  We use a large cosmological N-body simulation to study the origin of possible
correlations between the merging history and spin of cold dark matter halos. In
particular, we examine claims that remnants of major mergers tend to have
higher-than-average spins, and find that the effect is driven largely by
unrelaxed systems: equilibrium dark matter halos show no significant
correlation between spin and merger history. Out-of-equilibrium halos have, on
average, higher spins than relaxed systems, suggesting that the virialization
process leads to a net decrease in the value of the spin parameter. We find
that this decrease is driven by the internal redistribution of mass and angular
momentum that occurs during virialization, a process that is especially
efficient during major mergers, when high angular momentum material is pushed
beyond the virial radius of the remnant. Since such redistribution likely
affects the angular momentum of baryons and dark matter unevenly, our findings
question the common practice of identifying the specific angular momentum
content of a halo with that of its embedded luminous component. Further work is
needed to elucidate the true relation between the angular momentum content of
baryons and dark matter in galaxy systems assembled hierarchically.
