Dense Gas in the Outer Spiral Arm of M51

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 μm 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.