Black-hole hair from vector dark matter accretion

TL;DR

This paper models how black holes can acquire vector dark matter 'hair' through accretion, revealing significant density amplification near the horizon and potential mass growth, especially in supermassive black holes.

Contribution

It demonstrates the formation of vector-field hair around black holes due to accretion of vector dark matter, using linear perturbation theory and novel analytical approaches.

Findings

Density amplification near horizon exceeds 10^7 times.

Mass of dark matter cloud can reach about 1% of black hole mass.

Supermassive black holes can accrete dark matter at rates up to 10 solar masses per year.

Abstract

We model a single black hole in equilibrium with a dark photon-cold dark matter environment. Representing the dark photon as a Proca field, we show that a Schwarzschild black hole grows vector-field "hair" when allowed to accrete from an infinite homogeneous bath of particles far from the horizon. We solve the Proca equation in linear perturbation theory, separating it using the vector spherical harmonics and Frolov-Krtou\v{s}-Kubiz\v{n}\'{a}k-Santos approaches for the odd-parity and even-parity sectors, respectively. In the "particle" dark matter regime, the field is purely infalling and exhibits a sharply peaked density profile, in concordance with the particle dark matter "spikes" studied in the literature. In the "wave" regime, the field exhibits standing waves, and the profile is smeared. We find a dark-matter density amplification upward of $10^7$ near the horizon. Though small for most black holes, we find the mass enclosed in the cloud can reach $\sim 1 \%$ of the black hole mass for large supermassive black holes. These black holes are also most susceptible to vector dark matter accretion, with mass accretion rates as large as $10 M_\odot/$yr.