Indian researchers spot rare triple black hole system, Ministry of Science and Technology announce

In a rare occurrence, a merger of three supermassive black holes form as many galaxies in our nearby universe was spotted by a team of Indian researchers, the Ministry of Science and Technology announced.

This indicates that small merging groups are ideal laboratories to detect multiple accreting supermassive black holes and increases the possibility of detecting such rare occurrences. Supermassive black holes are difficult to detect because they do not emit any light. But they can reveal their presence by interacting with their surroundings.

The astrophysicists were studying a known interacting galaxy pair – NGC7733 and NGC7734 – when they detected unusual emissions from the center of the latter and a large, bright clump along the northern arm of NGC7733, moving with a different velocity compared to the galaxy itself. Further investigation led to the discovery of a small, separate galaxy behind the arm. They named this galaxy NGC7733N.

This study, published as a letter in the journal Astronomy and Astrophysics, used data from the Ultra-Violet 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.

The UV and H-alpha images also supported the presence of the third galaxy by revealing star formation along with the tidal tails, which could have formed from the merger of NGC7733N with the larger galaxy. Each of the galaxies hosts an active supermassive black hole in their nucleus and hence forms a very rare triple AGN system.

When the dust and gas from the surroundings fall onto a supermassive black hole, some of the mass is swallowed by the black hole, but some of it is converted into energy and emitted as electromagnetic radiation that makes the black hole appear very luminous. They are called active galactic nuclei (AGN) and release huge amounts of ionized particles and energy into the galaxy and its environment. Both of these ultimately contribute to the growth of the medium around the galaxy and ultimately the evolution of the galaxy itself.

According to the researchers from the Indian Institute of Astrophysics, a major factor impacting galaxy evolution is galaxy interactions, which happen when galaxies move close by each other and exert tremendous gravitational forces on each other. During such galaxy interactions, the respective supermassive black holes can get near each other. The dual black holes start consuming gas from their surroundings and become dual AGN.

Many AGN 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. However, the team of researchers expects such triple AGN systems to be more common in small merging groups of galaxies. Although this study focuses only on one system, results suggest that small merging groups are ideal laboratories to detect multiple supermassive black holes.



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