Gas kinematics, morphology, and angular momentum in the FIRE simulations
  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 $M_{\star}=10^{6-11}M_{\odot}$. Gas becomes increasingly
rotationally supported with increasing galaxy mass. In the lowest-mass galaxies
($M_{\star}<10^{8}M_{\odot}$), gas fails to form a morphological disk and is
primarily dispersion and pressure supported. At intermediate masses
($M_{\star}=10^{8-10}M_{\odot}$), galaxies display a wide range of gas
kinematics and morphologies, from thin, rotating disks, to irregular spheroids
with negligible net rotation. All the high-mass
($M_{\star}=10^{10-11}M_{\odot}$) galaxies form rotationally supported gas
disks. Many of the halos whose galaxies fail to form disks harbor 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 $j_{\rm gas}/j_{\rm DM}\sim0.1$ at
$M_{\star}=10^{6-7}M_{\odot}$ to $j_{\rm gas}/j_{\rm DM}\sim2$ at
$M_{\star}=10^{10-11}M_{\odot}$. 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
($M_{\star}=(10^{6}-2\times10^{7})M_{\odot}$). In our simulations, stars are
uniformly less rotationally supported than gas. The common assumption that
stars follow the same rotation curve as gas thus substantially overestimates
galaxies' stellar angular momentum, particularly at low masses.
