Pressure balance in the multiphase ISM of cosmologically simulated disk galaxies

TL;DR

This study uses cosmological simulations to examine how pressure balance is maintained in the multiphase interstellar medium of disk galaxies, revealing that total pressure correlates with gas weight and star formation rate.

Contribution

It demonstrates that in realistic simulations, the ISM's total pressure balances gravity and that this pressure correlates with star formation activity, advancing understanding of ISM dynamics.

Findings

Total ISM pressure balances the weight of overlying gas.

Different phases are in rough total pressure equilibrium, but not thermal equilibrium.

Total midplane pressure scales linearly with star formation rate surface density.

Abstract

Pressure balance plays a central role in models of the interstellar medium (ISM), but whether and how pressure balance is realized in a realistic multiphase ISM is not yet well understood. We address this question using a set of FIRE-2 cosmological zoom-in simulations of Milky Way-mass disk galaxies, in which a multiphase ISM is self-consistently shaped by gravity, cooling, and stellar feedback. We analyze how gravity determines the vertical pressure profile as well as how the total ISM pressure is partitioned between different phases and components (thermal, dispersion/turbulence, and bulk flows). We show that, on average and consistent with previous more idealized simulations, the total ISM pressure balances the weight of the overlying gas. Deviations from vertical pressure balance increase with increasing galactocentric radius and with decreasing averaging scale. The different phases are in rough total pressure equilibrium with one another, but with large deviations from thermal pressure equilibrium owing to kinetic support in the cold and warm phases, which dominate the total pressure near the midplane. Bulk flows (e.g., inflows and fountains) are important at a few disk scale heights, while thermal pressure from hot gas dominates at larger heights. Overall, the total midplane pressure is well-predicted by the weight of the disk gas, and we show that it also scales linearly with the star formation rate surface density (Sigma_SFR). These results support the notion that the Kennicutt-Schmidt relation arises because Sigma_SFR and the gas surface density (Sigma_g) are connected via the ISM midplane pressure.