Figure 1: Spatial distribution of molecular gas at the center of the Milky Way Galaxy, which is observed with wavelengths of 0.87 mm emitted from carbon monoxide molecules. The black cross mark indicates the position of "Sagittarius A*," the nucleus of the Milky Way Galaxy. (Credit: Keio University)
A research team at Keio University, led by Associate Professor Tomoharu Oka, has discovered intermediate-mass black hole (IMBH) candidates at the center of the Milky Way Galaxy. It is about 30,000 light-years from the solar system in the direction of Sagittarius. IMBH candidates are considered to be the "seeds" that form and grow massive black holes.
Using radio telescopes, the research team led by Associate Professor Oka has found four "warm, dense (more than 50 degrees Kelvin, more than 10,000 hydrogen molecules per cubic centimeter)" masses of molecular gas at the center of the Milky Way Galaxy. Three of those masses of molecular gas have been expanding.
Figure 2: Spatial distribution of "warm, dense gas" (above) at the center of the Milky Way Galaxy and its velocity distribution (below). Distribution of the whole molecular gas is indicated by pale white. It can be understood that "warm, dense gas" is localized in four areas, and all the gas is moving at a fast speed. (Credit: Keio University)
This research suggests that supernova explosions caused the expansion. It is estimated that the largest explosion that occurred in the masses of molecular gas is equivalent to 200 supernova explosions. On the other hand, the age of the gas masses is approximately 60,000 years old. Therefore, it can be inferred that a huge star cluster is buried in one of the gas masses. The mass of the cluster (more than 100,000 times the mass of the Sun) is comparable to the largest star cluster found in the Milky Way Galaxy.
It is thought that IMBHs are formed within such huge star clusters. Eventually, IMBHs born near the center of the Milky Way Galaxy form/expand into a supermassive black hole at the nucleus of the galaxy.
Education and Public Outreach Officer, NAOJ
Associate Professor, Keio University
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