Cores in warm dark matter haloes: a Catch 22 problem

Abstract

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