Stochastic modelling of star-formation histories II: star-formation variability from molecular clouds and gas inflow

TL;DR

This paper develops an analytical model to understand how small-scale molecular cloud processes and large-scale gas inflow influence the variability of star formation over different timescales in galaxies.

Contribution

It introduces a regulator model that links star-formation rate fluctuations to gas inflow, molecular cloud lifetimes, and galaxy-scale processes, providing a comprehensive framework for star-formation variability.

Findings

The PSD of SFR has three characteristic breaks related to inflow, gas reservoir, and molecular cloud lifetimes.

Short-term SFR variability can be modeled as a damped random walk with a power-law slope of ~2.

The model helps interpret star-formation burstiness and galaxy evolution around the main sequence.

Abstract

A key uncertainty in galaxy evolution is the physics regulating star formation, ranging from small-scale processes related to the life-cycle of molecular clouds within galaxies to large-scale processes such as gas accretion onto galaxies. We study the imprint of such processes on the time-variability of star formation with an analytical approach tracking the gas mass of galaxies ("regulator model"). Specifically, we quantify the strength of the fluctuation in the star-formation rate (SFR) on different timescales, i.e. the power spectral density (PSD) of the star-formation history, and connect it to gas inflow and the life-cycle of molecular clouds. We show that in the general case the PSD of the SFR has three breaks, corresponding to the correlation time of the inflow rate, the equilibrium timescale of the gas reservoir of the galaxy, and the average lifetime of individual molecular clouds. On long and intermediate timescales (relative to the dynamical timescale of the galaxy), the PSD is typically set by the variability of the inflow rate and the interplay between outflows and gas depletion. On short timescales, the PSD shows an additional component related to the life-cycle of molecular clouds, which can be described by a damped random walk with a power-law slope of $\beta\approx2$ at high frequencies with a break near the average cloud lifetime. We discuss star-formation "burstiness" in a wide range of galaxy regimes, study the evolution of galaxies about the main sequence ridgeline, and explore the applicability of our method for understanding the star-formation process on cloud-scale from galaxy-integrated measurements.