On the formation and stability of fermionic dark matter haloes in a cosmological framework
Abstract
The formation and stability of collisionless selfgravitating systems are longstanding problems, which date back to the work of D. LyndenBell on violent relaxation and extends to the issue of virialization of dark matter (DM) haloes. An important prediction of such a relaxation process is that spherical equilibrium states can be described by a FermiDirac phasespace distribution, when the extremization of a coarsegrained entropy is reached. In the case of DM fermions, the most general solution develops a degenerate compact core surrounded by a diluted halo. As shown recently, the latter is able to explain the galaxy rotation curves, while the DM core can mimic the central black hole. A yet open problem is whether these kinds of astrophysical corehalo configurations can form at all, and whether they remain stable within cosmological timescales. We assess these issues by performing a thermodynamic stability analysis in the microcanonical ensemble for solutions with a given particle number at halo virialization in a cosmological framework. For the first time, we demonstrate that the above corehalo DM profiles are stable (i.e. maxima of entropy) and extremely longlived. We find the existence of a critical point at the onset of instability of the corehalo solutions, where the fermioncore collapses towards a supermassive black hole. For particle masses in the keV range, the corecollapse can only occur for $M_{\rm vir} \gtrsim 10^{9}{\, \mathrm{M}_\odot}$ starting at z_{vir} ≍ 10 in the given cosmological framework. Our results prove that DM haloes with a corehalo morphology are a very plausible outcome within nonlinear stages of structure formation.
 Publication:

Monthly Notices of the Royal Astronomical Society
 Pub Date:
 April 2021
 DOI:
 10.1093/mnras/staa3986
 arXiv:
 arXiv:2012.11709
 Bibcode:
 2021MNRAS.502.4227A
 Keywords:

 methods: numerical;
 galaxies: haloes;
 galaxies: nuclei;
 galaxies: formation;
 galaxies: structure;
 dark matter;
 Astrophysics  Astrophysics of Galaxies
 EPrint:
 16+4 pages, 20 figures. This version matches the MNRAS published version