Molecular Gas and Star Formation in Nearby Disk Galaxies
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 Σ_(mol) and Σ_(SFR) but also find important second-order systematic variations in the apparent molecular gas depletion time, τ_(dep)^(mol) = ∑_(mol)/∑_(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 α_(CO) equivalent to the Milky Way value, our data yield a molecular gas depletion time, τ_(dep)^(mol)= ∑_(mol)∑_(SFR) ≈ 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, ∑_(SFR) ∝ ∑_(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 τ_(dep)^(mol) and other local and global quantities. The strongest of these are a decreased τ_(dep)^(mol) in low-mass, low-metallicity galaxies and a correlation of the kpc-scale τ_(dep)^(mol) with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H_2 conversion factor (α_(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 τ_(dep)^(mol) trends. After applying a D/G-dependent α_(CO), some weak correlations between τ_(dep)^(mol) and local conditions persist. In particular, we observe lower τ_(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 τ_(dep) does not appear bimodal in galaxy centers, τ_(dep) does appear multivalued at fixed Σ_(H2), supporting the idea of "disk" and "starburst" modes driven by other environmental parameters.
Additional Information
© 2013 American Astronomical Society. Received 2012 April 13; accepted 2012 December 24; published 2013 June 17. We thank the referee for a constructive report that led to significant improvements in this paper. We also thank Robert Feldmann, Scott Schnee, Guillermo Blanc, Harvey Liszt, and DavidWhelan for useful discussions and feedback. We thank the GALEX NGS, SINGS, LVL, and COLDGASS teams for making their outstanding data sets available. We thank staff of the IRAM 30 m for their assistance carrying out the HERACLES survey. We thank Deidre Hunter for sharing her Hα image of NGC4214. F.B.,A.K.L., and F.W. gratefully acknowledge the Aspen Center for Physics, where part of this work was carried out. Support for A.K.L. for part of this project was provided by NASA through Hubble Fellowship grant HST-HF-51258.01-A awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., for NASA, under contract NAS 5-26555. K.S. is supported by a Marie Curie International Incoming Fellowship. J.C.M.M. acknowledges financial support from NASA JPL/Spitzer grant RSA 1374189 provided for the S4G project. A.B. wishes to acknowledge partial support from grants NSF AST-0838178, NSF AST-0955836, as well as a Cottrell Scholar award from the Research Corporation for Science Advancement. We have made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. We also acknowledge use of the Lyon Extragalactic Database (LEDA) and NASA's Astrophysics Data System (ADS). The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc.Attached Files
Published - 1538-3881_146_2_19.pdf
Submitted - 1301.2328v1.pdf
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Additional details
- Eprint ID
- 41515
- Resolver ID
- CaltechAUTHORS:20130924-150103154
- HST-HF-51258.01-A
- NASA Hubble Fellowship
- NAS 5-26555
- NASA
- Marie Curie Fellowship
- RSA 1374189
- NASA/JPL/Spitzer
- AST-0838178
- NSF
- AST-0955836
- NSF
- Cottrell Scholar of Research Corporation
- NASA/JPL/Caltech
- Associated Universities, Inc.
- Created
-
2013-09-24Created from EPrint's datestamp field
- Updated
-
2021-11-10Created from EPrint's last_modified field
- Caltech groups
- Infrared Processing and Analysis Center (IPAC)