# Dense Gas in the Outer Spiral Arm of M51

1 FORMATION STELLAIRE 2017
LAB - Laboratoire d'Astrophysique de Bordeaux [Pessac]
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 $I_{HCN}/I_{CO}$ and $I_{HCO^+}/I_{CO}$ ratios are similar to the ratios in nearby galactic disks and the Galactic plane. Using the Herschel 70 $\mu$m data to trace the total IR luminosity at the resolution of the GMAs, we find that both the L$_{IR}$-L$_{HCN}$ and L$_{IR}$-L$_{HCO^+}$ relations in the outer disk GMAs are consistent with the proportionality between the L$_{IR}$ 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.
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Article dans une revue
Astrophysical Journal, American Astronomical Society, 2017, 836 (1), pp.id. 101. 〈10.3847/1538-4357/836/1/101〉

https://hal.archives-ouvertes.fr/hal-01409623
Contributeur : Marie-Paule Pomies <>
Soumis le : mardi 6 décembre 2016 - 09:27:05
Dernière modification le : vendredi 23 novembre 2018 - 01:11:20

### Citation

Hao Chen, J. Braine, Yu Gao, Jin Koda, Qiusheng Gu. Dense Gas in the Outer Spiral Arm of M51. Astrophysical Journal, American Astronomical Society, 2017, 836 (1), pp.id. 101. 〈10.3847/1538-4357/836/1/101〉. 〈hal-01409623〉

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