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 provides tidal torques to prevent the disc from the KH catastrophe and are led to the conclusion that there is a binary black hole at the center 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 \times 10 ^ { 7 } M _ { \odot } and a mass ratio of \sim 0.3 estimated from the angular momentum budget of the global system .
The KHI gives rise to a gap without cold gas at the velocity interface which 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 \gamma -rays .
Such a binary is shrinking on a timescale much longer than the local Hubble time via gravitational waves , however , the KHI leads to an efficient annihilation of the orbital angular momentum 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 .