Predicting HCN, HCO + , multi-transition CO, and dust emission of star-forming galaxies: Constraining the properties of resolved gas and dust disks of local spiral galaxies

TL;DR

This paper develops a detailed analytical model of the interstellar medium in local spiral galaxies, predicting emission lines and gas properties, and compares these predictions with observations to understand galaxy stability and star formation.

Contribution

It introduces a multi-phase, turbulent disk model that incorporates galactic scaling relations and compares predictions with extensive observational data, advancing understanding of gas dynamics and emission in spiral galaxies.

Findings

Model accurately predicts IR, HI, CO, HCN, and HCO+ emission profiles.

HCN and HCO+ detectable in low-density gas (~1000 cm$^{-3}$).

Conversion factors align with literature values.

Abstract

The ISM is a turbulent, multi-phase, and multi-scale medium following scaling relations. Analytical models of galactic gaseous disks need to take into account the multi-scale and multi-phase nature of the interstellar medium. They can be described as clumpy star-forming accretion disks in vertical hydrostatic equilibrium, with the mid-plane pressure balancing the gravity of the gaseous and stellar disk. ISM turbulence is taken into account by applying Galactic scaling relations to the cold atomic and molecular gas phases. Turbulence is maintained through energy injection by supernovae. With the determination of the gas mass fraction at a given spatial scale, the equilibrium gas temperature between turbulent heating and line cooling, the molecular abundances, and the molecular line emission can be calculated. The resulting model radial profiles of IR, H{\sc i}, CO, HCN, and HCO$^+$ emission are compared to THINGS, HERACLES, EMPIRE, SINGS, and GALEX observations of 17 local spiral galaxies. The Toomre parameter, which measures the stability against star formation (cloud collapse), exceeds unity in the inner disk of a significant number of galaxies. In two galaxies it also exceeds unity in the outer disk. Therefore, in spiral galaxies $Q_{\rm tot}=1$ is not ubiquitous. The model gas velocity dispersion is consistent with the observed H{\sc i} velocity dispersion where available. Within our model HCN and HCO$^+$ is already detectable in relatively low-density gas ($\sim 1000$~cm$^{-3}$). CO and HCN conversion factors and molecular gas depletion time were derived. Both conversion factors are consistent with values found in the literature. Whereas in the massive galaxies the viscous timescale greatly exceeds the star formation timescale, the viscous timescale is smaller than the star formation timescale within $\rm{R}~\sim~2~\rm{R}_{\rm d}$, the disk scale length, in the low-mass galaxies.