Dense Gas in the Outer Spiral Arm of M51

Journal Article · Fri Feb 10 04:00:00 EST 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/836/1/101· OSTI ID:22869381

Chen, Hao; Gu, Qiusheng ^[1]; Braine, Jonathan ^[2]; Gao, Yu ^[3]; Koda, Jin ^[4]

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1. School of Astronomy and Space Science, Nanjing University, Nanjing 210093 (China)
2. Univ. Bordeaux, Laboratoire d’Astrophysique de Bordeaux, F-33270, Floirac (France)
3. Purple Mountain Observatory, Chinese Academy of Sciences, 2 West Beijing Road, Nanjing 210008 (China)
4. Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800 (United States)

There is a linear relation between the mass of dense gas traced by the HCN(1–0) luminosity and the star formation rate (SFR) traced by the far-infrared luminosity. Recent observations of galactic disks have shown some systematic variations. In order to explore the SFR–dense gas link at high resolution (∼4″, ∼150 pc) in the outer disk of an external galaxy, we have mapped a region about 5 kpc from the center along the northern spiral arm of M51 in the HCN(1–0), HCO{sup +}(1–0), and HNC(1–0) emission lines using the Northern Extended Millimeter Array interferometer. The HCN and HCO{sup +} lines were detected in six giant molecular associations (GMAs), while HNC emission was only detected in the two brightest GMAs. One of the GMAs hosts a powerful H ii region, and HCN is stronger than HCO{sup +} there. Comparing observations of GMAs in the disks of M31 and M33 at similar angular resolution (∼100 pc), we find that GMAs in the outer disk of M51 are brighter in both the HCN and the HCO{sup +} lines by a factor of 3, on average. However, the I{sub HCN}/I{sub CO} and I{sub HCO{sup +}}/I{sub CO} ratios are similar to the ratios in nearby galactic disks and the Galactic plane. Using the Herschel 70 μm data to trace the total IR luminosity at the resolution of the GMAs, we find that both the L{sub IR}–L{sub HCN} and L{sub IR}–L{sub HCO{sup +}} relations in the outer disk GMAs are consistent with the proportionality between the L{sub IR} and the dense gas mass established globally in galaxies within the scatter. The IR/HCN and IR/HCO{sup +} ratios of the GMAs vary by a factor of 3, probably depending on whether massive stars are forming.

• OSTI ID: 22869381
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 836; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
COMPARATIVE EVALUATIONS
EMISSION
GALAXIES
INTERFEROMETERS
LUMINOSITY
MASS
MOLECULES
STARS