The evolution of the dark halo spin parameters λ and λ' in a ΛCDM universe:: the role of minor and major mergers

The evolution of the spin parameter of dark haloes and the dependence on the halo merging history in a set of dissipationless cosmological Lambda cold dark matter simulations are investigated. Special focus is placed on the differences of the two commonly used versions of the spin parameter, namely lambda = E(1/2)G(-1)M(-5/2) and lambda' = J(root 2M(vir)R(vir)V(vir)). Though the distribution of the spin transfer rate T(lambda)((')) =: lambda(('))(fin)/lambda(('))(init) (which is the ratio of spin parameters after and prior to 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 distribution of accreted material. The evolution of the spin parameter is strongly coupled with the virial ratio eta =: 2E(kin)/vertical bar E(pot vertical bar) of dark haloes. Major mergers (MMs) disturb haloes and increase their both virial ratio eta and spin parameter lambda((')) for 1-2 Gyr. At high redshifts z = 2-3, many haloes are disturbed with an average virial ratio of 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 MM events.