The properties of warm dark matter haloes

Abstract

Well-motivated elementary particle candidates for the dark matter, such as\nthe sterile neutrino, behave as warm dark matter (WDM).For particle masses of\norder a keV, free streaming produces a cutoff in the linear fluctuation power\nspectrum at a scale corresponding to dwarf galaxies. We investigate the\nabundance and structure of WDM haloes and subhaloes on these scales using high\nresolution cosmological N-body simulations of galactic haloes of mass similar\nto the Milky Way's. On scales larger than the free-streaming cutoff, the\ninitial conditions have the same power spectrum and phases as one of the cold\ndark matter (CDM) haloes previously simulated by Springel et al as part of the\nVirgo consortium Aquarius project. We have simulated four haloes with WDM\nparticle masses in the range 1.4-2.3keV and, for one case, we have carried out\nfurther simulations at varying resolution. N-body simulations in which the\npower spectrum cutoff is resolved are known to undergo artificial fragmentation\nin filaments producing spurious clumps which, for small masses (<10^7Msun in\nour case) outnumber genuine haloes. We have developed a robust algorithm to\nidentify these spurious objects and remove them from our halo catalogues. We\nfind that the WDM subhalo mass function is suppressed by well over an order\nmagnitude relative to the CDM case for masses <10^9Msun. Requiring that there\nshould be at least as many subhaloes as there are observed satellites in the\nMilky Way leads to a conservative lower limit to the (thermal equivalent) WDM\nparticle mass of ~1.5\\rmn{keV}. WDM haloes and subhaloes have cuspy density\ndistributions that are well described by NFW or Einasto profiles. Their central\ndensities are lower for lower WDM particle masses and none of the models we\nhave considered suffer from the "too big to fail" problem recently highlighted\nby Boylan-Kolchin et al.\n