Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have spotted streams of molecular gas orbiting around an invisible gravitational source with 32,000 times the mass of our Sun in the central region of the Milky Way. This intermediate-mass black hole is one of over 100 million quiet black holes expected to be lurking in our Galaxy.
An artist’s impression of a gas cloud swirling around a black hole. Image credit: NAOJ.
Astronomers know that black holes ranging from about 10 times to 100 times the Sun’s mass are the remnants of dying stars, and that supermassive black holes, more than 100,000 times the mass of the Sun, inhabit the centers of most galaxies.
But scattered across the Universe are a few apparent black holes of a more mysterious type.
Ranging from 100 to 100,000 solar masses, these intermediate-mass black holes are so hard to measure that even their existence is sometimes disputed.
Dr. Shunya Takekawa from the National Astronomical Observatory of Japan and co-authors used ALMA to perform high resolution observations of HCN-0.009-0.044, a gas cloud moving strangely near the center of the Milky Way.
They concluded that HCN-0.009-0.044 is swirling around an invisible massive object.
“Detailed kinematic analyses revealed that an enormous mass — 32,000 times that of the Sun — was concentrated in a region much smaller than our Solar System,” Dr. Takekawa said.
“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.”
HCN-0.009-0.044 is located at a projected distance of just 23 light-years from Sagittarius A*, the supermassive black hole at the center of our Galaxy.
“It is significant that this intermediate-mass black hole was found only 23 light-years from the supermassive black hole at the Galactic center,” said co-author Professor Tomoharu Oka, an astronomer at Keio University.
“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.”
The study appears in the Astrophysical Journal Letters.
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Shunya Takekawa et al. 2019. Indication of Another Intermediate-Mass Black Hole in the Galactic Center. ApJL 871, L1; doi: 10.3847/2041-8213/aafb07
