Title: Black Hole Binary Formation in AGN Discs: From Isolation to Merger
Authors: Connar Rowan, Tjarda Boekholt, Bence Kocsis, and Zoltán Haiman
First Author’s Institution: Rudolf Peierls Center for Theoretical Physics, Clarendon Laboratory
Status: Uploaded to ArXiV
The average stellar black hole leads a lonely and hungry existence. Too small to pull in gas from the interstellar medium to accrete, and not one of the lucky few with a helpful donor star, they orbit invisibly in the darkness, condemned to eternal silence by a lack of matter to eat.
Those that have gathered near the supermassive black hole (SMBH) at the center of their galaxy are more fortunate, for their far grander brother has the gravity necessary to pull in the interstellar gas and build an accretion disk around itself. And, it is willing to share. The stellar black holes can orbit inside the accretion disk of the SMBH and feed on the gas procured by it…and perhaps, according to today’s paper, in addition to filling their bellies, they can fill their hearts as well by finding a companion to merge with.
Through the Dust Clouds, I See Love Shine
In recent years, LIGO has detected gravitational waves from many merging stellar black holes in the distant Universe. However, most of the stellar black holes detected in this manner have been larger than the ones known in our own Milky Way. One possible way to explain this is that the mergers take place inside the accretion disk of an active galactic nucleus (AGN). In this scenario, stellar black holes in the vicinity of an actively accreting SMBH will be able to rapidly accrete matter from the AGN disk and grow to the larger sizes implied by LIGO. In addition to a source of food, the dense gas in an AGN disk may also enable two nearby stellar black holes to pair up into tight binaries by sapping the black holes’ orbital energy (a diagram of this process is shown in Figure 1).
However, models have disagreed under what conditions this process can actually happen. The authors of today’s paper run 15 different models, varying the initial separation between the two stellar black holes and the mass of the AGN accretion disk in order to get a better handle on this. They model the accretion disk of the AGN as an annulus with a width 20 times the stellar black holes’ Hill radii, centered on an SMBH weighing 2×106 solar masses. Two equal mass black holes 25 solar masses in weight are inserted into the accretion disk on a circular orbit, about 20 degrees in azimuth apart from each other. The simulations are then left to run, taking into account gravitational and hydrodynamic effects.
Looks Like Love has Finally Found Me
The authors start with a single model for comparison. In this model, the mass of the AGN accretion disk is typical, and the stellar black holes are separated by 2.5 Hill radii. Initially, the two stellar black holes are an unbound system. At around ~40 years of orbiting, they dramatically lose energy as they capture each other and become bound. The black holes eagerly gorge on the surrounding gas at a rate approaching 10 solar masses per year. Their orbits about each other progressively shrink, making the binary spiral tighter and tighter. It would seem in this model’s case, the black hole dinner date is successful.
Surprisingly, it is not dynamical friction with the gas itself that slows the black holes down enough to pull each other into a binary orbit. Rather, it is the momentum transfer from the process of accretion itself, as they tend to absorb gas moving directly into them and against their motion. The dynamical friction does play a role in shrinking the binary after it forms, but the initial work necessary to make the black holes pair up is done by accretion. Perhaps the best way to a black hole’s heart is through its stomach.
After discussing the initial model, the authors turn to the other 14 models. Their results are summarized in the table in Figure 2. They find that the stellar black holes are more likely to fail to pair up in systems with either larger initial separation or lower AGN disk masses. The former is self-explanatory, but the latter is somewhat interesting. They note it is due to the fact that a lower disk mass means there is less food for the black holes to consume and grow their masses. The mass growth increases the reach of their gravitational dominance and makes it easier for them to capture each other. Nevertheless, they find successful binary captures in all ranges of disk masses. Actual progress to merger is less certain, however, with whether it occurs strongly dependent on the specific moment at which the black holes make their closest approach.So while a supermassive black hole may be able to play matchmaker for its lower mass brethren, the dates may not always result in marriage. It would seem that even in highly-warped space-times, first impressions are everything.
Astrobite edited by Ali Crisp
Featured image credit: Lynnie Saade
Disclaimer: I have collaborated and currently collaborate with the final author on research.
