Contracting and Expanding Binary Black Holes in 3D Low-Mass AGN Disks:
  The Importance of Separation
  LIGO/Virgo has detected several binary black hole (BBH) merger events that
may have originated in the accretion disks of Active Galactic Nuclei (AGN).
These events require individual black hole masses that fall within the pair
instability supernova mass gap, and therefore these black holes may have been
grown from hierarchical mergers. AGN disks are a prime environment for
hierarchical mergers, and thus a potential location for the progenitors of BBH
gravitational wave events. Understanding how a BBH embedded in an AGN disk
interacts with the surrounding environment is thus crucial for determining if
this interaction can lead to its merger. However, there are few high fidelity
simulations of this process, and almost all are two-dimensional. We present the
results from 3D, high-resolution, local shearing-box simulations of an embedded
BBH interacting with an AGN disk. In these first simulations of their kind, we
focus on determining the mass accretion rate and the orbital evolution rate at
different BBH separations. We find that circular, equal-mass BBHs with
separations greater than 10% of their Hill radius contract while accreting at a
super-Eddington rate. At smaller separations, however, our 3D simulations find
that BBHs expand their orbits. This result suggests that it may be difficult
for an AGN disk to push a BBH to merger, but we discuss several mechanisms,
including MHD turbulence and radiative and mechanical feedback, that could
alleviate this difficulty.
