Equilibrium States of Galactic Atmospheres I: The Flip Side of Mass Loading

TL;DR

This paper introduces a new framework to analyze galaxy and circumgalactic medium interactions, revealing how heating, cooling, and feedback influence equilibrium states and star formation rates, especially in low-mass galaxies.

Contribution

It develops an algebraic model for galaxy equilibrium states considering all baryonic energy, including gravitational potential, and examines the impact of mass loading on star formation.

Findings

Equilibrium star formation rates in low-mass galaxies are largely insensitive to mass loading.

Increasing mass loading can lead to more star formation due to greater supernova feedback energy.

The framework challenges common assumptions about the role of feedback in galaxy evolution.

Abstract

This paper presents a new framework for understanding the relationship between a galaxy and its circumgalactic medium (CGM). It focuses on how imbalances between heating and cooling cause either expansion or contraction of the CGM. It does this by tracking \textit{all} of the mass and energy associated with a halo's baryons, including their gravitational potential energy, even if feedback has pushed some of those baryons beyond the halo's virial radius. We show how a star-forming galaxy's equilibrium state can be algebraically derived within the context of this framework, and we analyze how the equilibrium star formation rate depends on supernova feedback. We consider the consequences of varying the mass loading parameter etaM = Mdot_wind / Mdot_* relating a galaxy's gas mass outflow rate (Mdot_wind) to its star formation rate (Mdot_*) and obtain results that challenge common assumptions. In particular, we find that equilibrium star formation rates in low-mass galaxies are generally insensitive to mass loading, and when mass loading does matter, increasing it actually results in \textit{more} star formation because more supernova energy is needed to resist atmospheric contraction.