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The southern sky showing the Milky Way and its dark lanes of dust hiding Sagittarius A*, the supermassive black hole at the galactic center. Nearby is the gas cloud CO-0.40-0.22, probable home to an intermediate-mass black hole. 

ESO/S. Guisard

Long-rumored midsized black hole may be hiding out in the Milky Way

Astronomers have found the best evidence yet for the existence of a midsized black hole—long-rumored objects bigger than the small black holes formed from a single star, yet far smaller than the the giant ones lurking at the centers of galaxies—and it’s hiding out in our own Milky Way. If the discovery is confirmed, it could indicate that our galaxy has grown by cannibalizing its smaller neighbors.

“It’s a very careful paper and they have gorgeous data. It’s the most promising evidence so far” for an intermediate mass black hole, says astronomer Kevin Schawinski of the Swiss Federal Institute of Technology in Zurich.

Black holes are hard to see because they don’t emit their own light. But they can be detected by their influence on nearby objects, for example if the black hole is in a binary pair with a star, or if it is consuming gas which gets heated as it approaches and shines brightly. Astronomers have long found evidence for small, star-sized black holes—up to about 10 times the sun’s mass—and supermassive ones, containing millions or billions of solar masses, in galactic cores.

But intermediate-sized black holes have eluded detection. The best candidates so far have been so-called ultraluminous x-ray sources in nearby galaxies. But researchers are divided over whether these are really midsized black holes, shining bright as they imbibe lots of surrounding gas, or smaller ones ingesting at a superfast rate.

Last year, a team led by Tomoharu Oka of Keio University in Yokohama, Japan, reported finding a peculiar cloud of molecular gas, called CO-0.40-0.22, near the center of our Milky Way. Gas in the cloud, detected with the National Astronomical Observatory of Japan’s 45-meter Nobeyama radio telescope, was moving with a very wide range of velocities, some of it so fast that the team suspected something very massive was hiding there. Simulations of the gas movements suggested it harbored a black hole of 100,000 solar masses.

Since then, the team has studied the cloud with other instruments, in particular the Atacama Large Millimeter/submillimeter Array (ALMA), a collection of 66 dishes high in the Chilean Andes that observes shorter wavelengths than traditional radio observatories. The wide spacing of the dishes—they can be positioned up to 16 kilometers apart—gives the array the ability to see very fine detail in distant objects.

As Oka and colleagues report today in Nature Astronomy, when they studied CO-0.40-0.22 with ALMA they spotted a particularly dense clump of gas near the center of the cloud that again showed a distribution of velocities suggestive of a massive nearby object, again supported by simulations of the gas movements. And right next to the clump was a faint source of radio waves. The spectrum of that source appeared very similar to that of Sagittarius A* (Sgr A*), the radio source believed to be the supermassive black hole at the center of the Milky Way, but 500 times less luminous. This similarity with Sgr A* “supports the notion that CO-0.40-0.22* [the asterisk denoting the radio source] is an intermediate-mass black hole,” Oka says.

But how did it form and how did it get there? Some believe midsized black holes are born in the cores of dense star clusters, of which there are about 150 in the Milky Way, but Oka’s team says CO-0.40-0.22* is way too big to have arisen that way. Instead, the scientists suggest it is the former core of a dwarf galaxy that has been subsumed into the Milky Way, stripped of its stars, and is destined to one day fall into Sgr A*. There are some 50 dwarf galaxies in the vicinity of the Milky Way and if CO-0.40-0.22* is confirmed as a black hole, this would support the idea that galaxies grow through such cannibalism. Schawinski says that the Large Magellanic Cloud, a nearby dwarf galaxy visible in the southern sky, “if it has a black hole, could end up in a similar place.”

Oka says his team will continue to observe CO-0.40-0.22* at other wavelengths and keep an eye on it long-term to see whether it shows variations in brightness known as quasi-periodic oscillations, which are highly characteristic of accretion disks around black holes; that would give the scientists a better handle on the black hole’s mass. The team also has its eyes on several other compact molecular clouds that could harbor black holes.

“The most exciting thing is the likelihood that intermediate mass black holes are real,” Schawinski says. “We know very little about how black holes form.” But if Oka’s team or others are able to find a population of such objects, “we can put our ideas to the test.”