Gas kinematics, morphology and angular momentum in the FIRE simulations

Abstract

We study the z=0 gas kinematics, morphology, and angular momentum content of\nisolated galaxies in a suite of cosmological zoom-in simulations from the FIRE\nproject spanning $M_{\\star}=10^{6-11}M_{\\odot}$. Gas becomes increasingly\nrotationally supported with increasing galaxy mass. In the lowest-mass galaxies\n($M_{\\star}<10^{8}M_{\\odot}$), gas fails to form a morphological disk and is\nprimarily dispersion and pressure supported. At intermediate masses\n($M_{\\star}=10^{8-10}M_{\\odot}$), galaxies display a wide range of gas\nkinematics and morphologies, from thin, rotating disks, to irregular spheroids\nwith negligible net rotation. All the high-mass\n($M_{\\star}=10^{10-11}M_{\\odot}$) galaxies form rotationally supported gas\ndisks. Many of the halos whose galaxies fail to form disks harbor high angular\nmomentum gas in their circumgalactic medium. The ratio of the specific angular\nmomentum of gas in the central galaxy to that of the dark-matter halo increases\nsignificantly with galaxy mass, from $j_{\\rm gas}/j_{\\rm DM}\\sim0.1$ at\n$M_{\\star}=10^{6-7}M_{\\odot}$ to $j_{\\rm gas}/j_{\\rm DM}\\sim2$ at\n$M_{\\star}=10^{10-11}M_{\\odot}$. The reduced rotational support in the\nlowest-mass galaxies owes to (a) stellar feedback and the UV background\nsuppressing the accretion of high-angular momentum gas at late times, and (b)\nstellar feedback driving large non-circular gas motions. We broadly reproduce\nthe observed scaling relations between galaxy mass, gas rotation velocity,\nsize, and angular momentum, but may somewhat underpredict the incidence of\ndisky, high-angular momentum galaxies at the lowest observed masses\n($M_{\\star}=(10^{6}-2\\times10^{7})M_{\\odot}$). In our simulations, stars are\nuniformly less rotationally supported than gas. The common assumption that\nstars follow the same rotation curve as gas thus substantially overestimates\ngalaxies' stellar angular momentum, particularly at low masses.\n