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

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.

Dusty star-forming galaxies at high redshift

“…Davé et al, 2010Davé et al, , 2011Lagos et al, 2011;Popping et al, 2012Popping et al, , 2013, as well as observationally confirmed Saintonge et al, 2013;Tacconi et al, 2013;Santini et al, 2013). This trend is additionally seen in low-redshift galaxies, though Saintonge et al (2011) suggest that the gas fractions of low-z galaxies are more closely correlated with stellar density than stellar mass. We see this quantitatively for high-z SMGs in Figure 46, where we plot the gas fraction for all SMGs with CO (J=1-0) detections (to minimize the relatively large uncertainty in CO excitation; c.f.…”

“…The results from Bigiel et al (2008) and Leroy et al (2008) are denoted by the colored contours in Figure 44. Utilizing a sample of 222 galaxies, Saintonge et al (2011) found an increasing molecular depletion time scale (where the depletion time scale is the inverse of ) with galaxy mass, while a roughly constant atomic gas depletion time scale across their mass range of 10 < logM * /M < 11.5.…”

“…This is to be compared to local galaxies, which show typical gas fractions f gas < 10% (e.g. Saintonge et al, 2011). These results have come from both from CO inferred H 2 gas masses, as well as dust measurements (and associated dust-to-gas ratio assumptions Magdis et al, 2012a).…”

The formation and destruction of molecular clouds and galactic star formation

“…There are many reports that the Kennicutt-Schmidt correlation varies with some properties of galaxies (e.g., Saintonge et al 2011Saintonge et al , 2012Meidt et al 2013;Davis et al 2014). In addition, a simple relation does not fit to the center of our Galaxy (e.g., Longmore et al 2013).…”

SDSS-IV MaNGA-resolved Star Formation and Molecular Gas Properties of Green Valley Galaxies: A First Look with ALMA and MaNGA

“…Right panels: the star formation efficiency as a function of specific star formation rate. The dashed lines show the COLD GASS results based on a sample of galaxies with secure CO detections (see Table 1 of Saintonge et al 2011). The small dots in the upper two panels represent spaxels located in the bulges.…”