The angular momentum of dark halos: merger and accretion effects

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 find that the angular momentum grows in both cases, well into the nonlinear regime. For individual halos we observe strong correlation between the angular momentum variation and the mass variation. The rate of growth of both mass and angular momentum has a characteristic transition time at around z ∼ 1.5− 1.8, with an early fast phase followed by a late slow phase. Halos of the merger catalog acquire more angular momentum even when the scaling with mass is taken into account. The spin parameter has a different behavior for the two classes: there is a decrease with time for halos in the accretion catalog whereas a small increase is observed for the merger catalog. When the two catalogs are considered together, no significant variation of the spin parameter distribution with the redshift is obtained. We have also found that the spin parameter neither depends on the halo mass nor on the cosmological model. From our simple model developed for the formation of a disk galaxy similar to the Milky Way, we 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. The distribution of the angular momentum 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 10 − 5× 1011M⊙, clearly insufficient to explain the present observed abundance of these objects.