A Stray Black Hole that Swirls Gas Clouds ―More Reliable Evidence of an Intermediate-Mass Black Hole Lurking around the Center of the Milky Way―
February 28, 2019
National Astronomical Observatory of Japan Keio University
The Atacama Large Millimeter/submillimeter Array (ALMA) captured details of a previously unknown structure of the peculiar molecular cloud located near the center of the Milky Way. Analysis of its motion revealed the existence of a black hole with a mass 30,000 times that of the Sun. This result shows that there is a possibility that many other similar black holes are hidden around the center of the Milky Way.
Many galaxies are known to have a supermassive black hole at their nucleus, but their formation process is regarded as one of the big mysteries of the universe. In theory, it is thought that intermediate-mass black holes, which have masses of about several hundred to 100,000 times that of the Sun, act as "seeds," which then merge and grow to form a supermassive black hole. However, even though there have been several reported cases of intermediate-mass black holes, no definitive evidence proving their existence has yet been obtained.
For this research, a team consisting of Shunya Takekawa, a project researcher at the National Astronomical Observatory of Japan’s Nobeyama Radio Observatory, Professor Tomoharu Oka of the Keio University Faculty of Science and Technology, Department of Physics, and others, used ALMA to carry out a high resolution radio wave observation of a molecular gas cloud with an unusual velocity discovered about 20 light-years away from Sagittarius A*, the nucleus of the Milky Way. From this, it seemed that this molecular gas cloud was made up of multiple gas streams having orbital motions influenced by a strong pull of an "invisible gravitational source." Detailed kinematic analyses revealed that an enormous mass equivalent to 30,000 times that of the Sun was concentrated in a region much smaller than our Solar System. This strongly suggests that a heavy intermediate-mass black hole is drifting near the nucleus of the Milky Way. The findings of this research are extremely important not only because they lead to an explanation of the origins of supermassive black holes and an understanding of galactic evolution, but also because they lead to the possibility of opening new doors for the investigation of black holes.
The findings of this research were published in the January 20 issue of the American astrophysics journal, The Astrophysical Journal Letters.