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Abstract: N-body simulations of Cold Dark Matter CDM have shown that, in thishierarchical structure formation model, dark matter halo properties, such asthe density profile, the phase-space density profile, the distribution of axialratio, the distribution of spin parameter, and the distribution of internalspecific angular momentum follow `universal- laws or distributions. Here westudy the properties of the first generation of haloes in a Hot Dark MatterHDM dominated universe, as an example of halo formation through monolithiccollapse. We find all these universalities to be present in this case also.Halo density profiles are very well fit by the Navarro et al 1997 profileover two orders of magnitude in mass. The concentration parameter depends onmass as $c \propto M^{0.2}$,reversing the dependence found in a hierarchicalCDM universe. However, the concentration-formation time relation is similar inthe two cases: earlier forming haloes tend to be more concentrated than theirlater forming counterparts. Halo formation histories are also characterized bytwo phases in the HDM case: an early phase of rapid accretion followed byslower growth. Furthermore, there is no significant difference between the HDMand CDM cases concerning the statistics of other halo properties: thephase-space density profile; the velocity anisotropy profile; the distributionof shape parameters; the distribution of spin parameter, and the distributionof internal specific angular momentum are all similar in the two cases. Onlysubstructure content differs dramatically. These results indicate that mergersdo not play a pivotal role in establishing the universalities, thuscontradicting models which explain them as consequences of mergers.

Autor: Jie Wang, Simon D. M. White MPA


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