An Analytic Model for the Evolution of the Stellar, Gas, and Metal Content of Galaxies

TL;DR

This paper introduces an analytic model describing how galaxies evolve in terms of their stars, gas, and metals, based on a balance between inflow, outflow, and star formation, aligning well with simulation results.

Contribution

It presents a novel equilibrium formalism that captures galaxy evolution dynamics with three key parameters, linking theory with observations and simulations.

Findings

Galaxies tend to return to equilibrium after perturbations.

The model explains the correlation between star formation rate, gas fraction, and metallicity.

Most quiescent galaxies are in equilibrium over cosmic time.

Abstract

We present an analytic formalism that describes the evolution of the stellar, gas, and metal content of galaxies. It is based on the idea, inspired by hydrodynamic simulations, that galaxies live in a slowly-evolving equilibrium between inflow, outflow, and star formation. We argue that this formalism broadly captures the behavior of galaxy properties evolving in simulations. The resulting equilibrium equations for the star formation rate, gas fraction, and metallicity depend on three key free parameters that represent ejective feedback, preventive feedback, and re-accretion of ejected material. We schematically describe how these parameters are constrained by models and observations. Galaxies perturbed off the equilibrium relations owing to inflow stochasticity tend to be driven back towards equilibrium, such that deviations in star formation rate at a given mass are correlated with gas fraction and anti-correlated with metallicity. After an early gas accumulation epoch, quiescently star-forming galaxies are expected to be in equilibrium over most of cosmic time. The equilibrium model provides a simple intuitive framework for understanding the cosmic evolution of galaxy properties, and centrally features the cycle of baryons between galaxies and surrounding gas as the driver of galaxy growth.