The bridge between Supermassive and Stellar Mass Black Holes
Black holes have always been a point of fascination for everyone. Importantly, they help us in understanding the formation and evolution of the galaxies. They have been a part of Science Fiction since years. This interest towards these extremely dense objects, in the scientific community has led to several discoveries. I am sure we were all awed at the first ever image of a supermassive black hole unveiled last year. It was a joint effort of scientists and astronomers across the globe. The black hole in the talk, M87*, sits at the heart of the gargantuan elliptical galaxy, 53 million light years away, M87 (Messier 87).
Until recently, we only knew about two broad categories of black holes. One is the stellar-mass black holes, whose sizes range from 3-4 to tens of solar masses and another category is of the supermassive black holes that have masses of the order of millions to billions of solar masses. It is known that all the large galaxies have supermassive black holes at its centre.But how do the supermassive black holes which sit at the heart of the galaxies form and evolve over time? The answer to this question is still unknown but a possible explanation could be merger between the smaller black holes to form these gargantuan black holes. But then again, the question arises:
How many ants will you require to make an elephant?
That is, how many stellar mass black holes have to merge together to form a supermassive black hole. Is there any region so dense, that has thousands of stellar mass black holes? Questions, questions and questions. But now, scientists have discovered something which they predict as the answer to many questions like these- Intermediate Mass Black Holes (which have masses between 100 to 1000 Solar Masses).
On May 21, 2019, the ears that have been hearing the gravitational waves since 2015 (LIGO), when they were first discovered, heard something unusual. The Laser Interferometer Gravitational-Wave Observatory (LIGO) detected a brief signal, called GW190521, that came from 7 billion light years away and lasted for about one-tenth of a second. The International team of scientists has suspected various events that could have caused the signal but the explanation that fits the astrophysical data the most is a black hole merger. The team has predicted that the signal is caused due to a merger between two black holes of masses 85 and 66 times the mass of the Sun. The merger has resulted in a black hole of mass 142 Solar masses.
But, here is a twist in the tale. According to theoretical models, there exists a ‘pair instability mass gap’ in which black holes ranging between 65 and 120 solar masses cannot form by a collapsing star. No matter how massive the exploding star is, it can never result in a black hole of more than 65 solar masses. Then what has been the cause of formation of the two black holes whose merger have resulted in the formation of a black hole of 142 solar masses? Due to this mass limit, the two black holes might also have formed due to the merger of stellar mass black holes. Or there might be a whole new perspective to this tale, which is still unknown to us because we are a part of an infinite universe with infinite possibilities and uncountable probabilities.
References and Further Readings
 Bartels, Meghan. “Gravitational waves point scientists to elusive missing-link black hole” Space.com, Science and Astronomy News, 2nd Sept.2020,https://www.space.com/black-hole-intermediate-size-ligo-gravitational-waves-discovery.html
 By VOA News. “Researchers DIscover First Intermediate-size Black Hole”, voanews.com, Science and Health, 2nd Sept.2020, https://www.voanews.com/science-health/researchers-discover-first-intermediate-size-black-hole
 Buongiorno, Caitlyn. “Scientists Detect First Mid-Sized Black Hole via Gravitational Waves”, discovermagazine.com, The sciences, 3rd September 2020,https://www.discovermagazine.com/the-sciences/scientists-detect-first-mid-sized-black-hole-via-gravitational-waves
 LIGO Scientific Collaboration. “GW190521”, ligo.org, LSC, 2nd Sept, 2020, https://www.ligo.org/detections/GW190521.php
 NASA’s Goddard Space Flight Center, “A New Kind of Black Hole”, nasa.gov, Feature, 2ndSept.2020,