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 \pm 1 days from the 3-mm monitor observations of a giant elliptical galaxy ( catalog 3C 66B ) for which an orbital motion with a period of 1.05 \pm 0.03 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 \pm 0.03 years , an orbital radius of ( 3.9 \pm 1.0 ) \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 .