As part of a recent research, a team of Japanese astronomers has spotted what seems to be the second largest black hole in all of Milky Way. Weighing around 100 thousand times more than our Sun, it was detected with the help of the Nobeyama 45-m Radio Telescope, and is believed to be an “intermediate mass” black hole. According to the scientists, this rare discovery could help enhance our understanding about the formation of supermassive black holes, situated at the center of galaxies.
Led by professor Tomoharu Oka of Japan’s Keio University, the team has observed signs of an unusual gas cloud, which they named CO-0.40-0.22, just 200 light-years from the center of our galaxy. Characterized by surprisingly wide velocity dispersion, the cloud is composed of gases moving at widely differing speeds. For the research, the scientists took the help of two radio telescopes, namely the Nobeyama 45-m Telescope in Japan and Chile’s ASTE Telescope, both of which are operated by the National Astronomical Observatory of Japan.
Possessing an elliptical shape, the cloud was found to consist of two distinct components: a low density yet compact section with incredibly wide velocity dispersion of about 100 km/s, and a much denser part, spread over 10 light-years, with very narrow velocity dispersion. The absence of holes inside the cloud, as verified by X-ray and Infrared observations, indicates that the wide velocity dispersion is likely not the result of any local energy input, like supernova explosions.
For greater clarity, the team conducted a simulation of gas clouds, surrounded by a powerful gravity source. In the experiments, the gas clouds were first attracted by the source, as a result of which their speeds increased while approaching it, acquiring the maximum value at a point nearest to the object. As the clouds travelled past the object, their speeds were found to steadily decrease. Based on their observations, the researchers have concluded that a black hole with mass of nearly 100 thousand times that of the Sun, and area with radius of around 0.3 light-years, could likely explain the gas cloud’s wide velocity dispersion. Speaking about the research, recently published in the Astrophysical Journal Letters, Oka said:
Considering the fact that no compact objects are seen in X-ray or infrared observations, as far as we know, the best candidate for the compact massive object is a black hole.
If the observations by the scientists are correct, the discovery would be the first evidence of an intermediate mass black hole (IMBH), which until now was merely a hypothetical class of black holes. So far, astronomers are aware of two other sizes of black holes: supermassive black holes usually located at the center of galaxies, and stellar-mass black holes, which are formed as a result of massive explosions of gigantic stars. Possibly the largest of its kind in the universe, the mass of a supermassive black hole (SMBH) ranges from several million to billions of times the Sun’s mass. Although quite a few SMBHs have been discovered over the years, astronomers are still unsure as to how these strange bodies are formed in the first place.
Of the many hypotheses currently existing in the scientific world, one pertains to the belief that they are created from mergers of several intermediate black holes. If the cloud CO-0.40-0.22 does indeed contain an intermediate black hole, it might support the merger hypothesis, given that it is situated only 200 light-years away from Sagittarrius A* (or Sgr A*), the supermassive black hole present at the cente of Milky Way. The research is significant as it could improve our understanding of how SMBHs are formed.
The study, according to the scientists, could usher in a new way of detecting black holes using radio telescopes. As the team points out, there are a total of 100 million black holes in our galaxy. Of these, only a few dozens have been found via X-ray observations. The Milky Way is home to several wide-velocity-dispersion clouds, similar to CO-0.40-0.22, some of which might indeed contain black holes. Oka added:
Investigations of gas motion with radio telescopes may provide a complementary way to search for dark black holes. The on-going wide area survey observations of the Milky Way with the Nobeyama 45-m Telescope and high-resolution observations of nearby galaxies using the Atacama Large Millimeter/submillimeter Array (ALMA) have the potential to increase the number of black hole candidates dramatically.