Dynamic equilibrium sets atomic content of galaxies across cosmic time

TL;DR

This study uses high-resolution cosmological simulations to show that the atomic gas fraction in disk galaxies is governed by a stable relation with angular momentum, maintained across cosmic time due to equilibrium processes.

Contribution

It demonstrates that the analytic model relating atomic gas fraction and angular momentum holds from early galaxy formation to the present in simulated galaxies.

Findings

Simulated galaxies follow the $f_{atm}$-$q$ relation from $z oughly 4$ to today.

Over 90% of atomic gas remains stable at all times in the simulations.

Galaxies maintain a quasi-static atomic gas equilibrium throughout cosmic history.

Abstract

We analyze 88 independent high-resolution cosmological zoom-in simulations of disk galaxies in the NIHAO simulations suite to explore the connection between the atomic gas fraction and angular momentum of baryons throughout cosmic time. The study is motivated by the analytic model of \citet{obreschkow16}, which predicts a relation between the atomic gas fraction $f_{\rm atm}$ and the global atomic stability parameter $q \equiv j\sigma / (GM)$, where $M$ and $j$ are the mass and specific angular momentum of the galaxy (stars+cold gas) and $\sigma$ is the velocity dispersion of the atomic gas. We show that the simulated galaxies follow this relation from their formation ($z\simeq4$) to present within $\sim 0.5$ dex. To explain this behavior, we explore the evolution of the local Toomre stability and find that $90\%$--$100\%$ of the atomic gas in all simulated galaxies is stable at any time. In other words, throughout the entire epoch of peak star formation until today, the timescale for accretion is longer than the timescale to reach equilibrium, thus resulting in a quasi-static equilibrium of atomic gas at any time. Hence, the evolution of $f_{\rm atm}$ depends on the complex hierarchical growth history primarily via the evolution of $q$. An exception are galaxies subject to strong environmental effects.