Astron. J., 146, 19 (2013/August-0)
Molecular gas and star formation in nearby disk galaxies.
LEROY A.K., WALTER F., SANDSTROM K., SCHRUBA A., MUNOZ-MATEOS J.-C., BIGIEL F., BOLATTO A., BRINKS E., DE BLOK W.J.G., MEIDT S., RIX H.-W., ROSOLOWSKY E., SCHINNERER E., SCHUSTER K.-F. and USERO A.
Abstract (from CDS):
We compare molecular gas traced by 12CO (2-1) maps from the HERACLES survey, with tracers of the recent star formation rate (SFR) across 30 nearby disk galaxies. We demonstrate a first-order linear correspondence between Σmol and ΣSFRbut also find important second-order systematic variations in the apparent molecular gas depletion time, τdepmol = Σmol/ΣSFR. At the 1 kpc common resolution of HERACLES, CO emission correlates closely with many tracers of the recent SFR. Weighting each line of sight equally, using a fixed αCO equivalent to the Milky Way value, our data yield a molecular gas depletion time, τdepmol = Σmol/ΣSFR ≈ 2.2 Gyr with 0.3 dex 1σ scatter, in very good agreement with recent literature data. We apply a forward-modeling approach to constrain the power-law index, N, that relates the SFR surface density and the molecular gas surface density, ΣSFR ∝ ΣmolN. We find N = 1±0.15 for our full data set with some scatter from galaxy to galaxy. This also agrees with recent work, but we caution that a power-law treatment oversimplifies the topic given that we observe correlations between τdepmol and other local and global quantities. The strongest of these are a decreased τdepmol in low-mass, low-metallicity galaxies and a correlation of the kpc-scale τdepmol with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H2 conversion factor (αCO) that depends on dust shielding, and thus D/G, in the theoretically expected way. This is not a unique interpretation, but external evidence of conversion factor variations makes this the most conservative explanation of the strongest observedτdepmol trends. After applying a D/G-dependent αCO, some weak correlations between τdepmol and local conditions persist. In particular, we observe lower τdepmol and enhanced CO excitation associated with nuclear gas concentrations in a subset of our targets. These appear to reflect real enhancements in the rate of star formation per unit gas, and although the distribution of τdep does not appear bimodal in galaxy centers, τdepdoes appear multivalued at fixed ΣH2, supporting the idea of "disk" and "starburst" modes driven by other environmental parameters.
evolution - galaxies: ISM - galaxies: spiral - ISM: molecules - stars: formation
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