Modeling the Mass Distribution and Gravitational Potential of Nearby Disk Galaxies: Implications for the ISM Dynamical Equilibrium

TL;DR

This study models the mass distribution and gravitational potential of 17 nearby disk galaxies to understand the ISM's vertical equilibrium, revealing how stellar, gas, and dark matter components influence gas scale height and weight.

Contribution

It provides a detailed joint modeling approach combining stellar, gas, and dark matter mass profiles to analyze the gravitational potential and ISM equilibrium in nearby galaxies.

Findings

Gas scale height increases with radius, reaching over 500 pc.

ISM weight is mainly due to stellar gravity at small radii.

Results align with observations and theoretical predictions of star formation.

Abstract

We characterize stellar, gas, and dark matter mass distributions for 17 nearby massive disk galaxies from the PHANGS sample. This allows us to compute the gravitational potential that vertically confines the interstellar gas and determines its equilibrium scale height and weight. We first combine dynamical mass constraints from existing CO and HI rotation curves together with stellar and gas mass estimates from near-infrared, CO, and HI data. These estimates incorporate current best practices in modeling stellar mass-to-light ratios and CO-to-H2 conversion factor variations. Then, we fit joint stellar--gas--dark matter mass models to the rotation curves, adopting the classic maximal disk assumption to account for remaining zero-point uncertainties on the stellar mass-to-light ratio. After obtaining three-component radial mass profiles, we calculate the vertical equilibrium gas scale height and ISM weight in the combined gravitational potential. We find the gas scale height $H_\text{gas}$ increases from ${\lesssim}100$pc in the inner disks to ${>}500$pc at large radii, consistent with observations of our Galaxy and other edge-on galaxies. The gas weight is dominated by stellar gravity at small radii, but the gas and dark matter gravity often become important beyond 3-6 times the stellar disk radial scale length. Both our gas scale height and weight estimates are dependent on the treatment of stellar disk scale height $H_\star$, with $H_\text{gas}$ varying by 30-40% when $H_\star$ varies by a factor of 3. The relationship between our refined ISM weight estimates and local star formation surface density generally agrees with previous observations and predictions from theory and simulations.