The Rhythm of the ISM: Tracing the Timescales of Gas Evolution and Star Formation across Galactic Environments

TL;DR

This paper links kiloparsec-scale star formation relations to the underlying gas cycle timescales in galaxies, revealing how turbulence, density, and dynamics shape star formation efficiency and gas depletion across environments.

Contribution

It introduces an analytical framework connecting galaxy-scale star formation laws to local ISM gas cycle timescales, validated with idealized galaxy simulations across diverse environments.

Findings

Star-forming regions form on timescales of 3-30 Myr, decreasing with higher gas surface density.

Local depletion times range from 200 to 2000 Myr, decreasing with higher gas density.

Gas dispersal times are very short, around 0.4-1 Myr, weakly dependent on surface density.

Abstract

We investigate the physical origin of the star formation scaling relations between the gas depletion time, the star-forming gas mass fraction, and the gas surface density, $\Sigma_{\rm gas}$, on kiloparsec scales, all of which are the key ingredients of the observed Kennicutt-Schmidt relation. To elucidate these trends, we employ an analytical framework that explicitly connects these kiloparsec-scale properties to the timescales governing the rapid, continuous ISM gas cycle on the scales of individual star-forming regions, including the formation, dispersal, and local depletion of star-forming gas. Using a suite of idealized disk galaxy simulations spanning a range of environments from dwarf and Milky Way-mass systems to a gas-rich starburst analog, we measure the timescales of the gas cycle and relate them to the dynamical and turbulent properties of the interstellar medium (ISM). We find that star-forming regions form on a timescale close to the vertical turbulent crossing time of the galactic disk, $\sim$3-30 Myr, which decreases at higher $\Sigma_{\rm gas}$ due to the increase in turbulent velocities in the ISM and the decrease in the disk thickness. In contrast, the local star formation and dispersal of such gas are set by the local conditions. Specifically, the local depletion time, $\sim$200-2000 Myr, is decreasing at higher $\Sigma_{\rm gas}$, as star-forming gas becomes denser and more efficient in forming stars. The lifetime of such gas is very short, $\sim$0.4-1 Myr, and only weakly increases with $\Sigma_{\rm gas}$. Together, our results demonstrate how the star formation properties of galaxies on kiloparsec scales emerge directly from the interplay between the galaxy-scale dynamics, ISM turbulence, and the state of star-forming gas.