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Published August 20, 2010 | Published
Journal Article Open

Multi-transition Study of M51'S Molecular Gas Spiral Arms


Two selected regions in the molecular gas spiral arms in M51 were mapped with the Owens Valley Radio Observatory (OVRO) mm-interferometer in the ^(12)CO(2-1), ^(13)CO(1-0), C^(18)O(1-0), HCN(1-0), and HCO+(1-0) emission lines. The CO data have been combined with the ^(12)CO(1-0) data from Aalto et al. covering the central 3.5 kpc to study the physical properties of the molecular gas. All CO data cubes were short spacing corrected using IRAM 30 m (^(12)CO(1-0): NRO 45 m) single-dish data. A large velocity gradient analysis finds that the giant molecular clouds (GMCs) are similar to Galactic GMCs when studied at 180 pc (120 pc) resolution with an average kinetic temperature of T_(kin) = 20(16) K and H_2 density of n(H_2) = 120(240) cm^(–3) when assuming virialized clouds (a constant velocity gradient dv/dr). The associated conversion factor between H_2 mass and CO luminosity is close to the Galactic value for most regions analyzed. Our findings suggest that the GMC population in the spiral arms of M51 is similar to those of the Milky Way and therefore the strong star formation occurring in the spiral arms has no strong impact on the molecular gas in the spiral arms. Extinction inferred from the derived H_2 column density is very high (AV about 15-30 mag), about a factor of 5-10 higher than the average value derived toward H II regions. Thus, a significant fraction of the ongoing star formation could be hidden inside the dust lanes of the spiral arms. A comparison of MIPS 24 μm and Hα data, however, suggests that this is not the case and most of the GMCs studied here are not (yet) forming stars. We also present low (4".5) resolution OVRO maps of the HCN(1-0) and HCO+(1-0) emission at the location of the brightest ^(12)CO(1-0) peak.

Additional Information

© 2010 American Astronomical Society. Received 2010 February 5; accepted 2010 June 14; published 2010 August 2. Based on observations carried out with the IRAM 30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). We thank K. Schuster and C. Kramer for making available to us the HERA 12CO(2–1) data cube and G. Dumas for the HiRes version of the MIPS 24 μm image as well as her help with the SFRs. We are thankful for the public access to the 'Nobeyama CO Atlas of Nearby Spiral Galaxies'. The comments and suggestions by the anonymous refereewere highly appreciated and helped to improve the paper. The OwensValleyRadioObservatorywas funded in part byNSF grant AST 99-81546. Facilities: OVRO:40m, IRAM:30m

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