Cold vs. Hot Gas Accretion and Angular Momentum in FIRE Simulations: From Halo to Galaxy Scales

TL;DR

This study uses high-resolution FIRE simulations to analyze how cold and hot gas accretion influence angular momentum and galaxy formation across different halo masses and evolutionary states.

Contribution

It provides a systematic comparison of cold and hot gas inflows, their thermal states, angular momentum, and impact on galaxy morphology and star formation modes.

Findings

Cold inflows dominate in pre-virialized halos, hot inflows in virialized halos.

Hot inflows tend to circularize and cool at galaxy radii, forming disks.

Cold inflows lead to rapid star formation, hot inflows result in more extended star formation periods.

Abstract

We present a systematic study of gas accretion and angular momentum in the circumgalactic medium (CGM) using high-resolution FIRE cosmological simulations. Our analysis includes halos spanning the critical $\sim 10^{12}\ \mathrm{M}_{\odot}$ scale where several transitions have been identified, including inner CGM virialization, the transition from bursty to steady star formation, and the emergence of thin disks. We find that the temperature of inflowing gas is correlated with the virialization of the inner CGM. CGM inflows are almost entirely cold ($T < 10^5$ K) in pre-virialized halos, while hot inflows ($T > 10^5$ K) dominate in virialized halos. When hot inflows dominate, cooling generally occurs simultaneously with circularization at galaxy radii. The dominance of hot inflows onto massive galaxies persists even at high redshift, where cold streams may coexist. Consistent with previous studies, cold inflows have higher specific angular momentum than dark matter and hot gas. However, in bursty, low-mass galaxies, cold inflows do not circularize prior to star formation, while in steady, massive galaxies, hot inflows circularize, cool, and form stars with disk-like kinematics. We additionally find that in bursty galaxies, accreted gas typically forms stars after residing in the galaxy for less than $\sim 5$ galaxy free-fall times, while in steady galaxies, gas can persist for up to $\sim 25$ free-fall times before forming stars. This highlights a key difference between star formation in bursty galaxies fed by cold accretion and steady equilibrium disks fed by hot accretion.