The SWAN view of dense gas in the Whirlpool -- A cloud-scale comparison of N2H+, HCO+, HNC and HCN emission in M51

Abstract

Tracing dense molecular gas, the fuel for star formation, is essential for the understanding of the evolution of molecular clouds and star formation processes. We compare the emission of HCN(1-0), HNC(1-0) and HCO+(1-0) with the emission of N2H+(1-0) at cloud-scales (125 pc) across the central 5x7 kpc of the Whirlpool galaxy, M51a, from "Surveying the Whirlpool galaxy at Arcseconds with NOEMA" (SWAN). We find that the integrated intensities of HCN, HNC and HCO+ are more steeply correlated with N2H+ emission compared to the bulk molecular gas tracer CO, and we find variations in this relation across the center, molecular ring, northern and southern disk of M51. Compared to HCN and HNC emission, the HCO+ emission follows the N2H+ emission more similarly across the environments and physical conditions such as surface densities of molecular gas, stellar mass, star-formation rate, dynamical equilibrium pressure and radius. Under the assumption that N2H+ is a fair tracer of dense gas at these scales, this makes HCO+ a more favorable dense gas tracer than HCN within the inner disk of M51. In all environments within our field of view, even when removing the central 2 kpc, HCN/CO, commonly used to trace average cloud density, is only weakly depending on molecular gas mass surface density. While ratios of other dense gas lines to CO show a steeper dependency on the surface density of molecular gas, it is still shallow in comparison to other nearby star-forming disk galaxies. The reasons might be physical conditions in M51 that are different from other normal star-forming galaxies. Increased ionization rates, increased dynamical equilibrium pressure in the central few kpc and the impact of the dwarf companion galaxy NGC 5195 are proposed mechanisms that might enhance HCN and HNC emission over HCO+ and N2H+ emission at larger-scale environments and cloud scales.