Black holes are notoriously hard to find, since they don’t emit any light. But there’s one particular size of black hole that has been especially evasive, even though astronomers have theorized that they should be plentiful.

While there is ample evidence for the extra-large, supermassive kind as well as the much smaller, stellar-mass black holes, the in-between size, or so-called "intermediate mass" black holes, have thus far eluded detection.

Astronomers now think they have found one — a black hole 100,000 times more massive than the sun, lurking inside a gas cloud near the middle of the Milky Way. It's located not far from supermassive black hole called Sagittarius A* that lies at the dead center of our galaxy.

Researchers think that intermediate-mass black holes (IMBH) could be a missing link that helps explain how supermassive black holes are formed.

“The observations are rather compelling, and it's going to be exciting as astronomers follow up this source,” astronomer and black hole researcher Kevin Schawinski of the Swiss Federal Institute of Technology in Zurich, who was not involved in the current study, told Seeker.

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Last year, a team of researchers led by Tomoharu Oka from Keio University in Japan discovered “a peculiar molecular cloud” named CO–0.40–0.22, which was located 200 light-years away from the Milky Way’s center. The cloud had what the team called “extremely broad velocity width,” meaning it was moving very fast with varying velocities they could not explain. The researchers suspected a massive object was hiding inside, providing the gravitational kick for the variable and speedy gas flows.

Oka and his team followed up their observations using the Atacama Large Millimeter/submillimeter Array (ALMA) Observatory in Chile, and have now reported their findings in a paper published this week in the journal Nature Astronomy. With ALMA’s extremely high-precision data, the researchers were able to confirm the wide distribution of velocities inside the gas cloud, but they also found a telltale clue: a spectrum of radio waves very similar to what Sagittarius A* produces, but about 500 times fainter.

“Based on the careful analysis of gas kinematics, we concluded that a compact object with a mass of about [100,000] solar masses is lurking in this cloud,” Oka and his team wrote. They said numerical simulations suggest that CO–0.40–0.22 is one of the most promising candidates for an intermediate-mass black hole.

“If confirmed by others, having such an intermediate mass black hole in our Milky Way is going to open up so many exciting possibilities,” Schawinski said via email. “If there's one, maybe there are others?”

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While astronomers understand how stellar-mass black holes form, the origins of supermassive black holes remain unknown. A stellar-mass black hole forms when a massive star goes supernova. This explosion, which can outshine an entire galaxy of stars for a short period of time, leaves behind the small, heavy core of a star. If this core is massive enough, it will collapse on itself and form a black hole. A typical stellar-class of black hole can be between approximately three and 10 solar masses.

Supermassive black holes have masses ranging from millions to billions of solar masses. Why they are so incredibly massive isn't well understood, but astronomers think they may form out of the collapse of gigantic clouds of gas during the early stages of the formation of a galaxy. Since the SMBHs are at the center of galaxies, they have ample stars, gas, and dust to feed on, so they can grow quickly. And since many galaxies collide repeatedly during their long lifetimes, the supermassive black holes collide and coalesce into even heavier supermassive black holes.

How do intermediate-mass black holes form? One idea is that they come from runaway coalescence of stars in young compact star clusters. If these are plentiful, as astronomers theorize, they could merge at the center of a galaxy to form a supermassive black hole. In fact, since CO–0.40–0.22 is so close to the black hole at the center of our galaxy, it very likely could be gobbled up by Sagittarius A*.

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But Oko and his team posit that CO–0.40–0.22 used to be the nucleus of a dwarf galaxy that was slowly drawn into the Milky Way. This coincides with data that suggests that large galaxies and their supermassive black holes grow by cannibalizing their smaller neighbors. The Milky Way is thought to be currently absorbing several smaller dwarf galaxies, such as the Canis Major Dwarf Galaxy, and possibly the Magellanic Clouds.

Schawinski agrees.

“The most likely explanation is that this black hole was once in the core of a dwarf galaxy which got cannibalized by the Milky Way,” he said. “Knowing that nature has a way to make black holes with a mass of a hundred thousand solar masses really fills in a gap and may hold important clues to the formation of all massive black holes. Maybe we'll even be lucky enough to see it feed on some surrounding gas!”

Oka and his team said they will continue to observe CO-0.40-0.22 at multiple wavelengths to learn more about it, and they also have hints of other compact molecular clouds that may be hiding other intermediate mass black holes.

“Further detection of such compact high-velocity features in various environments may increase the number of non-luminous black hole candidate and thereby increase targets to search for evidential proof of general relativity,” Oka concludes. “This would make a considerable contribution to the progress of modern physics.”

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