Dynamical Equilibrium in the Molecular ISM in 28 Nearby Star-Forming Galaxies

Abstract

We compare the observed turbulent pressure in molecular gas, $P_{\mathrm{turb}}$, to the required pressure for the interstellar gas to stay in equilibrium in the gravitational potential of a galaxy, $P_{\mathrm{DE}}$. To do this, we combine arcsecond resolution CO data from PHANGS-ALMA with multi-wavelength data that traces the atomic gas, stellar structure, and star formation rate (SFR) for 28 nearby star-forming galaxies. We find that $P_{\mathrm{turb}}$ correlates with, but almost always exceeds the estimated $P_{\mathrm{DE}}$ on kiloparsec scales. This indicates that the molecular gas is over-pressurized relative to the large-scale environment. We show that this over-pressurization can be explained by the clumpy nature of molecular gas; a revised estimate of $P_{\mathrm{DE}}$ on cloud scales, which accounts for molecular gas self-gravity, external gravity, and ambient pressure, agrees well with the observed $P_{\mathrm{turb}}$ in galaxy disks. We also find that molecular gas with cloud-scale $P_{\mathrm{turb}}\approx P_{\mathrm{DE}}\gtrsim 10^5{k}_{\mathrm{B}}\mathrm{K}\mathrm{c}{\mathrm{m}}^{-3}$ in our sample is more likely to be self-gravitating, whereas gas at lower pressure appears more influenced by ambient pressure and/or external gravity. Furthermore, we show that the ratio between $P_{\mathrm{turb}}$ and the observed SFR surface density, $\hat{I}{\pounds }_{\mathrm{SFR}}$, is compatible with stellar feedback-driven momentum injection in most cases, while a subset of the regions may show evidence of turbulence driven by additional sources. The correlation between $\hat{I}{\pounds }_{\mathrm{SFR}}$ and kpc-scale $P_{\mathrm{DE}}$ in galaxy disks is consistent with the expectation from self-regulated star formation models. Finally, we confirm the empirical correlation between molecular-to-atomic gas ratio and kpc-scale $P_{\mathrm{DE}}$ reported in previous works.

28 pages + 3 appendices, ApJ in press. See https://www.youtube.com/watch?v=qxkd-RXB0Ek for a short video describing the main results. Data tables available at https://www.canfar.net/storage/list/phangs/RELEASES/Sun_etal_2020 prior to publication