Identification of a Fundamental Transition in a Turbulently-Supported Interstellar Medium

TL;DR

This study identifies a critical turbulent velocity (~35 km/s) in the interstellar medium that causes a transition from stable to unstable cooling regimes, influencing galaxy evolution by promoting gas ejection at higher turbulence levels.

Contribution

The paper reveals a fundamental transition in turbulent support of the interstellar medium at a specific velocity, impacting our understanding of galaxy evolution and gas dynamics.

Findings

Below 35 km/s turbulence, the medium remains stable with constant cooling times.

Above 35 km/s turbulence, the gas becomes unstable and prone to runaway heating.

High turbulence leads to gas ejection from the galaxy's interstellar medium.

Abstract

The interstellar medium in star-forming galaxies is a multiphase gas in which turbulent support is at least as important as thermal pressure. Sustaining this configuration requires continuous radiative cooling, such that the overall average cooling rate matches the decay rate of turbulent energy into the medium. Here we carry out a set of numerical simulations of a stratified, turbulently stirred, radiatively cooled medium, which uncover a fundamental transition at a critical one-dimensional turbulent velocity of ~ 35 km/s. At turbulent velocities below ~35 km/s, corresponding to temperatures below 300,000 K, the medium is stable, as the time for gas to cool is roughly constant as a function of temperature. On the other hand, at turbulent velocities above the critical value, the gas is shocked into an unstable regime in which the cooling time increases strongly with temperature, meaning that a substantial fraction of the interstellar medium is unable to cool on a turbulent dissipation timescale. This naturally leads to runaway heating and ejection of gas from any stratified medium with a one-dimensional turbulent velocity above ~35 km/s, a result that has implications for galaxy evolution at all redshifts.