Abstract
We analyse the properties of dark matter haloes in the cold-plus-warm dark matter (CWDM) cosmologies. We study their dependence on the fraction and velocity dispersion of the warm particle, keeping the free-streaming scale fixed. To this end, we consider three models with the same free-streaming: (1) a mixture of 90 per cent of CDM and 10 per cent of WDM with the mass 1 keV; (2) a mixture of 50 per cent of CDM and 50 per cent of WDM with the mass 5 keV and (3) pure WDM with the mass 10 keV. ‘Warm’ particles have rescaled –Dirac spectrum of primordial velocities (as non-resonantly produced sterile neutrinos would have). We compare the properties of haloes among these models and with a ΛCDM with the same cosmological parameters. We demonstrate that although these models have the same free-streaming length and the suppression of matter spectra are similar at scales probed by the Lyman α forest (comoving wave-numbers k < 3–5 h Mpc−1), the resulting properties of haloes with masses below ∼1011 M⊙ are different due to the different behaviour of matter power spectra at smaller scales. In particular, we find that while the number of galaxies remains the same as in the ΛCDM case, their density profiles become much less concentrated and hence in better agreement with current observational constraints. Our results imply that a single parameter (e.g. free-streaming length) description of these models is not enough to fully capture their effects on the structure formation process.
Highlights
It is usually said that cosmological data favour Cold Dark Matter
Interest in warm dark matter models has increased in the last years, mainly motivated by the possible need for cored dark matter halo density profiles for dwarf galaxies in the local group, or by solution of various Cold Dark Matter (CDM) “over-abundance problems” (Klypin et al 1999; Moore et al 1999), see e.g. (Maccio & Fontanot 2010; Strigari et al 2010; Boylan-Kolchin et al 2011; Papastergis et al 2011; Lovell et al 2012)
While it is still under debate what is the effects of baryons on the dark matter distribution on such small scales (e.g. Pontzen & Governato 2012; Maccioet al. 2012a and references therein); it is worth studying through numerical simulations the effects of a warm component on the inner structure of DM haloes
Summary
It is usually said that cosmological data favour Cold Dark Matter. The cosmological “concordance model” is often called ΛCDM. In many models (e.g. sterile neutrinos, gravitino, axino) the same DM particles can be produced via two co-existing mechanisms and generically primordial velocity spectra have “colder” and “warmer” components Such models can be called mixed or “cold plus warm” dark matter models (CWDM) (Boyarsky et al 2009a, see Palazzo et al 2007). The first results of Lovell et al (2012) demonstrate that the resonantly produced sterile neutrino DM models, compatible with the Lyman-α bounds of Boyarsky et al (2009b), do change the number of substructure of a Galaxy-size halo and their properties.
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