The Milky Way is expected to be teeming with black holes, but by their very nature they don’t emit light, so finding them is quite difficult. They are usually caught indirectly, either when studying the motion of things around them or if they're in a feeding frenzy. Now, a new black hole has been spotted thanks to the effects it had on an interstellar gas cloud.
As reported in The Astrophysical Journal Letters, researchers witnessed the peculiar movements of HCN–0.009–0.044, a galactic cloud near the Milky Way's center, roughly 25,000 light-years away. The team used ALMA (the Atacama Large Millimeter/submillimeter Array) to observe the gas in high resolution and discovered that the cloud was swirling around a massive and dense object.
“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," lead author Shunya Takekawa, from the National Astronomical Observatory of Japan, said in a statement. "By analyzing other anomalous clouds, we hope to expose other quiet black holes.”
Only a handful of intermediate-mass black holes are known. Most black holes are either a few times the mass of the Sun or supermassive, like the Milky Way’s Sagittarius A*. The supermassive black holes can be millions to billions of times the mass of the Sun, and we still have many questions about how they formed. One theory suggests that in the early universe, large black holes formed more easily and as they merged, they got bigger and bigger. It seems they might continue to grow by gobbling up intermediate-mass black holes such as this.
“It is significant that this intermediate-mass black hole was found only 20 light-years from the supermassive black hole at the galactic center," co-author Professor Tomoharu Oka, from Keio University, added. "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.”
This is not the first intermediate-mass black hole discovered by the team in this way. In September 2017, they published observations of a swirling gas cloud around a black hole 100,000 times the mass of the Sun.