The evolution of the spin parameter of dark halos and the dependence on the halo merging history in a set of dissipationless cosmological \Lambda CDM simulations is investigated .
Special focus is placed on the differences of the two commonly used versions of the spin parameter , namely \lambda = JE ^ { 1 / 2 } G ^ { -1 } M ^ { -5 / 2 } ( Peebles 1980 ) and \lambda ^ { \prime } = J / [ \sqrt { 2 } M _ { vir } R _ { vir } V _ { vir } ] ( Bullock et al .
2001 ) .
Though the distribution of the spin transfer rate T _ { \lambda } ^ { ( \prime ) } : = \lambda _ { fin } ^ { ( \prime ) } / \lambda _ { init } ^ { ( % \prime ) } ( which is the ratio of spin parameters after and prior to merger ) is similar to a high degree for both \lambda and \lambda ^ { \prime } , we find considerable differences in the time evolution : while \lambda ^ { \prime } 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: = 2 E _ { kin } / |E _ { pot } | of dark halos .
Major mergers disturb halos and increase both their virial ratio \eta and spin parameter \lambda ^ { ( \prime ) } for 1 - 2 Gyrs .
At high redshifts z = 2 - 3 many halos are disturbed with an average virial ratio of \overline { \eta } \approx 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 .