Do mergers spin-up dark matter haloes?

Abstract 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 haloes. 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 haloes show no significant correlation between spin and merging history. Out-of-equilibrium haloes have, on average, higher spin 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 due to the internal redistribution of mass and angular momentum that occurs during virialization. This process is especially efficient during major mergers, when high angular momentum material is pushed beyond the virial radius of the remnant. Because 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.