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Hubble may have found the ‘missing link’ in black hole formation

Researchers using the Hubble Space Telescope have found the first evidence for a type of black hole that has only been hypothesized so far.

Astronomers have found plenty of black holes which are either large, being hundreds of millions of times the mass of our sun, or small, being around five times the mass of our sun. But they have found no intermediate black holes, only indirect evidence of them, which raises questions of how black holes merge and grow from small to large.

These intermediate-mass black holes (or IMBHs) form a “missing link” in our understanding of black hole evolution, but they are particularly difficult to find. “Intermediate-mass black holes are very elusive objects,” Dacheng Lin of the University of New Hampshire, principal investigator of the new Hubble study, said in a statement, “and so it is critical to carefully consider and rule out alternative explanations for each candidate. That is what Hubble has allowed us to do for our candidate.”

artist’s impression depicts a star being torn apart by an intermediate-mass black hole
This artist’s impression depicts a star being torn apart by an intermediate-mass black hole (IMBH), surrounded by an accretion disc. This thin, rotating disc of material consists of the leftovers of a star that was ripped apart by the tidal forces of the black hole. ESA/Hubble, M. Kornmesser

As a starting point for their investigations, the researchers took data from NASA’s Chandra X-ray Observatory and the European Space Agency’s X-ray Multi-Mirror Mission, both of which observe the X-ray wavelength and had spotted a powerful flare of X-rays from an unknown source in 2006. By pointing Hubble toward the source of these X-rays, the team was able to see that they were generated not from the center of a galaxy, where you would expect to find a supermassive black hole, but off to one side.

It turned out that the source of the X-rays was a star cluster on the edge of the galaxy, and that the star cluster was around the size that would be expected to host an IMBH. The X-rays appear to have been emitted when an IMBH at the heart of the cluster fed on a star that strayed too close to it.

“Adding further X-ray observations allowed us to understand the total energy output,” team member Natalie Webb of the Université de Toulouse in France said in the statement. “This helps us to understand the type of star that was disrupted by the black hole.”

This research represents the best evidence yet of the identification of an IMBH. And that opens the door to answer many questions about black holes of all sizes: “Studying the origin and evolution of the intermediate mass black holes will finally give an answer as to how the supermassive black holes that we find in the centers of massive galaxies came to exist,” Webb said.

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