Black holes merging with low mass gap objects inside globular clusters

Abstract

Recently, the LIGO and Virgo collaborations have reported the coalescence of a binary involving a black hole and a low-mass gap object (LMGO) with mass in the range approximately $2.5--5\text{ }\text{ }{M}_{\ensuremath{\bigodot}}$. Such detections challenge our understanding of the black hole and neutron star mass spectrum, as well as how such binaries evolve especially if isolated. In this work, we study the dynamical formation of compact object pairs, via multiple binary-single exchanges that occur at the cores of globular clusters. We start with a population of binary star systems, which interact with single compact objects as first generation black holes and LMGOs. We evaluate the rate of exchange interactions leading to the formation of compact object binaries. Our calculations include all possible types of binary-single exchange interactions and also the interactions of individual stars with compact object binaries that can evolve their orbital properties, leading to their eventual merger. We perform our calculations for the full range of the observed Milky Way globular cluster environments. We find that the exchanges are efficient in forming hard compact object binaries at the cores of dense astrophysical stellar environments. Furthermore, if the population size of the LMGOs is related to that of neutron stars, the inferred merger rate density of black hole--LMGO binaries inside globular clusters in the local Universe is estimated to be about $0.1\text{ }\text{ }{\mathrm{Gpc}}^{\ensuremath{-}3}\text{ }{\mathrm{yr}}^{\ensuremath{-}1}$, with the full range of rate values being between ${10}^{\ensuremath{-}3}$ to $1\text{ }\text{ }{\mathrm{Gpc}}^{\ensuremath{-}3}\text{ }\text{ }{\mathrm{yr}}^{\ensuremath{-}1}$.