A Universal Angular Momentum Profile for Dark Matter Halos

The angular momentum distribution in dark matter halos and galaxies is a key ingredient in understanding their formation. Specifically, the internal distribution of angular momenta is closely related to the formation of disk galaxies. In this article, we use halos identified from a high-resolution simulation, the Bolshoi simulation, to study the spatial distribution of specific angular momenta, j(r, theta). We show that by stacking halos with similar masses to increase the signal-to-noise ratio, the profile can be fitted as a simple function, j(r, theta) = j(s)sin(2)(theta/theta(s)) (r/r(s))(2)/(1 + r/r(s))(4), with three free parameters, j(s), r(s), and theta(s). Specifically, j(s) correlates with the halo mass M-vir as j(s) alpha M-vir(2/3), r(s) has a weak dependence on the halo mass as r(s) alpha M-vir(0.040), and theta(s) is independent of M-vir. This profile agrees with that from a rigid shell model, though its origin is unclear. Our universal specific angular momentum profile j(r, theta) is useful in modeling the angular momenta of halos. Furthermore, by using an empirical stellar mass-halo mass relation, we can infer the average angular momentum distribution of a dark matter halo. The specific angular momentum-stellar mass relation within a halo computed from our profile is shown to share a similar shape as that from the observed disk galaxies.