Indian astrophysicists spot rare merger of three jumbo black holes

The study has been published as a letter in the ‘Astronomy and Astrophysics’ journal

Image of a black hole released by the Event Horizon Telescope team. Image for representation.

A rare merging of three supermassive black holes has been spotted by a team of astrophysicists in India. They were observing the merging of two galaxies named NGC7733 and NGC 7734 in our celestial neighbourhood when they detected unusual emissions from the centre of the latter and a curious movement of a large bright clump within it, having a different velocity than that of NGC7733. Inferring that this was a separate galaxy, the scientists named it NGC7733N. There are supermassive blackholes, which are several million solar masses in size, at the centres of galaxies, and these are known as Active Galactic Nuclei. Since they “accrete“ matter, they often have a glow around them which can be observed using light spectroscopy.

All three merging black holes were part of galaxies in the Toucan constellation. They are quite far away when you think that our nearest galactic neighbour – the Andromeda galaxy is 2.5 million light years away. Yet the paper describes these as nearby galaxies. “In Astronomy everything is relative. When we study solar system we say Mercury is closer and Jupiter is far… Compared to our nearest neighbour Andromeda galaxy, the galaxies NGC7733, 7734 and 7733 N are quite far away, but compared to the size of universe they are nearby galaxies,” says Jyoti Yadav, a PhD student at Indian Institute of Astrophysics and the first author of the paper published as a letter in the journal Astronomy and Astrophysics.

The study used data from the Ultraviolet Imaging Telescope (UVIT) onboard the first Indian space observatory ASTROSAT, the European integral field optical telescope called MUSE mounted on the Very Large Telescope (VLT) in Chile and infrared images from the optical telescope (IRSF) in South Africa.

In an email to The Hindu , Mousumi Das from Indian Institute of Astrophysics, an author of the paper says that they were studying the active galactic nuclei in the two massive barred spiral galaxies NGC7733 and NGC7734 and that the detection of the third was surprising. “It was a bit like buy two and get one free,” says Dr Das. “The PI of the project confirmed our suspicions using spectroscopic data from a European telescope called MUSE in Chile.”

The group observed these galaxies with a near infrared telescope in South Africa. “Then, later on, because they appeared interesting, we also observed them with the UVIT [onboard ASTROSAT]” says Dr Das. “We also found optical data in the MUSE archive. So, we did not have to do the optical spectroscopy.”

Final parsec

In a Press Information Bureau release, the team explains that if two galaxies collide, their black hole will also come closer by transferring the kinetic energy to the surrounding gas. The distance between the blackholes decreases with time until the separation is around a parsec (3.26 light-years). The two black holes are then unable to lose any further kinetic energy to get even closer and merge. This is known as the final parsec problem. The presence of a third black hole can solve this problem. “The two can come closer when another black hole or a star passes by and takes away some of their combined angular momentum,” explains Dr Das. Thus, the dual merging blackholes merge with each other in the presence of a third.

Many Active Galactic Nuclei (AGN, supermassive black hole at the centre of a galaxy) pairs have been detected in the past, but triple AGN are extremely rare, and only a handful has been detected before using X-ray observations. “Multiple accreting black holes [AGN] maybe more common in our universe and especially common in galaxy groups. So the growth of black holes may be driven by such mergers in groups,” she says. This is only the third detection of such a system.

“What is striking about this work is the use of several telescopes around the world (and one in space) to determine the presence of 3 AGN. In fact, this use of multiple telescopes and observing bands is essential to carry out good astronomical research. This also demonstrates how astrophysics is a truly collaborative science,” says Preeti Kharb  who is with National Centre for Radio Astrophysics - Tata Institute of Fundamental Research (NCRA-TIFR), Pune, who was not involved in this work.