Indian Researchers Spot Rare Merger Of 3 Supermassive Black Holes In Nearby Universe

Study published as a letter in Astronomy and Astrophysics journal. Scientists say observing the merging black holes will facilitate detection of multiple accreting supermassive black holes

Representative Photo: Getty Images

New Delhi: A team of researchers from the Indian Institute of Astrophysics has detected the merging of three supermassive black holes from three galaxies in our nearby universe, the Union Ministry of Science & Technology said in a statement Thursday.

Their findings, published as a letter in the Astronomy and Astrophysics Journal, explain that the black holes merge to form a triple active galactic nucleus, which is a compact region at the centre of a newly discovered galaxy, having a luminosity much higher than normal levels. Since this event occurred in our nearby universe, the merging black holes can serve as small laboratories because observing them will facilitate the detection of multiple accreting supermassive black holes, and also increase the chances of detection of such unusual phenomena, according to the scientists.

Supermassive black holes do not emit any light, as a result of which it is difficult to detect them. Black holes can be detected only due to radiations emitted from the accretion disc, which is created due to nearby galactic objects being bound to the gravitational field of a black hole. Everything, whether it be dust or gas, that comes in the vicinity of a black hole is either engulfed by it, or converted into energy and emitted in the form of electromagnetic radiation, which makes the black hole appear luminous. Large amounts of ionized particles and energy are released by the black holes, also known as active galactic nuclei (AGN), into the galaxy and the environment of the black hole. This causes the medium around the galaxy to grow, and results in the evolution of the galaxy.

Researchers Jyoti Yadav, Mousumi Das and Sudhanshu Barway, from the Indian Institute of Astrophysics, along with Francoise Combes of College de France, Chaire Galaxies et Cosmologie, Paris, studied a pair of galaxies, NGC7733 and NGC7734, known to be interacting from before. They detected unusual emissions being released from the centre of NGC7734 and a large, bright clump, moving at a velocity different from that of the galaxy NGC7733, along the northern arm of the latter galaxy. They found that this was a small galaxy behind the northern arm of NGC7733 and named it NGC7733N.

Data from the Ultra-Violet Imaging Telescope (UVIT), located in 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 were collected for the study.

About Supermassive Black Holes

Star formation and the creation of tidal tails, which were revealed by Ultra-Violet (UV) and H-alpha images, could have occured due to the merger of NGC7733N with the larger galaxy, indicating the presence of a third galaxy. 

Galaxies are said to interact when they come close to each other and exert enormous amounts of gravitational forces on each other, and this phenomenon results in galaxy evolution. When galaxy interactions occur, the respective supermassive black holes come close to each other, and the dual black holes begin engulfing gas from their surroundings and get converted into a dual Active Galactic Nucleus (AGN). 

A black hole transfers its kinetic energy to the surrounding gas when there is collision of the galaxies consisting of the black hole(s). With time, the distance between the black holes is reduced, until they are 3.26 light-years (around one parsec) apart. This phenomenon, which renders the black holes unable to lose further kinetic energy, in order to come even closer and merge, is called the final parsec problem. This problem can be resolved by the presence of a third black hole, to which the dual merging black holes can transfer their energy, and merge with each other.

The detection of triple AGNs is rare. This recent study by IIA states that triple AGN systems can be expected to be a common occurrence in small merging groups of galaxies.