Cores in warm dark matter haloes: a Catch 22 problem
  The free streaming of warm dark matter particles dampens the fluctuation
spectrum, flattens the mass function of haloes and imprints a fine grained
phase density limit for dark matter structures. The phase space density limit
is expected to imprint a constant density core at the halo center on the
contrary to what happens for cold dark matter. We explore these effects using
high resolution simulations of structure formation in different warm dark
matter scenarios. We find that the size of the core we obtain in simulated
haloes is in good agreement with theoretical expectations based on Liouville's
theorem. However, our simulations show that in order to create a significant
core, (r_c~1 kpc), in a dwarf galaxy (M~1e10 Msun), a thermal candidate with a
mass as low as 0.1 keV is required. This would fully prevent the formation of
the dwarf galaxy in the first place. For candidates satisfying large scale
structure constrains (m_wdm larger than 1-2 keV) the expected size of the core
is of the order of 10 (20) pc for a dark matter halo with a mass of 1e10 (1e8)
Msun. We conclude that "standard" warm dark matter is not viable solution for
explaining the presence of cored density profiles in low mass galaxies.
