February 12, 2019
National Astronomical Observatory of Japan
Keio University
The Atacama Large Millimeter/submillimeter Array (ALMA) has captured the unprecedentedly detailed structure of a peculiar molecular cloud near the center of the Milky Way. Analysis of its motion has revealed the presence of a black hole with a mass 30,000 times that of the Sun. This result suggests that many other such black holes may be lurking near the center of the Milky Way.
While it is known that many galaxies have supermassive black holes at their centers, with masses ranging from millions to tens of billions of times that of the Sun, how they formed is one of the great mysteries in the universe. Theoretically, it is believed that supermassive black holes are formed when "intermediate-mass black holes," with masses ranging from hundreds to one hundred thousand times that of the Sun, act as "seeds" that merge and grow. However, despite several reported cases, conclusive evidence for the existence of intermediate-mass black holes has not yet been obtained.
A research team led by Shunya Takekawa (a project researcher at the Nobeyama Radio Observatory, a branch of the National Astronomical Observatory of Japan) and Professor Tomoharu Oka of the Department of Physics, Faculty of Science and Technology, Keio University, used ALMA to conduct high-resolution radio observations of a molecular gas cloud with an anomalous velocity, discovered about 20 light-years away from the center of the Milky Way, Sagittarius A* (Sgr A*). They captured how this molecular gas cloud, composed of multiple gas streams, is orbiting as if being strongly pulled by an "invisible gravitational source." Detailed kinematic analysis revealed that an enormous mass, equivalent to 30,000 times that of the Sun, is concentrated in a region much smaller than our solar system. This strongly suggests that a massive intermediate-mass black hole is drifting near the galactic center.
This research is an extremely important achievement not only because it contributes to elucidating the origin of supermassive black holes and understanding galactic evolution, but also because it has the potential to open a new door for black hole exploration by studying the motion of surrounding gas. The results of this research were published in the January 20 issue of the American astrophysics journal "The Astrophysical Journal Letters."
For the full press release, please see below.