Dynamical evidence from the sub-parsec counter-rotating disc for a close binary of supermassive black holes in NGC 1068

Abstract

ABSTRACT A puzzle in NGC 1068 is how to secularly maintain the counter-rotating disc (CRD) from 0.2 to $7\,$ pc unambiguously detected by recent ALMA observations of molecular gas. Upon further dynamical analysis, we find that the Kelvin–Helmholtz (KH) instability (KHI) results in an unavoidable catastrophe for the disc developed at the interface between the reversely rotating parts. We demonstrate that a close binary of supermassive black holes (CB-SMBHs) provides tidal torques to prevent the disc from the KH catastrophe and are led to the conclusion that there is a CB-SMBH at the centre of NGC 1068. The binary is composed of black holes with a separation of $0.1\,$ pc from GRAVITY/VLTI observations, a total mass of 1.3 × 107 M⊙ and a mass ratio of ∼0.3 estimated from the angular momentum (AM) budget of the global system. The KHI gives rise to a gap without cold gas at the velocity interface that overlaps with the observed gap of hot and cold dust regions. Releases of kinetic energies from the KHI of the disc are in agreement with observed emissions in radio and γ-rays. Such a binary is shrinking on a time-scale much longer than the local Hubble time via gravitational waves, however, the KHI leads to an efficient annihilation of the orbital AM and a speed-up merge of the binary, providing a new mechanism for solving the long-standing issue of ‘final parsec problem’. Future observations of GRAVITY+/VLTI are expected to be able to spatially resolve the CB-SMBHs suggested in this paper.