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