Astronomers track two main weight classes of black holes, supermassive black holes and stellar-mass black holes.

Supermassive black holes such as the Sagittarius A* found at the center of the Milky Way are millions, even billions, of times the solar mass. Stellar-mass black holes, on the other hand, are only about 10 times the mass of the sun.

Intermediate-Mass Black Holes

Scientists expect that middleweight black holes, which fall between these two extremes, also exist. These intermediate-mass black holes (IMBH), expected to weigh between 100 and 10,000 the mass of the sun, however, are elusive.

No conclusive proof that this class of black hole exists has yet been found but astronomers studying a globular star cluster located about 13,000 light-years away have announced finding new evidence that hint the presence of an intermediate-mass black hole with solar mass of about 2,200.

Scientists reported in the journal Nature on Feb. 9 that an example of the elusive intermediate-mass black hole is hiding at the center of the 12-billion-year-old star cluster called 47 Tucanae.

IMBH At The Center Of 47 Tucanae Star Cluster

The cluster holds thousands of stars, which radiate a combined luminosity half a million times more than that of the sun. The cluster also contains about two dozen pulsar stars that produce precisely timed radio signals.

The middleweight black hole does not have a surrounding gas that emits X-rays or radio waves, which means that it can only be observed based on its effects on the stars orbiting it.

"The existence of an intermediate-mass black hole in the centre of one of the densest clusters with no detectable electromagnetic counterpart suggests that the black hole is not accreting at a sufficient rate to make it electromagnetically bright and therefore, contrary to expectations, is gas-starved," researchers wrote in their study.

Scientists looked at the overall motions of the stars throughout the stellar cluster: where the environment is very dense, heavier stars tend to get to the center.

A black hole at the heart of the cluster acts like a sort of a cosmic spoon that stirs the pot and causes the stars to get flung to greater distances and at higher speeds providing astronomers a subtle signal to measure.

Study researcher Bülent Kızıltan, from the Harvard-Smithsonian Center for Astrophysics in Cambridge, and colleagues also tracked the motion of the 23 pulsars in the cluster and found the positions and accelerations of the pulsars imply the presence of a black hole that is about 2,200 solar masses.

Pulsars get flung about by the gravity of the central IMBH so they are found at greater distances from the center of the cluster than when a black hole does not exist.

Researchers said that intermediate-mass black holes may grow into supermassive black holes.

During the early days of the Milky Way, a globular cluster may have passed near the center of the galaxy, spilling the stars in it and an IMBH there. That black hole may have grown into the supermassive Sagittarius A*.

"We want to find intermediate-mass black holes because they are the missing link between stellar-mass and supermassive black holes. They may be the primordial seeds that grew into the monsters we see in the centers of galaxies today," Kiziltan said.

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