Gap formation and its consequence in the evolution of SMBHs binaries in galaxy mergers

Abstract

Motivated by the theoretical and observational evidence that after a major merger of gas-rich galaxies a massive gaseous disk with a SMBH binary will be formed in the nuclear region of the remnant, we study the interaction between an unequal mass binary and an isothermal circumbinary disk. We focus our study in the transport of angular momentum from the binary to the disk and how this transport can result on the formation of a cavity or gap in the gaseous disk. We propose that, for comparable mass binaries, this exchange of angular momentum is driven by the gravitational interaction of the binary and a strong non-axisymmetric density perturbation that is formed in the disk as response to the presence of the gravitational field of the binary. We compare the efficiency of this gravitational torque on extract angular momentum from the binary with the efficiency of redistribution of the extracted angular momentum in the disk to derive a gap-opening criterion. We run a set of SPH simulations of binaries embedded in isothermal gaseous disks to test our gap-opening criterion. We find that our gap-opening criterion successfully predicts in which simulations a gap will form on the disk. We also run simulations of merging galaxies and we apply our criterion to the more real SMBHB-Disk systems that are formed in situ in these simulations. We find that, in the conditions of our galaxy mergers simulations, the formation of a circumbinary gap is unlikely.