Does the HCN/CO Ratio Trace the Star-forming Fraction of Gas? I. A Comparison with Analytical Models of Star Formation

Abstract We use archival ALMA observations of the HCN and CO J = 1–0 transitions, in addition to radio continuum at 93 GHz, assess relation between dense gas, star formation, gas dynamics 10 nearby (ultra)luminous IR galaxies (U)LIRGs late-type galaxy centers. frame our results context turbulent gravoturbulent models formation whether HCN/CO ratio tracks gravitationally bound star-forming molecular clouds ( f grav ) subkiloparsec scales galaxies. confirm that is a tracer above n SF ? 4.5 cm ?3 , but variations do not universally track . find strong evidence for varying density-threshold models, which are able reproduce trends observed t dep ? ff fixed-threshold reproduce. Composite lognormal power-law outperform pure reproducing trends, even when fixed slope used. The ability composite better properties provides additional indirect efficiency per freefall time proportional fraction gas.