Indian researchers have discovered three supermassive black holes from three galaxies merging together to form a triple active galactic nucleus, a compact region at the centre of a newly discovered galaxy that has a much-higher-than-normal luminosity.

What are Supermassive black holes?

• A supermassive black hole is the largest type of black hole, with mass on the order of millions to billions of times the mass of the Sun.
• Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light.
• They are difficult to detect because they do not emit any light. But they can reveal their presence by interacting with their surroundings.

Active galactic nuclei (AGN) from such black holes

• 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.
• This is emitted back 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.

How does merger of black holes occur?

• 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.

What happens when galaxies collide?

• 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 in order to get even closer and merge.
• This is known as the final parsec problem.

Here comes the third black hole

• 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.
• The presence of a third black hole can solve this problem.
• The dual merging blackholes can transfer their energy to the third blackhole and merge with each other.

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