Mergers of black holes and ‘kicks’ that hold a key to puzzles

The fact of black holes having masses over 100 times the solar mass has puzzled the community

October 09, 2021 09:15 pm | Updated October 10, 2021 11:23 am IST

An aerial view of the LIGO detector site near Livingston, Louisiana, U.S. that was released by Caltech/MIT/LIGO Laboratory in 2016.

An aerial view of the LIGO detector site near Livingston, Louisiana, U.S. that was released by Caltech/MIT/LIGO Laboratory in 2016.

Scientists from Chennai Mathematical Institute, with their collaborators, have analysed data from the LIGO-VIRGO observatories and estimated the fraction of the binary black hole mergers detected so far that show potential to form intermediate mass black holes. This throws light on the puzzle of how intermediate mass black holes form.

Black holes form when a massive star undergoes a supernova explosion towards the end of its lifetime. The black hole forms from the remnants of the explosion. However, there are factors that place limits on the mass of a black hole so formed. According to physicist K.G. Arun of Chennai Mathematical Institute, black holes with masses between approximately 45-135 times the solar mass are unlikely to be produced by standard stellar evolution as the pair-instability process either limits the max mass of the black hole or completely disrupts the star during the supernova explosion. What puzzles astronomers and cosmologists is that gravitational wave detectors have seen several such “intermediate mass black holes”.


The two detectors of the Laser Interferometer Gravitational Wave Observatory (LIGO) made the first observation of a pair of binary black holes on September 19, 2014.

Since then with other gravitational wave observatories about 40 mergers have been detected, of which nearly five have masses above 100 times solar mass.

One of the theories of intermediate mass black hole formation has to do with ‘hierarchical growth’. That is, if the black holes exist among a dense cluster of stars, the remnant (black hole) of a merger can pair up with another black hole close by to form a binary. This can eventually merge to form a second remnant which is more massive. This process, happening in a hierarchical manner, can explain intermediate mass black hole formation.

Kicks in mergers

During the mergers, gravitational waves take away energy and linear momentum, as a reaction, the remnant black hole acquires an opposite momentum. This is the “kick” it receives. These kicks can be quite large, giving it a velocity of up to 1000 kilometres per second. If this kick velocity is above the escape velocity of the star cluster in which the black hole is formed, it literally escapes from the environment and moves out. This prevents it from undergoing further hierarchical mergers.

The extent of the kick received by the remnant can be calculated from the masses of the merging black holes and their spin. “As GW observations give an estimate of these, we can calculate the kick imparted to every remnant black hole in the population of binary black holes reported by LIGO/Virgo till date,” says Parthapratim Mahapatra, Ph. D. student, Chennai Mathematical Institute, who is the first author of a paper on this work published in The Astrophysical Journal Letters.

The kick estimates help understand which mergers have the possibility of undergoing further hierarchical mergers and forming into intermediate mass black holes.

There have been recent studies using astrophysical models to understand whether the components of some of the binaries are formed hierarchically. “This is a complementary approach as we are interested in the prospects of the remnants participating in further mergers and not whether the observed binaries contain one or more of the black holes which are hierarchically formed,” says Prof. Arun, in whose lab the work was done, in an email to The Hindu .

Using the state-of-the-art understanding of the escape speeds of star clusters and using the kick magnitudes they have inferred for different observed events, the group has calculated what fraction of the remnants may remain in-cluster (provided they originally merged in the cluster). “We find that as many as 17 out of 40 remnants may be retained by the nuclear star clusters,” says Prof. Arun.

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