The angular momentum of dark haloes: merger and accretion effects

We present new results on the angular momentum evolution of dark matter haloes. Haloes, from N-body simulations, are classified according to their mass growth histories into two categories: the accretion category contains haloes whose mass has varied continuously and smoothly, while the merger category contains haloes 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 non-linear regime. For individual haloes 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 similar to 1.5-1.8, with an early fast phase followed by a late slow phase. Haloes of the merger catalogue acquire more angular momentum even when the scaling with mass is taken into account. The spin parameter has a different behaviour for the two classes: there is a decrease with time for haloes in the accretion catalogue whereas a small increase is observed for the merger catalogue. When the two catalogues are considered together, no significant variation of the spin parameter distribution with the fedshift 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 disc 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 disc around z similar to 1.6. The distribution of the angular momentum indicates that at z similar to 1.6 only 22 per cent of the haloes have angular momentum of magnitude comparable to that of disc galaxies in the mass range 10(10)-5 x 10(11) M., clearly insufficient to explain the present observed abundance of these objects.