A Very Close Binary Black Hole in a Giant Elliptical Galaxy 3C 66B and its Black Hole Merger

TL;DR

This paper reports the detection of a very close binary black hole in galaxy 3C 66B, providing evidence for black hole mergers and insights into supermassive black hole formation.

Contribution

It presents the first observational evidence of a close binary black hole with detailed orbital parameters in a giant elliptical galaxy.

Findings

Detected a 93-day flux variation indicating a binary black hole orbit

Estimated the black hole masses and orbital separation

Calculated the black hole merger decay time of a few hundred years

Abstract

Recent observational results provide possible evidence that binary black holes (BBHs) exist in the center of giant galaxies and may merge to form a supermassive black hole in the process of their evolution. We first detected a periodic flux variation on a cycle of $93\pm1$ days from the 3-mm monitor observations of a giant elliptical galaxy \object{3C 66B} for which an orbital motion with a period of $1.05\pm0.03$ years had been already observed. The detected signal period being shorter than the orbital period can be explained by taking into consideration the Doppler-shifted modulation due to the orbital motion of a BBH. Assuming that the BBH has a circular orbit and that the jet axis is parallel to the binary angular momentum, our observational results demonstrate the presence of a very close BBH that has the binary orbit with an orbital period of $1.05\pm0.03$ years, an orbital radius of $(3.9\pm1.0) \times 10^{-3}$ pc, an orbital separation of $(6.1^{+1.0}_{-0.9}) \times 10^{-3}$ pc, the larger black hole mass of $(1.2^{+0.5}_{-0.2}) \times 10^9$ $M_{\sun}$, and the smaller black hole mass of $(7.0^{+4.7}_{-6.4}) \times 10^8$ $M_{\sun}$. The BBH decay time of $(5.1^{+60.5}_{-2.5})\times 10^2$ years provides evidence for the occurrence of black hole mergers. This Letter will demonstrate the interesting possibility of black hole collisions to form a supermassive black hole in the process of evolution, one of the most spectacular natural phenomena in the universe.