I Want to Know What Gravity Is

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).

Figure 1: A drawing of the scenario explored in this paper. Two stellar black holes orbit inside a supermassive black hole’s accretion disk (Panel 1) and absorb material from it, gathering their own smaller accretion disks around themselves (Panel 2). As they approach each other (Panel 3), the small accretion disks around each stellar black hole collide and violently interact, absorbing the energy of the stellar black holes’ motion and expelling it in powerful outflows (Panel 4). This energy loss allows them to form a hard binary (a binary which cannot be easily pulled apart by encounters with other celestial objects; Panel 4). This binary then can decay through gravitational wave emission. Source: Figure 1 in the paper

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.

Figure 2: Table with the 15 models and their outcomes. The second column stands for mass ratio of the AGN disk relative to the Shakura-Sunyaev disk, and the third column stands for how widely the black holes are separated in terms of Hill radii. The fourth column indicates whether the binary forms, the fifth column indicates whether the binary orbits prograde or retrograde with respect to the AGN disk, the sixth column indicates the final fate of the binary in the simulation, the seventh column indicates the final fate of the binary including gravitational waves, and the final column indicates whether the eccentricity of the binary is increased or decreased with time after it forms.

Astrobite edited by Ali Crisp

Featured image credit: Lynnie Saade

Disclaimer: I have collaborated and currently collaborate with the final author on research.

About Lynnie Saade

I'm a PhD student at UCLA who works on X-ray observations of supermassive black holes and their active galactic nuclei. I have an unusual hobby of drawing comics and writing stories about personified natural phenomena. I really want to see a story with a black hole being used as an actual character, just as how they (almost) are characters with great impact on their galaxies in real life.

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