The effect of interstellar medium on LVK's black holes

TL;DR

This paper explores how the interstellar medium can influence the evolution and merger rates of binary black holes detected by LIGO-Virgo-KAGRA, highlighting a potential additional pathway for their orbital hardening.

Contribution

It introduces a toy model demonstrating the ISM's role in binary black hole dynamics and its potential impact on gravitational wave observations.

Findings

ISM can significantly harden BBH orbits over billion-year timescales.

Black hole mass growth rate is proportional to the square of its mass.

ISM properties can influence the population of BBH mergers observed by LVK.

Abstract

Gravitational radiation alone is not efficient in hardening the orbit of a wide binary black hole (BBH). By employing a toy model for the interstellar medium (ISM) surrounding BBHs, here we discuss the effect of this baryonic medium on BBH dynamics. Depending on the BBH's mass, we show that a binary surrounded by an isotropic cold neutral medium (i.e., an asymptotic temperature $T_{\infty} \approx 100$ K) with a time-averaged particle density of $\langle n_H \rangle = \mathcal{O}(1)$ cm$^{-3}$ can play a significant role in hardening the binary orbit over a $\mathcal{O}(10^9)$ yr time scale. Additionally, this causes the black hole's mass to grow at a rate $\propto m^2$. We thus discuss the impact of the ISM on the LIGO-Virgo-KAGRA (LVK) observables and quantify the properties of the ISM under which the latter could act as an additional important pathway for driving a subset of LVK's BBH mergers.