This black hole is evidently a hundred thousand times more massive than the sun.
Molecular clouds swirl around a possible black hole. [Image by Keio University/NOAJ]
Astronomers have found the best evidence yet of a long-rumored black hole that could be the second-largest one in the Milky Way galaxy so far. Its existence will also serve as evidence that our galaxy has grown by consuming other smaller galaxies.
Last year, a team of astronomers at Keio University in Yokohama reported that they detected a strange molecular gas cloud, which the team named CO-0.40-0.22, near the center of the Milky Way. The gas in the cloud was moving at vastly different velocities, indicating that the cloud was concealing something massive. According to simulations, a black hole one hundred thousand times more massive than the sun could be the culprit behind the strange movements of the gas.
Since making this discovery, the team has been studying the cloud with other instruments, including the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.
Sagittarius A*, the supermassive black hole at the center of the Milky Way [Image by NASA - CXC - MIT - Frederick K. Baganoff et al.]
The cloud is 150 trillion kilometers wide and lies 200 light years away from the heart of the Milky Way. According to the simulations, the purported black hole in the cloud is about 1.4 trillion kilometers across.
The black hole, named CO-0.40-0.22*, is therefore mid-sized, which makes it a rarity. Astronomers have found small black holes, which are as massive as 10 suns, or supermassive black holes, which are as massive as millions or billions of suns. So far, this may be the first mid-sized black hole ever found.
Looking for a black hole isn't easy, as they don't emit their own light. Usually, astronomers can detect the presence of a black hole by the way it affects the things around it. If astronomers detect peculiar gravitational disruptions, it may mean that there's something massive nearby, likely a black hole.
The researchers say that the black hole may have formed when the Milky Way cannibalized a dwarf galaxy. Some scientists think that mid-sized black holes arise at the hearts of dense star clusters in the Milky Way, but the researchers that found CO-0.40-0.22* think that the black hole didn't arise that way. They suggest that the Milky Way subsumed a smaller galaxy, stripped it of its stars, and kept its black hole core. If the researchers confirm that CO-0.40-0.22* is indeed a black hole, then they have evidence that galaxies grow by cannibalizing smaller galaxies.
The Milky Way may be growing by consuming smaller galaxies.
This, however, isn't the end of CO-0.40-0.22*. Eventually, it will fall into Sagittarius A*, the purported supermassive black hole at the center of the Milky Way. When some smaller black holes get consumed by a massive black hole, it makes the massive black hole even more massive than it already is. Thus, at some point in the future, CO-0.40-0.22* will feed into Sagittarius A*.
For now, however, the researchers will keep observing CO-0.40-0.22*. There's still a lot left to find out about CO-0.40-0.22*—for example, we aren't even sure yet whether or not it is indeed a black hole. Also, the researchers are set to confirm how massive CO-0.40-0.22* really is. They're looking at other molecular clouds that could harbor black holes as well.
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