Orbital evolution of binary black holes in active galactic nucleus disks: a disk channel for binary black hole mergers?

Abstract

We perform a series of high-resolution 2D hydrodynamical simulations of equal-mass binary black holes (BBHs) embedded in active galactic nucleus (AGN) accretion disks to study whether these binaries can be driven to merger by the surrounding gas. We find that the gravitational softening adopted for the BBH has a profound impact on this result. When the softening is less than ten percent of the binary separation, we show that, in agreement with recent simulations of isolated equalmass binaries, prograde BBHs expand in time rather than contract. Eventually, however, the binary separation becomes large enough that the tidal force of the central AGN disrupts them. Only when the softening is relatively large do we find that prograde BBHs harden. We determine through detailed analysis of the binary torque, that this dichotomy is due to a loss of spiral structure in the circum-single disks orbiting each BH when the softening is a significant fraction of the binary separation. Properly resolving these spirals -both with high resolution and small softening -results in a significant source of binary angular momentum. Only for retrograde BBHs do we find consistent hardening, regardless of softening, as these BBHs lack the important spiral structure in their circum-single disks. This suggests that the gas-driven inspiral of retrograde binaries can produce a population of compact BBHs in the gravitational-wave-emitting regime in AGN disks, which may contribute a large fraction to the observed BBH mergers.

The hydrodynamic evolution of binary black holes embedded within the vertically stratified disks of active galactic nuclei

“…In the limiting case of an effectively infinite angular momentum reservoir, one might expect accretion to resemble the commonly studied thin circumbinary disk ('CBD', Muñoz et al 2019Muñoz et al , 2020, where BBHs are expected to expand rather than contract due to positive gravitational torques from the surrounding gas. This is similar to the scenario explored by Baruteau et al (2011) and more recently by Li et al (2021), who performed global simulations of an AGN disk with an embedded binary system in two dimensions. In Li et al (2021), they found that prograde embedded binaries also expand, as in the case of CBDs.…”

“…This is similar to the scenario explored by Baruteau et al (2011) and more recently by Li et al (2021), who performed global simulations of an AGN disk with an embedded binary system in two dimensions. In Li et al (2021), they found that prograde embedded binaries also expand, as in the case of CBDs. We emphasize, however, that for these works to globally resolve the AGN disk and the binary, they are required to use large values of q and model the system in two dimensions.…”

“…However, as we will find in the following section, for much of the q − H/R parameter space, H R w and the resulting flow is inherently threedimensional, supported partially by rotation and partially by pressure gradients. This suggests a dichotomy in the geometry of accretion flows surrounding embedded binaries: in thin accretion disks hosted by lower mass ( 10 6 − 10 7 M ) SMBHs, the embedded binary may be better represented by the more traditional two-dimensional accretion flows studied by Baruteau et al (2011) and Li et al (2021). In either thicker AGN disks (i.e., for high Eddington ratios, or in the outer Toomre-stabilized regions discussed in the next section) or for more massive ( 10 7 M ) SMBHs, the embedded binaries may be better represented by the vertically stratified accretion flows studied here.…”

“…These prescriptions are often motivated by more detailed hydrodynamic simulations of accreting binaries (e.g., Antoni et al 2019), yet hydrodynamic studies of embedded BBHs remain rare. Notable exceptions include Baruteau et al (2011) and more recently Li et al (2021), who both simulated the two-dimensional hydrodynamic evolution of the AGN disk coupled with the orbital evolution of the embedded BBH. However, these works focused on large mass ratios (appropriate for 10 6 − 10 7 M SMBHs) and thin AGN disks.…”

Accretion-Induced Collapse of Neutron Stars in the Disks of Active Galactic Nuclei

“…Although the binary orbit has been found to expand in some cases (i.e. for circular, near-equal-mass, prograde binaries embedded in relatively warm circumbinary disks; Tang et al 2017;Miranda et al 2017;Muñoz et al 2019;Tiede et al 2020;Duffell et al 2020;Heath & Nixon 2020), a circumbinary disk, fueled by the AGN disk, is likely to decrease the binary's semi-major axis in more realistic thin disks (Tiede et al 2020;Heath & Nixon 2020) and for all retrograde binaries (Nixon et al 2011;Li et al 2021). We therefore assume that the semi-major axis and eccentricity further evolve due to type I/II migration torques from a circumbinary disk.…”