Gas dynamical friction as a binary formation mechanism in AGN disks

With analysis of GW190521 potentially indicating the existence of a high eccentricity Stellar Black Hole Binary, the idea that Active Galactic Nuclei (AGN) are the birthplaces for Binary Black Holes (BBHs) has picked up a lot of steam in recent months.

An accretion disk of gas circling around the Super Massive Black Hole (SMBH), AGN disks are potential hosts to many black holes. sBHs in the AGN are either pulled in, through gas torques, from the surrounding nuclear stellar cluster, or are simply born in the disk through in-situ star formation and evolution. Playing host to these BHs, the environment of the AGN disk can form BBHs through both dynamical and gaseous processes. Our study sheds new light on how gaseous dynamical friction, the slowing of bodies in a medium by the gravitational pull of their wakes, can form bound sBH binaries from two single, close approach, sBHs.

Performing 2D 3-body integrations with REBOUND we find that Gaseous Dynamical Friction not only can form BBHs but it does so quite effectively, being able to capture close approaches of different impact parameters, and in different astrophysical systems. We discover that, unlike previous prescriptions, high levels of Dynamical Friction (equivalently over-dense gas) can actually hinder binary formation. We determine characteristics of the captured binaries, reporting general trends in eccentricity, semi major axis, direction of rotation, precession, and more across astrophysical parameters. We provide an analytic formula that determines the strength of Dynamical Friction in any astrophysical system and subsequently provide numerical limits to such formula, dictating when and where binaries form.

Our work not only provides a strong case for the importance of gaseous capture as a method for BBH formation in AGN, but it also yields a detailed analytic formula that will provide much aid to population synthesis projects in their study of intermediate mass black hole growth, updating older, and not entirely accurate, semi-analytic models. In future work we hope to further study the implications our newly powerful dynamical friction has on the picture of AGN as source for GW events and BBH mergers.