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takyon writes:

A larger intermediate mass black hole may have been discovered in the Milky Way galaxy:

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.

[...] 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 [open, DOI: 10.1038/s41550-017-0224-z] [DX], 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.

By comparison, the black hole at Sagittarius A* is thought to have a mass of around 4 million solar masses.

Previously: Possible Intermediate-Mass Black Hole Discovered (2,200 solar masses)

Original Submission