Long-term Evolution of Tightly-Packed Stellar Black Holes in AGN Disks: Formation of Merging Black-Hole Binaries via Close Encounters

Understand

We study the long-term evolution of two or more stellar black holes (BHs) on initially separated but unstable circular orbits around a supermassive BH (SMBH).

• Such a close-packed orbital configuration can naturally arise from BH migrations in the AGN disk.
• Dynamical instability of the orbits leads to recurring close encounters between two BHs, during which the BH separation $r_{\rm p}$ becomes less than the Hill radius $R_{\rm H}$.
• In the rare very close encounters (with $r_{\rm p}$ several orders of magnitude less than $R_{\rm H}$), a tight merging BH binary can form with the help of gravitational wave emission.