期刊
ASTROPHYSICAL JOURNAL
卷 764, 期 2, 页码 -出版社
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/764/2/117
关键词
galaxies: abundances; galaxies: ISM; galaxies: spiral; ISM: clouds; ISM: molecules
资金
- Norman Hackerman Advanced Research Program (NHARP) [ARP-03658-0234-2009]
- Sigma Xi
- Scientific Research Society
- NSF [AST 1109116, AST-0838178, AST-0955836]
- Research Corporation for Science Advancement
- PAPIIT [IA-100212]
- Gordon and Betty Moore Foundation
- Eileen and Kenneth Norris Foundation
- Caltech Associates
- state of California
- state of Illinois
- state of Maryland
- CARMA partner universities
- National Aeronautics and Space Administration
- Direct For Mathematical & Physical Scien
- Division Of Astronomical Sciences [0955836, 1139998] Funding Source: National Science Foundation
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [1140063, 1109116] Funding Source: National Science Foundation
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.
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