Title: Runaway Eccentricity Growth: A Pathway for Binary Black Hole Mergers in AGN Disks
Authors: Josh Calcino, Adam M. Dempsey, Alexander J. Dittmann, and Hui Li
First Author’s Institution: Los Alamos National Laboratory, Los Alamos, NM
Status: Published in ApJ [open access]:
The Ligo-Virgo-Kagra (LVK) collaboration has observed roughly 90 stellar mass gravitational wave merger events, with a few that push our fundamental understanding of how black holes form and evolve. For example, GW190521 was an event in which black holes of 85 solar masses and 65 solar masses merged to form a black hole of 142 solar masses, with 8 solar masses radiating away as gravitational waves. The pair-instability supernova gap is a theory that puts an upper limit of about 50 solar masses on the masses of black holes that can form from single stars. We usually refer to black holes above this mass as ‘intermediate-mass black holes‘ or IMBHs (though the exact definition of an IMBH is contentious in some astronomical circles). The black holes of GW190521 exceed this limit. Since the LVK has observed a “too-heavy-to-exist from stars alone” IMBH merger, how might it have happened?
If the pair-instability supernova gap does prevent individual stellar black holes from forming in this heavier mass range, researchers have a few theories about what would cause these IMBH mergers. One idea is hierarchical merging. Two black holes from a binary stellar system merge, and the resultant black hole finds another merging companion elsewhere. Additionally, if the stellar neighborhood is dense enough, multiple black holes might naturally find each other, though this only happens in very dense regions of stars called nuclear star clusters near the centers of galaxies. Another idea is that these mergers are from primordial black holes, a different type of black that formed in the very early universe but whose existence has yet to be confirmed. A final idea, the focus of today’s paper, is that these mergers happen in the disks of active galactic nuclei, or AGN.
We observe AGN from some supermassive black holes (SMBHs) at the centers of some galaxies. Though we believe that nearly every galaxy has an SMBH at its center, not every one of these SMBHs is an AGN. An AGN is characterized by its massive brightness, often on par with the rest of its entire host galaxy. Of course, SMBHs don’t themselves emit light. The enormous amount of light comes from the superheated gas swirling around the region of high gravitational attraction near the SMBH. This gas forms a disk around the black hole called an accretion disk. Generally, the term “AGN” refers to the whole SMBH/accretion disk system (though this is a simplification – an AGN usually has a few additional parts).
We have three reasons to think AGNs are a rich area to produce IMBH mergers:
- AGNs, located in the centers of galaxies, usually have dense nuclear star clusters around them, so they are already in a high-density stellar mass black hole environment.
- When these stellar black holes end up in the AGN accretion disk, they gain mass from attracting the gas around them, which can increase their masses beyond what they would be from the stellar black hole formation process alone.
- The accretion disk has a friction effect on the stellar black holes, which causes stellar black holes that fall into the disk to find each other more easily and merge more quickly once they are in a binary system with another stellar black hole that ends up in the disk.
Today’s authors model the mergers of stellar black hole binaries in an AGN accretion disk to evaluate how plausible these mergers are as IMBH candidates. First, they investigate the effect of a prograde vs. a retrograde orientation. In this case, prograde and retrograde refer to the orientation of the black hole binary’s angular momentum to the AGN’s angular momentum. In a prograde orientation, the angular momenta of the binary black hole system aligns with the angular momentum of the AGN. In a retrograde orientation, the angular momentum of the binary black hole system is anti-aligned with the angular momentum of the AGN, meaning the angular momentum vector of the AGN is pointing in the exact opposite direction as the total angular momentum vector of the binary black hole system. The authors also investigate the impact of the eccentricity of the binary black holes’ orbits. In the case of a binary system, eccentricity is how much the two objects’ orbits deviate from being circular around the center of mass of the two-body system. An eccentricity of 0 means each body has a nearly perfectly circular orbit around the center of mass. In contrast, an eccentricity close to but less than 1 means the objects follow highly elliptical orbits around the center of mass. For this paper, the authors investigate both prograde and retrograde orbits of eccentricity 0.0, 0.1, 0.3, 0.5, and 0.7. Figures 1 and 2 show snapshots of the binary system in the prograde and retrograde orbits for some of these eccentricities.
Figure 1 (Figure 3 in the paper) – Gas density and velocity for the prograde binary black hole simulations. Gas density is represented by color, with dark purple being the least dense and yellow/tan being the most dense. The blue lines with arrows represent the gas speed and direction. Each column represents a specific orbital eccentricity, with 0.3 on the left, 0.5 in the middle, and 0.7 on the right. Each x and y axis refers to the distance in units of the orbit’s elliptical semi-major axis. The top row represents the orbits at the pericenter, the point in the orbit where the orbiting objects are closest, and the bottom row shows the orbits at the apocenter, where they are furthest apart.
Figure 2 (Figure 3 in the paper) – Similar to Figure 1, but with the binary black holes in a retrograde orientation (opposite angular momentum vectors) with respect to the AGN instead of a prograde orientation (aligned angular momentum vectors).
Figure 3 (Figure 8 from the paper) – the amount of mass accreted for six selected binary black hole systems over twenty orbits. The x-axis represents the number of orbits of the binary system, and the y-axis represents the amount of mass accreted. The cold colors represent the prograde orbits at the three eccentricities: 0.1 (light blue), 0.3 (mid-blue), and 0.5 (dark blue). The warm colors represent mass accreted by the retrograde orbits at three eccentricities: 0.1 (yellow). 0.3 (orange) and 0.7 (red). The authors find that retrograde binaries accrete more mass in shorter times on average than prograde binaries and higher eccentricity orbits accrete more than lower eccentricity orbits. .
One quantity they examine is how much mass the binaries accrete. This accretion is an indicator that these binaries may acquire enough mass to be an infamous IMBH. Their simulations show that retrograde-oriented orbits tend to accrete mass more quickly. In addition to this mass accretion rate, they also find that retrograde binaries increase in eccentricity over multiple obits (typically, isolated black hole binaries – binary systems out in space with no other major influence like an AGN – have orbits that become smaller and more circular through a process called ‘hardening‘). This unexpected growth in eccentricity brings them closer at the pericenter of the orbit, which means that gravitational wave emission can decay the orbit more quickly, allowing the merge to happen sooner. From this observation, the authors conclude that their simulations show further evidence that AGN disks may be a rich soil to form the IMBHs above the stellar mass range, especially if those black hole systems have retrograde orientations to their host AGN and have highly eccentric orbits.
Astrobite edited by: Erica Sawczynec
Featured image credit: Caltech/R. Hurt (IPAC)
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