Does the HCN/CO ratio trace the star-forming fraction of gas? I. A comparison with analytical models of star formation

TL;DR

This study uses ALMA observations to evaluate if the HCN/CO ratio effectively traces star-forming gas in galaxies, finding it correlates with dense gas but not universally with gravitationally-bound gas, supporting variable threshold models.

Contribution

It demonstrates that composite star formation models better explain observed gas properties, indicating star formation efficiency relates to gravitationally-bound gas fraction.

Findings

HCN/CO ratio traces dense gas above a certain density threshold

Varying star formation density thresholds reproduce observed trends

Composite models outperform pure lognormal models in matching data

Abstract

We use archival ALMA observations of the HCN and CO $J=1-0$ transitions, in addition to the radio continuum at 93 GHz, to assess the relationship between dense gas, star formation, and gas dynamics in ten, nearby (U)LIRGs and late-type galaxy centers. We frame our results in the context of turbulent and gravoturbulent models of star formation to assess if the HCN/CO ratio tracks the gravitationally-bound, star-forming gas in molecular clouds ($f_\mathrm{grav}$) at sub-kpc scales in nearby galaxies. We confirm that the HCN/CO ratio is a tracer of gas above $n_\mathrm{SF}\approx10^{4.5}$ cm$^{-3}$, but the sub-kpc variations in HCN/CO do not universally track $f_\mathrm{grav}$. We find strong evidence for the use of varying star formation density threshold models, which are able to reproduce trends observed in $t_\mathrm{dep}$ and $\epsilon_\mathrm{ff}$ that fixed threshold models cannot. Composite lognormal and powerlaw models outperform pure lognormal models in reproducing the observed trends, even when using a fixed powerlaw slope. The ability of the composite models to better reproduce star formation properties of the gas provides additional indirect evidence that the star formation efficiency per free-fall time is proportional to the fraction of gravitationally-bound gas.