Constraining the population of intermediate-mass black holes by gamma-ray data of the Fornax cluster

Recent studies of merging black holes suggest that the cosmological mass fraction of primordial black holes (PBHs) is not completely negligible. The mass of a PBH can be as massive as an intermediate-mass black hole (IMBH). In this article, we assume that dark matter particles are thermal relic particles and they can self-annihilate. The dark matter around an IMBH may have formed a density spike so that the rate of dark matter annihilation would be greatly enhanced. Using the gamma-ray data of the Fornax cluster and assuming merging events in the cluster are not important, we show that the amount of IMBHs (including PBHs) is very subdominant compared with the amount of baryons and dark matter. The upper limit on the IMBH-to-baryon ratio is ∼7 × 10⁻⁴ for mDM ≤ 10 TeV. Copyright © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society.