A team of astronomers believe that they have discovered the second largest black hole in our Milky Way Galaxy. If proven true, this is the first detection of an intermediate mass black hole.
Astronomers already know about two sizes of black holes: stellar-mass black holes, formed after the gigantic explosions of very massive stars; and supermassive black holes (SMBH) often found at the centers of galaxies. The mass of SMBH ranges from several million to billions of times the mass of the Sun.
A number of SMBHs have been found, but no one knows how the SMBHs are formed. One idea is that they are formed from mergers of many intermediate mass black holes. But this raises a problem because so far no firm observational evidence for intermediate mass black holes has been found.
If the cloud contains an intermediate mass black hole, it might support the intermediate mass black hole merger scenario of SMBH evolution.
The scientific team led by Tomoharu Oka, a professor at Keio University in Japan, found the enigmatic gas cloud, called CO-0.40-0.22, only 200 light years away from the center of the Milky Way Galaxy. What makes the cloud unusual is its surprisingly wide velocity dispersion: the cloud contains gas with a very wide range of speeds.
The team performed a simple simulation of gas clouds flung by a strong gravity source. In the simulation, the gas clouds are first attracted by the source and their speeds increase as they approach it, reaching maximum at the closest point to the object. After that the clouds continue past the object and their speeds decrease. The team found that a model using a gravity source with 100 thousand times the mass of our Sun inside an area with a radius of 0.3 light years provided the best fit to the observed data.
"Considering the fact that no compact objects are seen in X-ray or infrared observations," Oka, the lead author of the paper that appeared in the Astrophysical Journal Letters, explains "as far as we know, the best candidate for the compact massive object is a black hole."
These results could 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 also 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. "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."
ABOVE IMAGE: Molecular clouds scattered by an intermediate black hole show very wide velocity dispersion. This scenario well explains the observational features of a peculiar molecular cloud CO-0.40-0.22. Credit: Keio University