Merging Black Holes in Dwarf Galaxies: Calculating Binary Black Hole Coalescence Timescales from Simulations for LISA Detection

TL;DR

This study estimates the timescales for black hole mergers in dwarf galaxies using simulations and models of dynamical processes, predicting several mergers detectable by LISA within a Hubble time.

Contribution

It introduces a method to calculate black hole coalescence timescales from cosmological simulations, accounting for dynamical friction, loss-cone scattering, and gravitational radiation.

Findings

Five black hole pairs will merge within 0.8-8 Gyr and be detectable by LISA.

Six black hole pairs are unresolved due to simulation resolution limits.

Close SMBH pairs in dwarf galaxies are likely to merge and be detectable by LISA.

Abstract

Supermassive black holes (SMBHs) merging in dwarf galaxies will be detectable by the Laser Interferometer Space Antenna (LISA) in the mid-2030s. Previous cosmological hydrodynamic simulations have shown the prediction of massive black holes merging in dwarf galaxies, but these simulations are limited by their resolution and cannot follow black hole pairs all the way to coalescence. We calculate the delay time between black hole pairing and merger based on the properties of the black holes and their host galaxies, and use these properties to calculate gravitational wave strains for eleven different binary black holes that merge inside dwarf galaxies from eight cosmological simulations. This delay time calculation accounts for dynamical friction due to gas and stars, loss-cone scattering, and hardening of the binary due to gravitational radiation. Out of the eleven black hole mergers in the simulations, five black hole pairs will merge within 0.8 - 8 Gyr of forming a close pair and could be observed by LISA, and the remaining six are unresolved due to resolution limitations of the simulation. As all five of the resolved close pairs merge within a Hubble time, we make the broad estimate that close SMBH pairs in dwarf galaxies will merge and be detectable by LISA, but this estimate depends on either the presence of gas during orbital decay or a solution to the dynamical buoyancy problem in cored potentials.