The Pairing Probability of Massive Black Holes in Merger Galaxies in the Presence of Radiative Feedback

TL;DR

This study investigates how radiative feedback, specifically negative dynamical friction, reduces the likelihood of massive black hole pairs forming in galaxy mergers, impacting gravitational wave source predictions.

Contribution

It introduces a semi-analytic model to quantify the impact of negative dynamical friction on MBH pairing probabilities in merger remnants, highlighting conditions that suppress pairing.

Findings

Negative DF reduces MBH pairing probability by ~46%.

Gas-rich, rotating disks and low-mass MBHs are most affected.

Implications for LISA gravitational wave source predictions.

Abstract

Dynamical friction (DF) against stars and gas is thought to be an important mechanism for orbital evolution of massive black holes (MBHs) in merger remnant galaxies. Recent theoretical investigations however show that DF does not always lead to MBH inspiral. For MBHs evolving in gas-rich backgrounds, the ionizing radiation that emerges from the innermost parts of their accretion flow can affect the surrounding gas in such a way to cause the MBHs to accelerate and gain orbital energy. This effect was dubbed "negative DF". We use a semi-analytic model to study the impact of negative DF on pairs of MBHs in merger remnant galaxies evolving under the combined influence of stellar and gaseous DF. Our results show that for a wide range of merger galaxy and MBH properties negative DF reduces the MBH pairing probability by $\sim 46\%$. The suppression of MBH pairing is most severe in galaxies with one or more of these properties: (1) a gas fraction of $f_g \geq 0.1$; (2) a galactic gas disk rotating close to the circular velocity; (3) MBH pairs in prograde, low eccentricity orbits, and (4) MBH pairs with mass $< 10^8\,$M$_\odot$. The last point is of importance because MBH pairs in this mass range are direct progenitors of merging binaries targeted by the future space-based gravitational wave observatory LISA.