The Pressure of an Equilibrium Interstellar Medium in Galactic Disks

TL;DR

This study models the equilibrium gas pressure in galactic disks of various galaxies, revealing how pressure correlates with molecular gas content and highlighting the influence of galactic structure on interstellar medium properties.

Contribution

It provides a self-consistent method to estimate gas pressure in galactic disks considering dark halos and gas self-gravity, improving understanding of interstellar medium equilibrium.

Findings

SMC and M51 have the highest pressure at a given radius.

A positive correlation exists between pressure and molecular gas fraction.

Pressure estimates are less accurate in galaxy bulge regions.

Abstract

Based on an axisymmetric galactic disk model, we estimate the equilibrium gas pressure P/k in the disk plane as a function of the galactocentric distance R for several galaxies (MW, M33, M51, M81, M100, M101, M106, and the SMC). For this purpose, we solve a self-consistent system of equations by taking into account the gas self-gravity and the presence of a dark pseudo-isothermal halo. We assume that the turbulent velocity dispersions of the atomic and molecular gases are fixed and that the velocity dispersion of the old stellar disk corresponds to its marginal stability (except for the Galaxy and the SMC). We also consider a model with a constant disk thickness. Of the listed galaxies, the SMC and M51 have the highest pressure at a given relative radius R/R_25, while M81 has the lowest pressure. The pressure dependence of the relative molecular gas fraction confirms the existence of a positive correlation between these quantities, but it is not so distinct as that obtained previously when the pressure was estimated very roughly. This dependence breaks down for the inner regions of M81 and M106, probably because the gas pressure has been underestimated in the bulge region. We discuss the possible effects of factors other than the pressure affecting the relative content of molecular gas in the galaxies under consideration.