Efficiency and Timescale of SMBH Coalescence Following Galaxy Mergers

Determine whether mergers of galaxies efficiently result in mergers of their central supermassive black holes (SMBHs) by quantifying the characteristic timescale required to reduce the separation between two SMBHs from kiloparsec scales to milliparsec scales (the gravitational-wave radiation regime) so that coalescence occurs within a Hubble time.

Background

The paper argues that the spin properties of Sgr A* are best explained by a past major black hole merger rather than by coherent or chaotic accretion alone. However, even if mergers can produce the observed high spin and misalignment, a key uncertainty is whether typical galaxy mergers actually progress to SMBH mergers within cosmological timescales.

This uncertainty is related to the classical "final parsec problem," namely whether dynamical mechanisms (e.g., stellar scattering, gas torques) can efficiently shrink SMBH binaries from galactic (kiloparsec) separations to the gravitational-wave regime (milliparsecs) within a Hubble time. Establishing this efficiency and timescale is essential to validate the merger-origin scenario for Sgr A* and to predict detection rates for space-borne gravitational-wave observatories.