The Interplay between Galactic Angular Momentum and Morphology

In both observed galaxies and in galaxy formation simulations, the degree of rotation vs. dispersion support declines with increasing mass, at least up to $L\sim L_{\star}$. While most isolated Milky-Way mass galaxies have thin, centrifugally supported gas disks, many lower-mass galaxies are puffy and irregular, supported by a mix of thermal pressure and feedback-driven turbulence. Cosmological simulations show that 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 $j_{\rm galaxy} / j_{\rm halo} \sim 0.1$ at $ M_{\rm star}=10^{6-7} \rm M_{\odot}$ to $j_{\rm galaxy} / j_{\rm halo} \sim 2$ at $ M_{\rm star}=10^{10-11}\,M_{\odot}$. Such a decrease in the angular momentum retention factor is also required to reproduce the observed $j_{\rm galaxy} - M_{\rm vir}$ relation at low masses.  In simulations, the reduced rotational support in the lowest-mass galaxies is a result of (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.