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Date and Time of the Query: 2019-06-19 T04:59:59 PDT
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For refcode 2013AJ....146...19L:
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2013AJ....146...19L Molecular Gas and Star Formation in nearby Disk Galaxies Leroy, Adam K.; Walter, Fabian; Sandstrom, Karin; Schruba, Andreas; Munoz-Mateos, Juan-Carlos; Bigiel, Frank; Bolatto, Alberto; Brinks, Elias; de Blok, W. J. G.; Meidt, Sharon; Rix, Hans-Walter; Rosolowsky, Erik; Schinnerer, Eva; Schuster, Karl-Friedrich; Usero, Antonio Abstract. We compare molecular gas traced by ^12^CO (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 Sigma_mol_ and Sigma_SFR_ but also find important second-order systematic variations in the apparent molecular gas depletion time, {tau}_dep^mol = {SIGMA}_mol / {SIGMA}_SFR. 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 {alpha}_CO_ equivalent to the Milky Way value, our data yield a molecular gas depletion time, {tau}_dep^mol={SIGMA}_mol/{SIGMA}_SFR ~ 2.2 Gyr with 0.3 dex 1sigma 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, {SIGMA}_SFR is proportional to {SIGMA}_mol^N. 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 {tau}_dep^mol and other local and global quantities. The strongest of these are a decreased {tau}_dep^mol in low-mass, low-metallicity galaxies and a correlation of the kpc-scale {tau}_dep^mol with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H_2_ conversion factor ({alpha}_CO_) 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 observed {tau}_dep^mol trends. After applying a D/G-dependent {alpha}_CO_, some weak correlations between {tau}_dep^mol and local conditions persist. In particular, we observe lower {tau}_dep^mol 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 tau_dep_ does not appear bimodal in galaxy centers, tau_dep_ does appear multivalued at fixed Sigma_H2_, supporting the idea of "disk" and "starburst" modes driven by other environmental parameters. Key words: evolution, galaxies: ISM, galaxies: spiral, ISM: molecules, stars: formation
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