A Constant Molecular Gas Depletion Time in Nearby Disk Galaxies

Abstract

We combine new sensitive, wide-field CO data from the HERACLES survey with ultraviolet and infrared data from GALEX and Spitzer to compare the surface densities of H 2 , Σ H2 , and the recent star formation rate, Σ SFR , over many thousands of positions in 30 nearby disk galaxies. We more than quadruple the size of the galaxy sample compared to previous work and include targets with a wide range of galaxy properties. Even though the disk galaxies in this study span a wide range of properties, we find a strong, and approximately linear correlation between Σ SFR and Σ H2 at our common resolution of 1 kpc. This implies a roughly constant median H 2 consumption time, τ H2 Dep = Σ H2 /Σ SFR , of ∼ 2.35 Gyr (including heavy elements) across our sample. At 1 kpc resolution, there is only a weak correlation between Σ H2 and τ H2 Dep over the range Σ H2 ≈ 5-100 M ⊙ pc −2 , which is probed by our data. We compile a broad set of literature measurements that have been obtained using a variety of star formation tracers, sampling schemes and physical scales and show that overall, these data yield almost exactly the same results, although with more scatter. We interpret these results as strong, albeit indirect evidence that star formation proceeds in a uniform way in giant molecular clouds in the disks of spiral galaxies.

CO-dark molecular gas at high redshift: very large H2 content and high pressure in a low-metallicity damped Lyman alpha system

“…This value is comparable with the values of Σ SFR for the measured Σ H2 in local galaxies with kpc-resolution emission observations (Bigiel et al 2011). This supports the idea that the H 2 -bearing system in J 0843+0221 probes a star-forming medium, similar to the GMCs in the local Universe.…”

An Introduction to Gas Accretion onto Galaxies

“…The high redshift observations are limited to those galaxies with accessible gas and star formation tracers (Shapley, 2011), but this result is found for a wide variety of tracers. At lower redshift, where there is a more complete census of the gas content and star formation rate of galaxies, the measured gas depletion times are longer, but still typically less than a few Gyrs (Kennicutt & Evans, 2012;Bigiel et al, 2011;Leroy et al, 2013;Schiminovich et al, 2010). The primary exception is the gas-rich dwarf galaxies that can have depletion times closer to a Hubble time (van Zee, 2001;Hunt et al, 2015;Huang et al, 2012).…”

Evolution of star formation and gas

“…The most recent compilations have been done by Bigiel et al (2011) and Krumholz et al (2012). The former advocate a simple linear dependence on molecular gas surface density; the latter propose a 'volumetric' laẇ Σ * = f H 2 ff Σ/τ ff where the factors entering are the H 2 mass fraction, the efficiency of star formation per free-fall time, the total gas surface density and the free-fall time.…”