Researchers discover rare occurrence of triple black holes merging to form center of newly discovered galaxy

Vijay Mohan

Tribune News Service

Chandigarh, August 27

Indian researchers have discovered a rare occurrence of three super massive black holes from three galaxies merging together to form an active galactic nucleus, a compact region at the center of a newly discovered galaxy, which has a much-higher-than-normal luminosity.

Super massive black holes are difficult to detect because they do not emit any light, but can reveal their presence by interacting with their surroundings. When the dust and gas from the surroundings fall onto a super massive black hole, some of the mass is swallowed by the black hole, while 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 to the evolution of the galaxy itself.

A team of researchers from the Indian Institute of Astrophysics, along with a researcher from College de France, Chaire Galaxies et Cosmologie, Paris, while studying a known interacting galaxy pair, labelled NGC7733, and NGC7734, detected unusual emissions from the centre of NGC7734 and a large, bright clump along the northern arm of NGC7733.

Their investigations showed that the clump is moving with a different velocity compared to the galaxy NGC7733 itself. It was inferred that this clump was not a part of NGC7733; but a small separate galaxy behind the arm. This galaxy was named NGC7733N.

The study, published as a letter in the journal, Astronomy and Astrophysics, used data from the Ultra-Violet Imaging Telescope (UVIT) on board the first Indian space observatory, ASTROSAT, as well as data from the European integral field optical telescope mounted on the Very Large Telescope (VLT) in Chile and infrared images from the optical telescope (IRSF) in South Africa, according to a statement issued by the Ministry of Science and Technology today.

The data and 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 super massive black hole in their nucleus and hence form a very rare triple AGN system.

According to the researchers, 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 interactions, the respective super massive 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 few have been detected so far using X-ray observations. However, the IIA team 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 super massive black holes.