The inner structure of haloes in Cold+Warm dark matter models

We analyze the properties of dark matter halos in the cold-plus-warm dark matter cosmologies (CWDM). 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% of CDM and 10% of WDM with the mass 1 keV/ (2) a mixture of 50% of CDM and 50% of WDM with the mass 5 keV/ and (3) pure WDM with the mass 10 keV. 'Warm' particles have rescaled Fermi-Dirac spectrum of primordial velocities (as non-resonantly produced sterile neutrinos would have). We compare the properties of halos among these models and with a LCDM 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-alpha forest comoving wave-numbers k<3-5 h/Mpc), the resulting properties of halos with masses below 1e11 Msun 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 LCDM 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.