Density spikes near black holes in self-interacting dark matter halos and indirect detection constraints

TL;DR

This paper investigates how self-interacting dark matter (SIDM) forms density spikes near black holes, affecting indirect detection signals, and explores observational constraints and tests, including implications for the Event Horizon Telescope.

Contribution

It demonstrates that SIDM creates shallower density spikes than collisionless dark matter, relaxing indirect detection constraints and enabling new observational tests.

Findings

SIDM produces a $r^{-7/4}$ density spike near black holes.

Constraints on dark matter annihilation are weaker in SIDM scenarios.

Event Horizon Telescope could test SIDM-induced density spikes.

Abstract

Self-interacting dark matter (SIDM) naturally gives rise to a cored isothermal density profile, which is favored in observations of many dwarf galaxies. The dark matter distribution in the presence of a central black hole in an isothermal halo develops a density spike with a power law of $r^{-7/4}$, which is shallower than $r^{-7/3}$ as expected for collisionless dark matter (CDM). Thus, indirect detection constraints on dark matter annihilations from the density spike could be relaxed in SIDM. Taking the most dense satellite galaxy of the Milky Way Draco as an example, we derive upper limits on the annihilation cross section and the black hole mass for both SIDM and CDM halos. For the former case, Draco could host an intermediate mass black hole even if dark matter is composed of thermal relics. We further explore the constraints from the Milky Way and M87, which host supermassive black holes, and show the upper limits on the annihilation cross section are significantly weakened in SIDM. Our results also indicate that the Event Horizon Telescope could provide a unique test of SIDM spikes.