The closest ever binary supermassive black hole system in a spiral galaxy called NGC 7674, located about 400 million light years from earth, has been discovered by Dr. Preeti Kharb and Dr. Dharam Vir Lal from NCRA-TIFR, Pune, and David Merritt from the Rochester Institute of Technology, USA.
The obvious separation of the two black holes in the binary system is less than one light year. This indeed is much less than the earlier recordholder, which was a black hole binary with a separation of almost 24 light years.
The two compact radio sources separated by less than a light year at the center of the galaxy NGC7674. The two sources correspond to the location of the two active supermassive black holes which form a binary and orbit around each other. (CREDIT: TIFR-NCRA and RIT, USA)
This discovery is extremely important since this is a direct observational proof of the presence of close supermassive black hole binary systems within galaxies, which are promising sources of gravitational waves.
Black holes are considered to be amongst the most fascinating objects and even more the binary black-holes (two black holes orbiting around each other). The presence of binary black holes, approximately 10 times the mass of Sun, got established in 2015 by the latest detection of gravitational waves by the LIGO telescope.
Astrophysicists have long predicted the presence of a second class of binaries, comprising of supermassive black holes, each containing a mass upwards of one million times the mass of the Sun. Single supermassive black holes are believed to be present at the centers of most galaxies and it is possible to develop gravitationally bound black hole pairs since galaxies are observed to merge with other galaxies. As time moves on, these two supermassive black holes will be able to merge through the emission of gravitational waves.
Very long baseline interferometry (VLBI) is a technique that was used to detect the above binary system. In this technique, it is possible to separate radio telescopes from over the world to function together as a single huge telescope, attaining an angular resolution of milli- or microarcseconds - approximately 10 million times the angular resolution of the human eye. Two compact sources of radio emission were detected at the center of NGC 7674 by using the VLBI techniques.
The two radio sources have properties that are known to be associated with massive black holes that are accreting gas, implying the presence of two black holes.
Dr Preeti Kharb, NCRA-TIFR, Pune
The combined mass of the two black holes is approximately 40 million times the mass of the Sun. Kharb et al. assume that the orbital period of the binary will be about one hundred thousand years.
"Detection of a binary supermassive black hole in this galaxy also confirms a theoretical prediction that such binaries should be present in so-called Z-shaped radio sources," David Merritt stated. NGC 7674 is indeed such a radio source. The very name "Z-shaped" refers to the twisted morphology of the galaxy's radio emission on much bigger scales. This morphology is believed to result from the joint effects of the galaxy merger followed by the development of the massive binary.