Dense Gas in Star Forming Galaxies traced by HCN and CN

We use archival data from the Atacama Large Millimeter Array to assess the relationship between dense gas, star formation, and gas dynamics in 10 nearby (ultra-)luminous infrared galaxies and spiral galaxy centers on sub-kiloparsec scales. The data from this same sample of galaxies shows a nearly constant CN/HCN line intensity ratio. This result conflicts with simple models of photon dominated regions, which predict that HCN emission traces shielded regions with high column densities while CN traces dense gas exposed to high ultraviolet radiation fields. We use gravo-turbulent models of star formation to measure the dense gas traced by emission from HCN. We find that the HCN/CO ratio in galaxies is a good tracer of gas above densities log(n) = 4.5 but that this ratio does not universally track the ratio of gravitationally bound, star-forming gas. We also find that the depletion time of the dense gas increases with the dense gas fraction.