Dense Gas in the Outer Spiral Arm of M51

Hao Chen; Jonathan Braine; Yu Gao; Jin Koda; Qiusheng Gu

doi:10.3847/1538-4357/836/1/101

paper

Dense Gas in the Outer Spiral Arm of M51

arXiv:1612.00459 · doi:10.3847/1538-4357/836/1/101

Abstract

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 ( $\sim 4"$ , $\sim 150$ 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 $^{+}$ (1-0) and HNC(1-0) emission lines using the Northern Extended Millimeter Array (NOEMA) interferometer. The HCN and HCO $^{+}$ lines were detected in 6 giant molecular associations (GMAs) while HNC emission was only detected in the two brightest GMAs. One of the GMAs hosts a powerful HII region and HCN is stronger than HCO $^{+}$ there. Comparing with observations of GMAs in the disks of M31 and M33 at similar angular resolution ( $\sim 100$ pc), we find that GMAs in the outer disk of M51 are brighter in both HCN and HCO $^{+}$ lines by a factor of 3 on average. However, the $I_{H C N} / I_{C O}$ and $I_{H C O^{+}} / I_{C O}$ ratios are similar to the ratios in nearby galactic disks and the Galactic plane. Using the Herschel 70 $μ$ m data to trace the total IR luminosity at the resolution of the GMAs, we find that both the L $_{I R}$ -L $_{H C N}$ and L $_{I R}$ -L $_{H C O^{+}}$ relations in the outer disk GMAs are consistent with the proportionality between the L $_{I R}$ and the dense gas mass established globally in galaxies within the scatter. The IR/HCN and IR/HCO $^{+}$ ratios of the GMAs vary by a factor of 3, probably depending on whether massive stars are forming or not.

arXiv comment8 pages, 5 figures, accepted by ApJ

Dense Gas in the Outer Spiral Arm of M51

Abstract

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