The cosmological distribution of compact object mergers from dynamical interactions with SMBH binaries

Abstract

ABSTRACT We combine sophisticated high-precision scattering experiments, together with results from the Millenium-II simulation, to compute the cosmic merger rate of bound compact object (CO) binaries dynamically interacting with supermassive black hole binaries (SMBHBs). We consider binaries composed of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). The overall merger rates for WD–WD, NS–NS, BH–BH, BH–NS binaries, and EBBH (eccentric binaries of BHs) from redshift ∼5 are found to be $4.32\times 10^3\, \mathrm{yr}^{-1}(5.93\times 10^2\, \mathrm{yr}^{-1}$ for Type Ia SNe), $82.7\, \mathrm{yr}^{-1}$, $96.3\, \mathrm{yr}^{-1}$, $13.1\, \mathrm{yr}^{-1}$, and $148\, \mathrm{yr}^{-1}$, respectively, for a nominal CO binary fraction in the Galactic centre of 0.1. We calculate the distance (R) distribution of the merger sites with respect to the host galaxies of the binaries. The distribution shows a wide range of distances up to ∼Mpc; this tail is produced by escaped hypervelocity CO binaries. Due to the differences in the matter density of the surrounding environment, merger events with different R are expected to display significantly different signatures in their EM counterparts. In particular, merger events (and especially NS–NS) producing a relativistic jet but occurring in the intergalactic medium will have very weak afterglow radiation relative to their prompt emission. These events, which we call ‘off-centre’, can only be produced from a close encounter between CO binaries and SMBHBs; hence the detection of such merger events would indicate the existence of nearby SMBHBs, and in particular with high mass ratio, produced in the aftermath of a major galaxy merger.