Astrophysicists have discovered a dozen black holes —“invisible one-way doors out of our universe” —gathered around Sagittarius A* (Sgr A*), the supermassive black hole in the center of the Milky Way Galaxy, which in May, 2019 suddenly brightened, appearing like a massive, dormant volcano.
Tens of millions of the enigmatic, dark objects in the Milky Way
After conducting a cosmic inventory to calculate and categorize stellar-remnant black holes, astronomers from the University of California Irvine concluded that there are probably tens of millions of the enigmatic, dark objects in the Milky Way – far more than expected. The prevalence of so many massive compact objects has also led researchers to expect more frequent detections of gravitational waves from merging black holes in the near future.
“Weirdness of the LIGO discovery”
“We think we’ve shown that there are as many as 100 million black holes in our galaxy,” said UCI chair and professor of physics & astronomy James Bullock, referring to UCI’s celestial census that began in 2015, shortly after the news that the Laser Interferometer Gravitational-Wave Observatory, or LIGO, had detected ripples in the space-time continuum created by the distant merger of two black holes, each the mass of 30 suns.
“Fundamentally, the detection of gravitational waves was a huge deal, as it was a confirmation of a key prediction of Einstein’s general theory of relativity,” Bullock said. “But then we looked closer at the astrophysics of the actual result, a merger of two 30-solar-mass black holes. That was simply astounding and had us asking, ‘How common are black holes of this size, and how often do they merge?”
UCI’s work was a theoretical investigation into the “weirdness of the LIGO discovery,” Bullock said. The research was an attempt to interpret the gravitational wave detections through the lens of what is known about galaxy formation and to form a framework for understanding future occurrences.
“Based on what we know about star formation in galaxies of different types, we can infer when and how many black holes formed in each galaxy,” said UCI doctoral candidate, Oliver Elbert. “Big galaxies are home to older stars, and they host older black holes too.”
Detect a Massive Invisible Object
In early 2019, a research team led by Shunya Takekawa at the National Astronomical Observatory of Japan noticed HCN-0.009-0.044, a gas cloud moving strangely near the center of the Milky Way 25,000 light-years away from Earth in the constellation Sagittarius. They used ALMA (Atacama Large Millimeter/submillimeter Array) to perform high-resolution observations of the cloud and found that it is swirling around a massive invisible object.
Astronomers detected the stealthy black hole from its effects on an interstellar gas cloud. This intermediate-mass black hole is one of over 100 million quiet black holes expected to be lurking in the galaxy. These results provide a new method to search for other hidden black holes and help us understand the growth and evolution of black holes.
Discovery of an Intermediate-mass Black Hole
Astronomers think that small black holes merge and gradually grow into large ones, but no one had ever found an intermediate-mass black hole weighing hundreds or thousands of times the mass of the sun.
“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, said Takekawa. “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.”
Found Only 20 Light Years from Sgr A*
“It is significant that this intermediate-mass black hole was found only 20 light-years from the supermassive black hole at the galactic center,” adds Tomoharu Oka, a professor at Keio University and co-leader of the team. “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.”
These results were published as Takekawa et al. “Indication of Another Intermediate-mass Black Hole in the Galactic Center” in The Astrophysical Journal Letters on January 20, 2019.
Image top of page: artist’s conception shows two merging black holes similar to those detected by LIGO. LIGO/Caltech/MIT/Sonoma State (Aurore Simonnet)