Science Daily reports that Japanese observers have just found the second-largest black hole in the Milky Way – by looking at clouds of gas:
Astronomers using the Nobeyama 45-m Radio Telescope have detected signs of an invisible black hole with a mass of 100 thousand times the mass of the Sun around the center of the Milky Way. The team assumes that this possible “intermediate mass” black hole is a key to understanding the birth of the supermassive black holes located in the centers of galaxies.
A team of astronomers led by Tomoharu Oka, a professor at Keio University in Japan, has found an enigmatic gas cloud, called CO-0.40-0.22, only 200 light years away from the center of the Milky Way. What makes CO-0.40-0.22 unusual is its surprisingly wide velocity dispersion: the cloud contains gas with a very wide range of speeds.
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These results open a new way to search for black holes with radio telescopes. Recent observations have revealed that there are a number of wide-velocity-dispersion compact clouds similar to CO-0.40-0.22. The team proposes that some of those clouds might contain black holes. A study suggested that there are 100 million black holes in the Milky Way Galaxy, but X-ray observations have only found dozens so far. Most of the black holes may be “dark” and very difficult to see directly at any wavelength. “Investigations of gas motion with radio telescopes may provide a complementary way to search for dark black holes” said Oka.