The Evolution of the Dark Halo Spin Parameters lambda and lambda' in a LCDM Universe: The Role of Minor and Major Mergers

The evolution of the spin parameter of dark halos and the dependence on the halo merging history in a set of dissipationless cosmological LCDM simulations is investigated. Special focus is placed on the differences of the two commonly used versions of the spin parameter, namely lambda=J*E^1/2/(G*M^5/2) (Peebles 80) and lambda'=J/(sqrt(2)*M_vir*R_vir*V_vir) (Bullock et al. 01). Though the distribution of the spin transfer rate defined as the ratio of the spin parameters after and prior to a merger is similar to a high degree for both, lambda and lambda', we find considerable differences in the time evolution: while lambda' is roughly independent of redshift, lambda turns out to increase significantly with decreasing redshift. This distinct behaviour arises from small differences in the spin transfer during accretion events. The evolution of the spin parameter is strongly coupled with the virial ratio eta:=2*E_kin/|E_pot| of dark halos. Major mergers disturb halos and increase both their virial ratio and spin parameter for 1-2 Gyrs. At high redshifts (z=2-3) many halos are disturbed with an average virial ratio of eta = 1.3 which approaches unity until z=0. We find that the redshift evolution of the spin parameters is dominated by the huge number of minor mergers rather than the rare major merger events.