The Clustering Dynamics of Primordial Black Boles in <em>N</em>-Body Simulations

• Main Authors: Manuel Trashorras, Juan García-Bellido, Savvas Nesseris
• Format: Article
• Language: English
• Published: MDPI AG 2021-01-01
• Series: Universe
• Subjects: primordial black holes; dark matter; black hole binaries; black hole mergers; <em>N</em>-body simulations
• Online Access: https://www.mdpi.com/2218-1997/7/1/18

We explore the possibility that Dark Matter (DM) may be explained by a nonuniform background of approximately stellar mass clusters of Primordial Black Holes (PBHs) by simulating the evolution from recombination to the present with over 5000 realisations using a Newtonian \({ N }\)-body code. We compute the cluster rate of evaporation and extract the binary and merged sub-populations along with their parent and merger tree histories, lifetimes and formation rates, the dynamical and orbital parameter profiles, the degree of mass segregation and dynamical friction and power spectrum of close encounters. Overall, we find that PBHs can constitute a viable DM candidate, and that their clustering presents a rich phenomenology throughout the history of the Universe. We show that binary systems constitute about 9.5% of all PBHs at present, with mass ratios of \({ \bar{q}_{\rm B} = 0.154 }\), and total masses of \({ \bar{m}_{\rm T,\,B} = 303\,M_\odot}\). Merged PBHs are rare, about 0.0023% of all PBHs at present, with mass ratios of \({ \bar{q}_{\rm B}= 0.965 }\) with total and chirp masses of \({ \bar{m}_{\rm T,\,B}= 1670\,M_\odot}\) and \({ \bar{m}_{c,{\rm M}} = 642\,M_\odot }\), respectively. We find that cluster puffing up and evaporation leads to bubbles of these PBHs of order 1 kpc containing at present times about 36% of objects and mass, with one-hundred pc-sized cores. We also find that these PBH sub-haloes are distributed in wider PBH haloes of order hundreds of kpc, containing about 63% of objects and mass, coinciding with the sizes of galactic halos. We find at last high rates of close encounters of massive Black Holes (\({ M \sim 1000\,M_\odot}\)), with \({ \Gamma^{\mathrm{S}} = (1.2_{-0.9}^{+5.9}) \times 10^{7}~{\rm yr^{-1}~\rm Gpc^{-3}}}\) and mergers with \({\Gamma^{\mathrm{M}} = 1337 \pm 41~{\rm yr^{-1}~\rm Gpc^{-3}} }\).