# The physical origin of long gas depletion times in galaxies

## TL;DR

This paper introduces a model explaining why galaxies have long gas depletion times, emphasizing the role of feedback and multiple gas cycling through star-forming states, without assuming equilibrium.

## Contribution

The model provides a non-equilibrium framework for understanding long depletion times and explains how feedback self-regulates star formation rates in galaxies.

## Key findings

- Gas evolves quickly from non-star-forming to star-forming states.
- Star formation rate is limited by the fraction of gas converted before dispersal.
- The model reproduces observed Kennicutt-Schmidt relations for molecular and atomic gas.

## Abstract

We present a model that explains why galaxies form stars on a time scale significantly longer than the time scales of processes governing the evolution of interstellar gas. We show that gas evolves from a non-star-forming to a star-forming state on a relatively short time scale and thus the rate of this evolution does not limit the star formation rate. Instead, the star formation rate is limited because only a small fraction of star-forming gas is converted into stars before star-forming regions are dispersed by feedback and dynamical processes. Thus, gas cycles into and out of star-forming state multiple times, which results in a long time scale on which galaxies convert gas into stars. Our model does not rely on the assumption of equilibrium and can be used to interpret trends of depletion times with the properties of observed galaxies and the parameters of star formation and feedback recipes in simulations. In particular, the model explains how feedback self-regulates the star formation rate in simulations and makes it insensitive to the local star formation efficiency. We illustrate our model using the results of an isolated $L_*$-sized galaxy simulation that reproduces the observed Kennicutt-Schmidt relation for both molecular and atomic gas. Interestingly, the relation for molecular gas is almost linear on kiloparsec scales, although a nonlinear relation is adopted in simulation cells. We discuss how a linear relation emerges from non-self-similar scaling of the gas density PDF with the average gas surface density.