×1011 M⊙. 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 are on the stellar mass of the galaxy [parametrized as ], and on the specific star formation rate (sSFR). A single 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 yr. Normal galaxies at z = 1–2 are displaced with respect to the local galaxy population in the 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.