Accretion of self-interacting dark matter onto supermassive black holes

¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, D-21029 Hamburg, Germany
² Deutsches Elektronen-Synchrotron DESY, Notkestr. 85, 22607 Hamburg, Germany
³ Universitäts-Sternwarte, Fakultät für Physik, Ludwig-Maximilians-Universität München, Scheinerstr. 1, D-81679 München, Germany
⁴ Excellence Cluster ORIGINS, Boltzmannstrasse 2, D-85748 Garching, Germany

^⋆ Corresponding author: sabarish.venkataramani@uni-hamburg.de.

Received: 20 May 2025
Accepted: 4 August 2025

Abstract

Context. Dark matter (DM) spikes around supermassive black holes (SMBHs) can lead to interesting physical effects such as enhanced DM annihilation signals or dynamical friction within binary systems, shortening the merger time and possibly addressing the ‘final parsec problem’. They can also be promising places to study the collisionality of DM because their velocity dispersion is higher than in DM halos, allowing us to probe a different velocity regime.

Aims. We aim to understand the evolution of isolated DM spikes for collisional DM and compute the black hole (BH) accretion rate as a function of the self-interaction cross-section.

Methods. We have performed the first N-body simulations of self-interacting dark matter (SIDM) spikes around SMBHs and studied the evolution of the spike with an isolated BH starting from profiles similar to the ones that have been shown to be stable in analytical calculations.

Results. We find that the analytical profiles for SIDM spikes remain stable over the timescales of hundreds of years that we have covered with our simulations. In the long-mean-free-path regime, the accretion rate onto the BHs grows linearly with the cross-section and flattens when we move towards the short-mean-free-path regime. In both regimes, our simulations match analytic expectations, which are based on the heat conduction description of SIDM. A simple model of the accretion rate allows us to calibrate the heat conduction in the gravothermal fluid prescription of SIDM. Using this prescription, we determine the maximum allowed accretion rate that occurs when r_ISCOρ(r_ISCO)σ/m_χ ∼ 1, where σ/m_χ is the self-interaction cross-section and r_ISCO the radius of the innermost stable orbit.

Conclusions. Our calibrated DM accretion rates could be used for statistical analysis of SMBH growth and incorporated into subgrid models to study BH growth in cosmological simulations.