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arXiv · 1301.2328

Molecular Gas and Star Formation in Nearby Disk Galaxies

Adam K. Leroy·Fabian Walter·Karin Sandstrom·Andreas Schruba·Juan-Carlos Munoz-Mateos·Frank Bigiel·Alberto Bolatto·Elias Brinks·W. J. G. de Blok·Sharon Meidt·Hans-Walter Rix·Erik Rosolowsky·Eva Schinnerer·Karl-Friedrich Schuster·Antonio Usero
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Abstract

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 Sig_mol and Sig_SFR but also find important second-order systematic variations in the apparent molecular gas depletion time, t_dep^mol = Sig_mol / Sig_SFR. At our 1 kpc common resolution, CO correlates closely with many tracers of the recent SFR. Weighting each line of sight equally and using a fixed, Milky Way alpha_CO, our data yield a molecular gas depletion time, t_dep^mol=Sig_mol/Sig_SFR ~ 2.2 Gyr with 0.3 dex scatter, in good agreement with 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 and find N=1+/-0.15 for our full data set with some variation from galaxy to galaxy. However, we caution that a power law treatment oversimplifies the topic given that we observe correlations between t_dep^mol and other local and global quantities. The strongest of these are a decreased t_dep^mol in low-mass, low-metallicity galaxies and a correlation of the kpc-scale t_dep^mol with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H2 conversion factor that depends on D/G in the theoretically expected way. This is not a unique interpretation, but external evidence of conversion factor variations makes it a conservative one. After applying a D/G-dependent alpha_CO, some weak correlations between t_dep^mol and local conditions persist. In particular, we observe lower t_dep^mol and enhanced CO excitation associated with some nuclear gas concentrations. These appear to reflect real enhancements in the SFR/H2 and t_dep appears multivalued at fixed Sig_mol, supporting the the idea of "disk" and "starburst" modes driven by environmental factors.

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Adam K. Leroy, Fabian Walter, Karin Sandstrom, Andreas Schruba, Juan-Carlos Munoz-Mateos, Frank Bigiel, Alberto Bolatto, Elias Brinks, W. J. G. de Blok, Sharon Meidt, Hans-Walter Rix, Erik Rosolowsky, Eva Schinnerer, Karl-Friedrich Schuster, Antonio Usero. 2013-01-10. Molecular Gas and Star Formation in Nearby Disk Galaxies. https://doi.org/10.1088/0004-6256%2F146%2F2%2F19

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