Core mass -- halo mass relation of bosonic and fermionic dark matter halos harbouring a supermassive black hole

TL;DR

This paper investigates the relationship between core mass and halo mass in bosonic and fermionic dark matter halos with supermassive black holes, revealing stability conditions and maximum core masses for different dark matter models.

Contribution

It introduces a unified analytical framework for core-halo relations in bosonic and fermionic dark matter halos, including effects of black holes and self-interactions.

Findings

Core mass increases with halo mass up to a maximum before decreasing.

Series of equilibria are stable for noninteracting and repulsively interacting bosons.

Fermionic halos behave similarly to bosonic halos with repulsive interactions.

Abstract

We study the core mass -- halo mass relation of bosonic dark matter halos, in the form of self-gravitating Bose-Einstein condensates, harbouring a supermassive black hole. We use the ``velocity dispersion tracing'' relation according to which the velocity dispersion in the core $v_c^2\sim GM_c/R_c$ is of the same order as the velocity dispersion in the halo $v_h^2\sim GM_h/r_h$ (this relation can be justified from thermodynamical arguments) and the approximate analytical mass-radius relation of the quantum core in the presence of a central black hole obtained in our previous paper [P.H. Chavanis, Eur. Phys. J. Plus 134, 352 (2019)]. For a given minimum halo mass $(M_h)_{\rm min}\sim 10^8\, M_{\odot}$ determined by the observations, the only free parameter of our model is the scattering length $a_s$ of the bosons (their mass $m$ is then determined by the characteristics of the minimum halo). For noninteracting bosons and for bosons with a repulsive self-interaction, we find that the core mass $M_c$ increases with the halo mass $M_h$ and achieves a maximum value $(M_c)_{\rm max}$ at some halo mass $(M_h)_{*}$ before decreasing. The whole series of equilibria is stable. For bosons with an attractive self-interaction, we find that the core mass achieves a maximum value $(M_c)_{\rm max}$ at some halo mass $(M_h)_{*}$ before decreasing. The series of equilibria becomes unstable above a maximum halo mass $(M_h)_{\rm max}\ge (M_h)_{*}$. In the absence of black hole $(M_h)_{\rm max}=(M_h)_{*}$. At that point, the quantum core (similar to a dilute axion star) collapses. We perform a similar study for fermionic dark matter halos. We find that they behave similarly to bosonic dark matter halos with a repulsive self-interaction, the Pauli principle for fermions playing the role of the repulsive self-interaction for bosons.