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Title: MOLECULAR GAS AND STAR FORMATION IN NEARBY DISK GALAXIES

We compare molecular gas traced by {sup 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}{sub mol} and {Sigma}{sub SFR} but also find important second-order systematic variations in the apparent molecular gas depletion time, {tau}{sub dep}{sup mol}={Sigma}{sub mol}/{Sigma}{sub 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}{sub CO} equivalent to the Milky Way value, our data yield a molecular gas depletion time, {tau}{sub dep}{sup mol}={Sigma}{sub mol}/{Sigma}{sub SFR}{approx}2.2 Gyr with 0.3 dex 1{sigma} 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}{sub SFR}{proportional_to}{Sigma}{sub mol}{sup 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}{sub dep}{sup mol} and other local and global quantities. Themore » strongest of these are a decreased {tau}{sub dep}{sup mol} in low-mass, low-metallicity galaxies and a correlation of the kpc-scale {tau}{sub dep}{sup mol} with dust-to-gas ratio, D/G. These correlations can be explained by a CO-to-H{sub 2} conversion factor ({alpha}{sub 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}{sub dep}{sup mol} trends. After applying a D/G-dependent {alpha}{sub CO}, some weak correlations between {tau}{sub dep}{sup mol} and local conditions persist. In particular, we observe lower {tau}{sub dep}{sup 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}{sub dep} does not appear bimodal in galaxy centers, {tau}{sub dep} does appear multivalued at fixed {Sigma}{sub H2}, supporting the idea of ''disk'' and ''starburst'' modes driven by other environmental parameters.« less
Authors:
;  [1] ; ; ; ; ;  [2] ;  [3] ;  [4] ;  [5] ;  [6] ;  [7] ;  [8] ;  [9] ;  [10]
  1. National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903 (United States)
  2. Max Planck Institute fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg (Germany)
  3. California Institute for Technology, 1200 East California Boulevard, Pasadena, CA 91125 (United States)
  4. Theoretische Astrophysik, Albert-Ueberle-Str. 2, D-69120 Heidelberg (Germany)
  5. Department of Astronomy, University of Maryland, College Park, MD (United States)
  6. Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB (United Kingdom)
  7. Astrophysics, Cosmology and Gravity Centre, Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701 (South Africa)
  8. University of British Columbia, Okanagan Campus, Kelowna, BC (Canada)
  9. IRAM, 300 rue de la Piscine, F-38406 St. Martin d'Heres (France)
  10. Observatorio Astronomico Nacional, C/ Alfonso XII, 3, E-28014 Madrid (Spain)
Publication Date:
OSTI Identifier:
22118719
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astronomical Journal (New York, N.Y. Online); Journal Volume: 146; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ASTRONOMY; ASTROPHYSICS; CARBON MONOXIDE; COMPARATIVE EVALUATIONS; CORRELATIONS; COSMIC DUST; DENSITY; ELEMENT ABUNDANCE; EXCITATION; HYDROGEN; INDEXES; MILKY WAY; MOLECULES; PHOTON EMISSION; STAR EVOLUTION; STARS; VARIATIONS