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Physical Conditions in Molecular Clouds in the Arm and Interarm Regions of M51

Koda, Jin and Scoville, Nick and Hasegawa, Tetsuo and Calzetti, Daniela and Donovan Meyer, Jennifer and Egusa, Fumi and Kennicutt, Robert and Kuno, Nario and Louie, Melissa and Momose, Rieko and Sawada, Tsuyoshi and Sorai, Kazuo and Umei, Michiko (2012) Physical Conditions in Molecular Clouds in the Arm and Interarm Regions of M51. Astrophysical Journal, 761 (1). Art. No. 41. ISSN 0004-637X.

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We report systematic variations in the emission line ratio of the CO J = 2-1 and J = 1-0 transitions (R_(2-1/1-0)) in the grand-design spiral galaxy M51. The R_(2-1/1-0) ratio shows clear evidence for the evolution of molecular gas from the upstream interarm regions into the spiral arms and back into the downstream interarm regions. In the interarm regions, R_(2-1/1-0) is typically <0.7 (and often 0.4-0.6); this is similar to the ratios observed in Galactic giant molecular clouds (GMCs) with low far-infrared luminosities. However, the ratio rises to >0.7 (often 0.8-1.0) in the spiral arms, particularly at the leading (downstream) edge of the molecular arms. These trends are similar to those seen in Galactic GMCs with OB star formation (presumably in the Galactic spiral arms). R_(2-1/1-0) is also high, ~0.8-1.0, in the central region of M51. Analysis of the molecular excitation using a Large Velocity Gradient radiative transfer calculation provides insight into the changes in the physical conditions of molecular gas between the arm and interarm regions: cold and low-density gas (≾ 10 K, ≾ 300 cm^(–3)) is required for the interarm GMCs, but this gas must become warmer and/or denser in the more active star-forming spiral arms. The ratio R_(2-1/1-0) is higher in areas of high 24 μm dust surface brightness (which is an approximate tracer of star formation rate surface density) and high CO(1-0) integrated intensity (i.e., a well-calibrated tracer of total molecular gas surface density). The systematic enhancement of the CO(2-1) line relative to CO(1-0) in luminous star-forming regions suggests that some caution is needed when using CO(2-1) as a tracer of bulk molecular gas mass, especially when galactic structures are resolved.

Item Type:Article
Related URLs:
URLURL TypeDescription
Koda, Jin0000-0002-8762-7863
Scoville, Nick0000-0002-0438-3323
Calzetti, Daniela0000-0002-5189-8004
Donovan Meyer, Jennifer0000-0002-3106-7676
Kennicutt, Robert0000-0001-5448-1821
Additional Information:© 2012 American Astronomical Society. Received 2012 April 20; accepted 2012 October 22; published 2012 November 20. The Nobeyama 45 m telescope is operated by the Nobeyama Radio Observatory, a branch of the National Astronomical Observatory of Japan. We thank Seiichi Sakamoto and Jim Barrett for valuable comments. We also thank the anonymous referee for valuable comments. J.K. acknowledges support from the NSF through grant AST-1211680 and NASA through grant NNX09AF40G, a Hubble Space Telescope grant, and a Herschel Space Observatory grant.
Group:Infrared Processing and Analysis Center (IPAC)
Funding AgencyGrant Number
Hubble Space TelescopeUNSPECIFIED
Herschel Space ObservatoryUNSPECIFIED
Subject Keywords:evolution; galaxies: individual (NGC 5194, M51); ISM: clouds; ISM: molecules
Issue or Number:1
Record Number:CaltechAUTHORS:20130108-095035129
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Official Citation:Physical Conditions in Molecular Clouds in the Arm and Interarm Regions of M51 Jin Koda et al. 2012 ApJ 761 41
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:36230
Deposited By: Ruth Sustaita
Deposited On:08 Jan 2013 18:15
Last Modified:09 Mar 2020 13:18

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