2025A&A...702A..66S

Tracing dense molecular gas, the fuel for star formation, is essential for understanding the evolution of molecular clouds and star-formation processes. We compared 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 5 × 7 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 closely across the environments and physical conditions, such as the 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 the central 2 kpc are removed, the ratio HCN/CO, which is commonly used to trace average cloud density, is only weakly dependent on molecular gas mass surface density. While ratios of other dense gas lines to CO show a steeper dependence on the surface density of molecular gas, this relation is still shallow in comparison to other nearby star-forming disk galaxies. One reason might be physical conditions in M51, which are different from other normal star-forming galaxies. Increased ionization rates, increased dynamical equilibrium pressure in the central few kiloparsecs, 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.