Critical configurations for a system of semidegenerate fermions

Abstract

We study an isothermal system of semi-degenerate self-gravitating fermions in general relativity. Such systems present mass density solutions with a central degenerate core, a plateau and a tail which follows a power law behaviour $r^{-2}$. The different solutions are governed by the free parameters of the model: the degeneracy and temperature parameters at the center, and the particle mass $m$. We then analyze in detail the free parameter space for a fixed $m$ in the keV regime, by studying the one-parameter sequences of equilibrium configurations up to the critical point, which is represented by the maximum in a central density ($\rho_0$) Vs. core mass ($M_c$) diagram. We show that for fully degenerate cores, the known expression for the critical core mass $M_c^{cr}\propto m_{pl}^3/m^2$ is obtained, while instead for low degenerate cores, the critical core mass increases showing the temperature effects in a non linear way. The main result of this work is that when applying this theory to model the distribution of dark matter in galaxies from the very center up to the outer halos, we do not find any critical core-halo configuration of self-gravitating fermions, which be able to explain the super massive dark object in their centers together with an outer halo simultaneously.