Japanese astronomers using two radio telescopes have produced a map of carbon monoxide distribution in the central region of our Galaxy. While mapping the region, they identified three huge gas clumps and a number of intermediate-mass black hole candidates.

From 2005 to 2010, the team using the Atacama Submillimeter Telescope Experiment in Chile and the Nobeyama Radio Observatory (NRO) 45-m telescope observed emission lines at wavelengths of 0.87 and 2.6 mm, emitted from carbon monoxide molecules in an area of several degrees that includes the center of the Milky Way Galaxy.

By comparing intensity values of emission lines at different wavelengths, the astronomers were able to estimate temperature and density of molecular gas. In this way, they succeeded in drawing detailed distribution maps of ‘warm, dense’ molecular gas at the center of the Milky Way Galaxy for the first time ever.

“The results are astonishing,” said Dr Tomoharu Oka of Keio University, lead author of a paper reporting the results in the Astrophysical Journal Supplement Series. “The ‘warm, dense’ molecular gas in that area is concentrated in four clumps (called Sgr A, L=+1.3°, L=–0.4°, L=–1.2°). Moreover, it turns out that these four gas clumps are all moving at a very fast speed of more than 100 km/s. Sgr A – one of the four gas clumps – contains ‘Sagittarius A*,’ the nucleus of the Milky Way Galaxy.”

“The remaining three gas clumps are objects we discovered for the very first time. It is thought that ‘Sagittarius A*’ is the location of a supermassive black hole that is approximately 4 million times the mass of the Sun. It can be inferred that the gas clump ‘Sgr A’ has a disk-shaped structure with radius of 25 light-years and revolves around the supermassive black hole at a very fast speed,” Dr Oka added.

On the other hand, the team found signs of expansion other than rotation in the remaining three gas clumps. This means that the gas clumps, L=+1.3°, L=–0.4°and L=–1.2°, have structures that were formed by supernova explosions that occurred within the gas clumps. The gas clump “L=+1.3°” has the largest amount of expansion energy, equivalent to 200 supernova explosions. The age of the gas masses is estimated at about 60,000 years.

The researchers used the NRO 45-m Telescope again to further examine the molecular gas’s distribution, motion and composition to determine whether supernova explosions caused the expansion.

“Observation clearly showed that the energy source of L=+1.3° is multiple supernova explosions. We detected multiple expansion structures and molecules attributed to shock waves,” Dr Oka said. “Based on the observation of L=+1.3°, it is also natural to think that the expanding gas clumps L=–0.4° and L=–1.2° derived energy from multiple supernova explosions.”

A supernova is a massive explosion that occurs when a star with more massive than eight to ten times the mass of the Sun ends its life. Such a high occurrence of supernova explosions – once per 300 years – indicates that many young, massive stars are concentrated in the gas clumps. In other words, this means that there is a massive ‘star cluster’ in each gas clump. Based on the frequency of the supernova explosions, the team estimated the mass of the star cluster buried in L=+1.3°as more than 100,000 times the mass of the Sun, which is equivalent to that of the largest star cluster found in the Milky Way.

“The Solar System is located at the edge of the Milky Way Galaxy’s disk, and is about 30,000 light-years away from the center of the Milky Way Galaxy. The huge amount of gas and dust lying between the Solar System and the center of the Milky Way Galaxy prevent not only visible light, but also infrared light, from reaching the Earth. Moreover, innumerable stars in the bulge and disc of the Milky Way Galaxy lie in the line of sight. Therefore, no matter how large the star cluster is, it is very difficult to directly see the star cluster at the center of the Milky Way Galaxy,” Dr Oka explained.

“Huge star clusters at the center of the Milky Way Galaxy have an important role related to formation and growth of the Milky Way Galaxy’s nucleus,” he said.

According to theoretical calculations, when the density of stars at the center of star clusters increases, the stars are merged together, one after another. Then, it is expected that intermediate-mass black holes (IMBHs) with several hundred times the mass of the Sun are formed. Eventually, these IMBHs and star clusters sink into the nucleus of the Milky Way Galaxy. It can be thought that the IMBHs and star clusters are then merged further, and form a massive black hole at the Milky Way Galaxy’s nucleus. Alternatively, the IMBHs and star clusters could help expand an existing massive black hole.

It can be thought that the supermassive black hole at Sagittarius A*, the nucleus of our Galaxy, has also been grown up through these processes. In summary, the new discovery is the finding of ‘cradles’ of IMBHs that become ‘seeds’ of the supermassive black hole at the nucleus.

“We would like to observe IMBHs in the star cluster. Actually, our observation data have already indicated traces of IMBHs,” Dr Oka said. One of the newly discovered gas masses, ‘L=–0.4°,’ contains two small gas clumps moving at a very fast speeds. If it is confirmed that these small gas clumps are rotating, it can be inferred that there are ‘invisible huge masses’ at the center of the gas clumps.

These ‘invisible huge masses’ are likely to be IMBHs hidden in the center of the star cluster.

“In order to confirm the existence of IMBHs, we are planning to conduct further observations,” Dr Oka said. “The new discovery is an important step toward unraveling the formation and growth mechanism of the supermassive black hole at the Milky Way Galaxy’s nucleus, which is a top-priority issue in galactic physics.”

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Bibliographic information: Oka et al. 2012. ASTE CO J = 3-2 Survey of the Galactic Center. ApJS 201, 14; doi: 10.1088/0067-0049/201/2/14