Cold versus hot gas accretion and angular momentum in FIRE simulations: from halo to galaxy scales

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 haloes crossing the critical (Formula presented) 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 discs. We find the temperature of gas inflowing on to galaxies is correlated with the virialization of the inner CGM. CGM inflows are almost entirely cold ((Formula presented)  K) in pre-virialized haloes, while hot inflows ((Formula presented)  K) dominate in virialized haloes. When hot inflows dominate, cooling generally occurs simultaneously with circularization at galaxy radii. The dominance of hot inflows on to 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 disc-like kinematics. We additionally find that in bursty galaxies, accreted gas typically forms stars after residing in the galaxy for less than (Formula presented) galaxy free-fall times, while in steady galaxies, gas can persist in the galaxy for up to (Formula presented) free-fall times before forming stars. This highlights a key difference between star formation in bursty galaxies fed by cold accretion and steady equilibrium discs fed by hot accretion.