Gas kinematics, morphology and angular momentum in the FIRE simulations

Abstract

We study the z = 0 gas kinematics, morphology and angular momentum content of isolated galaxies in a suite of cosmological zoom-in simulations from the FIRE project spanning M_(star) = 10^(6–11) M_⊙. Gas becomes increasingly rotationally supported with increasing galaxy mass. In the lowest mass galaxies (M_(star) < 10^8 M_⊙), gas fails to form a morphological disc and is primarily dispersion and pressure supported. At intermediate masses (M_(star) = 10^(8–10) M_⊙), galaxies display a wide range of gas kinematics and morphologies, from thin, rotating discs to irregular spheroids with negligible net rotation. All the high-mass (M_(star) = 10^(10–11) M_⊙) galaxies form rotationally supported gas discs. Many of the haloes whose galaxies fail to form discs harbour high angular momentum gas in their circumgalactic medium. The ratio of the specific angular momentum of gas in the central galaxy to that of the dark matter halo increases significantly with galaxy mass, from 〈j_(gas)〉/〈j_(DM)〉 ∼ 0.1 at M_(star) = 10^(6-7) M_⊙ to 〈j_(gas)〉/〈j_(DM)〉 ∼ 2 at M_(star) = 10^(10–11) M_⊙. The reduced rotational support in the lowest mass galaxies owes to (a) stellar feedback and the UV background suppressing the accretion of high angular momentum gas at late times, and (b) stellar feedback driving large non-circular gas motions. We broadly reproduce the observed scaling relations between galaxy mass, gas rotation velocity, size and angular momentum, but may somewhat underpredict the incidence of disky, high angular momentum galaxies at the lowest observed masses (M_(star) = (10^6–2 × 10^7) M_⊙). Stars form preferentially from low angular momentum gas near the galactic centre and are less rotationally supported than gas. The common assumption that stars follow the same rotation curve as gas thus substantially overestimates the simulated galaxies’ stellar angular momentum, particularly at low masses.