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    The Virus-P Exploration Of Nearby Galaxies (Venga): The X-Co Gradient In NGC 628

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    Date
    2013-02
    Author
    Blanc, Guillermo A.
    Schruba, Andreas
    Evans, Neal J.
    Jogee, Shardha
    Bolatto, Alberto
    Leroy, Adam. K.
    Song, Mimi
    van den Bosch, Remco C. E.
    Drory, Niv
    Fabricius, Maximilian
    Fisher, David
    Gebhardt, Karl
    Heiderman, Amanda
    Marinova, Irina
    Vogel, Stuart
    Weinzirl, Tim
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    Abstract
    We measure the radial profile of the (CO)-C-12(1-0) to H-2 conversion factor (X-CO) in NGC 628. The H alpha emission from the VENGA integral field spectroscopy is used to map the star formation rate (SFR) surface density (Sigma(SFR)). We estimate the molecular gas surface density (Sigma(H2)) from Sigma(SFR) by inverting the molecular star formation law (SFL), and compare it to the CO intensity to measure X-CO. We study the impact of systematic uncertainties by changing the slope of the SFL, using different SFR tracers (H alpha versus far-UVplus 24 mu m), and CO maps from different telescopes (single-dish and interferometers). The observed X-CO profile is robust against these systematics, drops by a factor of two from R similar to 7 kpc to the center of the galaxy, and is well fit by a gradient Delta log(X-CO) = 0.06 +/- 0.02 dex kpc(-1). We study how changes in X-CO follow changes in metallicity, gas density, and ionization parameter. Theoretical models show that the gradient in X-CO can be explained by a combination of decreasing metallicity, and decreasing Sigma(H2) with radius. Photoelectric heating from the local UV radiation field appears to contribute to the decrease of X-CO in higher density regions. Our results show that galactic environment plays an important role at setting the physical conditions in star-forming regions, in particular the chemistry of carbon in molecular complexes, and the radiative transfer of CO emission. We caution against adopting a single X-CO value when large changes in gas surface density or metallicity are present.
    Department
    Astronomy
    Subject
    galaxies: abundances
    galaxies: ism
    galaxies: spiral
    ism: clouds
    ism:
    molecules
    star-formation law
    co-to-h-2 conversion factor
    luminous infrared
    galaxies
    molecular interstellar-medium
    integral field spectroscopy
    hubble-space-telescope
    gamma-ray emission
    spiral galaxies
    forming
    galaxies
    schmidt law
    astronomy & astrophysics
    URI
    http://hdl.handle.net/2152/34627
    Citation
    Blanc, Guillermo A., Andreas Schruba, Neal J. Evans, Shardha Jogee, Alberto Bolatto, Adam K. Leroy, Mimi Song et al. "The VIRUS-P exploration of nearby galaxies (VENGA): the X CO gradient in NGC 628." The Astrophysical Journal, Vol. 764, No. 2 (Feb., 2013): 117.
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