Modeling dark matter halos with self-interacting fermions

Abstract: In this work we study the possibility of modeling the dark matter content in galaxies as a core-halo model consisting of self-gravitating, self-interacting fermions. For the core of the halo, the dark matter fermions are degenerate, while for the halo we have considered two possibilities: the fermions have thermalized as a perfect fluidor they will follow a standard cold dark matter Navarro-Frenk-White profile. The core density profile is obtained by solving the Tolman-Oppenheimer-Volkoff equations, and their properties are determined by the fermion mass, the central density and the interaction strength. The mass of the fermion and the strength of the fermion self-interaction is fixed by doing a $χ^2$ analysis to fit that fit the rotational curves of Low Surface Brightness galaxies. It was found that the fermion mass should be in the range $38.73~\rm{eV}< m_{f} < 42.11~\rm{eV}$ and the interparticle strength in the range $269.69 < y <348.48$ at $68$ C.L. in order to reproduce the rotational curves adequately, in the case when the halo is modeled as a thermalized ideal gas. Similar values are obtained if the halo is modeled following a Navarro-Frenk-White case, namely $41.54 ~\rm{eV} < m_{f} <49.87 ~\rm{eV}$ and $5606.06< y < 17484.84$. Once fixed the values of the mass of the fermion $m_f$ and the interaction strength $y$, we tested the core-halo model with data from the Milky Way and the SPARC database. We have found good agreement between the data and the core-halo models, varying only one free parameter: the central density. Thus a single fermion can fit hundreds of galaxies. Nevertheless, the dark matter halo surface density relation or the halo total mass and radius depend strongly on the model for the halo.