Dense Gas in the Outer Spiral Arm of M51
Hao Chen, Jonathan Braine, Yu Gao, Jin Koda, Qiusheng Gu

TL;DR
This study maps dense gas tracers in the outer spiral arm of M51 at high resolution, revealing brighter emission compared to other galactic disks and confirming the IR-dense gas relation at sub-galactic scales.
Contribution
First high-resolution mapping of dense gas tracers in the outer disk of M51, demonstrating the IR-dense gas relation at ~150 pc scale.
Findings
GMAs in M51's outer disk are brighter in HCN and HCO+ than in M31 and M33.
Ratios of dense gas tracers to CO are similar to those in galactic disks.
IR luminosity correlates with dense gas tracers, consistent with global galaxy relations.
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 (, 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…
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