Pairing of Massive Black Holes in Merger Galaxies Driven by Dynamical Friction

TL;DR

This study models the orbital evolution of massive black hole pairs in merger galaxies, revealing how galaxy properties influence the likelihood and timescale of close pair formation, with implications for gravitational wave sources and dual AGNs.

Contribution

Develops a semi-analytic model to predict MBH pairing timescales based on galaxy and black hole properties, covering a wide parameter space.

Findings

Over 80% pairing probability within a Hubble time in certain galaxy conditions.

Fastest MBH pair formation occurs in galaxies with large stellar bulges, comparable mass MBHs, or rotating gas disks.

Short inspiral times are associated with circular prograde or eccentric retrograde orbits.

Abstract

Motivated by observational searches for massive black hole (MBH) pairs at kiloparsec separations we develop a semi-analytic model to describe their orbital evolution under the influence of stellar and gaseous dynamical friction (DF). The goal of this study is to determine how the properties of the merger remnant galaxy and the MBHs affect the likelihood and timescale for formation of a close MBH pair with separation of < 1 pc. We compute approximately 40,000 configurations that cover a wide range of host galaxy properties and investigate their impact on the orbital evolution of unequal mass MBH pairs. We find that the percentage for MBH pairing within a Hubble time is larger than 80% in remnant galaxies with a gas fraction < 20% and in galaxies hosting MBH pairs with total mass > 10^6 solar mass and mass ratios > 1/4. Among these, the remnant galaxies characterized by the fastest formation of close, gravitationally bound MBHs have one or more of the following properties: (1) large stellar bulge, (2) comparable mass MBHs and (3) a galactic gas disk rotating close to the circular speed. In such galaxies, the MBHs with the shortest inspiral times, which are likely progenitors of coalescing MBHs, are either on circular prograde orbits or on very eccentric retrograde orbits. Our model also indicates that remnant galaxies with opposite properties, that host slowly evolving MBH pairs, are the most likely hosts of dual AGNs at kiloparsec separations.