Astronomers observe largest-ever merger between ‘impossible’ black holes

Researchers have observed the most massive black hole collision ever detected, using the gravitational wave observatories LIGO and VIRGO. The two monstrous black holes crashing into each other are the 88 times the mass of the sun and 66 times the mass of the sun respectively, and their collision gave off such tremendous energy that it was detectable through ripples in space-time called gravitational waves.

The finding is surprising not only because of its epic scale — there’s also a strange mystery surrounding the larger of the black holes. Astronomers often find black holes that are relatively small at less than 100 times the mass of the sun, called stellar black holes, and or those which are very large at millions or billions of times the mass of the sun, called supermassive black holes. But those in between these two sizes, called intermediate-mass black holes, are rare.

Artist’s impression of binary black holes about to collide
Artist’s impression of binary black holes about to collide Mark Myers, ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav)

At 85 times the mass of the sun, the larger black hole seems to be impossible because when a star of that size collapses, it typically reaches a stage called pair instability supernova in which it blows apart in an enormous explosion, leaving nothing behind. So this black hole can’t have been created by the collapse of a star and must have been formed in some other way.

Astronomers therefore think that the larger black hole must have been formed by a previous merger of two smaller black holes.

“We think of black holes as the vacuum cleaners of the universe,” co-author Susan Scott from the Australian National University explained in a statement. “They suck in everything in their paths, including gas clouds and stars. They also suck in other black holes and it is possible to produce bigger and bigger black holes by the ongoing collisions of earlier generations of black holes. The heavier ‘impossible’ black hole in our detected collision may have been produced in this way.”

This also means the final black hole created by the collision has a mass of around 142 times the mass of the sun, which puts it in the rare intermediate black hole (IMBH) zone. This is the first time that a black hole of this size has been observed forming through a merger.

“Every observation we make of two black holes colliding gives us new and surprising information about the lives of black holes throughout the universe,” postdoctoral researcher Vaishali Adya said in the statement. “We are beginning to populate the black hole mass gaps previously thought to exist, with `impossible’ black holes that have been revealed through our detections.”

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