Abstract
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 sub-dominant compared with the amount of baryons and dark matter. The upper limit on the IMBH-to-baryon ratio is $\sim 7\times 10^{-4}$ for $m_{\rm DM} \le 10$ TeV.
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