The angular momentum of dark halos: merger and accretion effects

Abstract

We present new results on the angular momentum evolution of dark matter halos. Halos, from N-body simulations, are classified according to their mass growth histories into two categories: the accretion category contains halos whose mass has varied continuously and smoothly, while the merger category contains halos which have undergone sudden and significant mass variations (greater than 1/3 of their initial mass per event). We observe for the growth of angular momentum that it continues well into the nonlinear regime, that it is correlated with mass growth and that it occurs in two phases: a fast followed by a slow phase. The spin parameter decreases with time for the accretion catalog whereas slightly increases for the merger catalog, but does not change when two catalogs are considered together, and furthermore it does not depend on halo mass or cosmological model. We also develop a simple model for the formation of a disk galaxy similar to Milky Way, and conclude that our own halo must have captured satellites in order to acquire the required angular momentum and to achieve most of the disk around z ~ 1.6. Angular momentum distribution indicates that at z ~ 1.6 only 22% of the halos have angular momentum of magnitude comparable to that of disk galaxies in the mass range of $(0.1-5)*10^{11}$ of solar mass, clearly insufficient to explain the present observed abundance of these objects.