A team of Japanese astronomers using the ALMA telescope has captured details of a previously unknown structure located near the center of the Milky Way. Their analysis of its motion revealed that it’s an intermediate-mass black hole – nowhere near as massive as the supermassive, 4-million-solar-mass black hole directly in the galaxy’s center – but still a powerfully massive object with some 30,000 times more heft that of our sun.
These astronomers said they think this intermediate-mass black hole might be one of over 100 million such quiet black holes lurking – unseen – in our Milky Way galaxy. Their work was published in the peer-reviewd journal The Astrophysical Journal Letters on January 20, 2019.
Black holes tend to fall into two categories: fairly small – about five times the mass of the sun – or supermassive – millions of times the mass of the sun. But in recent years astronomers have found evidence of another class of black hole somewhere in between. These are the intermediate-mass black holes, which are thought to have masses of about 100 to 100,000 times that of the sun.
Until now, no one had yet found one.
Black holes in general are defined by their massiveness. They’re objects with such strong gravity that everything, including light, that passes too near them is sucked inside and cannot escape. Because black holes don’t emit light, astronomers have to infer their existence from the effects a black hole’s gravity produces in other objects. The astronomers detected this intermediate-mass black hole from its effects on an interstellar gas cloud.
The cloud is labeled HCN–0.009–0.044. Astronomers had noticed it was moving strangely, near the center of the Milky Way. A team from the National Astronomical Observatory of Japan used the ALMA telescope in northern Chile to perform high-resolution observations of the cloud. In other words, they saw it more clearly than anyone else had seen it before. In so doing, they found that the cloud was swirling around an invisible massive object.
The team’s discovery suggests that many other similar black holes could be hidden around the center of the Milky Way. Why hidden? Unless the black holes are affecting some nearby object, we might never have a reason to suspect they are there.
Detailed kinematic analyses revealed that an enormous mass, 30,000 times that of the sun, was concentrated in a region much smaller than our solar system. This and the lack of any observed object at that location strongly suggests an intermediate-mass black hole.
By analyzing other anomalous clouds, we hope to expose other quiet black holes.
Astronomers know of many supermassive black holes. They appear to reside at the centers of most galaxies. A theory suggests that intermediate-mass black holes might merge with each other and grow by swallowing surrounding material in order to form supermassive black holes, and the new discovery supports that theory. Tomoharu Oka is a professor at Keio University and a study co-author. Oka said:
It is significant that this intermediate mass black hole was found only 20 light-years from the supermassive black hole at the galactic center. In the future, it will fall into the supermassive black hole; much like gas is currently falling into it.
This supports the merger model of black hole growth.
Bottom line: Astronomers have discovered an intermediate mass black hole near the center of our Milky Way galaxy.
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