Dense Gas in the Outer Spiral Arm of M51

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 (∼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 + (1–0), and HNC(1–0) emission lines using the Northern Extended Millimeter Array interferometer. The HCN and HCO + 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 + 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 + lines by a factor of 3, on average. However, the and 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 – and – relations in the outer disk GMAs are consistent with the proportionality between the 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.