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

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ID: 317338
2026
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Abstract
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 crossing the critical $\sim 10^{12}\ \mathrm{M}_{\odot }$ mass scale where several transitions have been found, including inner CGM virialization, the transition from bursty to steady star formation, and the emergence of thin disks. We find the temperature of gas inflowing onto galaxies is correlated with the virialization of the inner CGM. CGM inflows are almost entirely cold (T < 105 K) in pre-virialized halos, while hot inflows (T > 105 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. Yet, we find 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 in the galaxy 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.
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Authors Imran Sultan, Claude-André Faucher-Giguére, Jonathan Stern, Guochao Sun
Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag1117
URL
https://doi.org/10.1093/mnras/stag1117
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