ago, numerous black-hole candidates have been found. These consist of both low-mass black holes, which have several times the mass of the Sun, and supermassive black holes (SMBHs), which are billions of times heavier. While many candidates exist for stellar-mass black holes and SMBHs, the latter being thought to occupy the centres of galaxies, candidates for black holes in the intermediate mass range were lacking.

A group of researchers from Keio University in Japan has now shown strong evidence for the existence of an intermediate-mass black hole (IMBH) within the Milky Way, which could shed light on the formation of black holes and of our galaxy.

While there is a consensus that stellar-mass black holes form when massive stars die, the source of SMBHs – one of which is thought to be at the centre of the Milky Way – is not well known. It is believed that large galaxies such as the Milky Way grew to their current size by cannibalising smaller dwarf galaxies containing IMBHs at their centres. Finding a candidate IMBH would provide evidence for this theory.

First, using the Nobeyama radio telescope, the team detected a gas cloud in the Milky Way with a peculiar velocity profile, hinting that an IMBH exists near the centre of our galaxy. This then prompted a more precise observation of the area using the Atacama Submillimeter Telescope Experiment (ASTE) and Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. The cloud, named CO-0.40-0.22, was found to consist of one dense cloud in the centre with a large velocity profile, surrounded by 20 smaller clouds, the velocity profiles of which are aligned. Since the probability of these clouds being aligned by chance is less than one part in 108, it suggests there is some other object close to the cloud interacting with it. Within the gas cloud the data also revealed a point source emitting weak electromagnetic radiation at submillimetre wavelengths and none at higher wavelengths, ruling out a massive star cluster.

Based on these striking observations, the group simulated the gravitational interactions of the cluster and found that the measured velocity profiles are consistent with a gravitational kick by a dense object of 105 solar masses. Combined with the lack of high-energy emission and the spectrum of the object measured in radio wavelengths, the object matches all the characteristics of an IMBH – the first one ever observed. Two further IMBHs are now under study. The finding opens a new research avenue in understanding both massive and supermassive black holes, and strengthens the hypothesis that our galaxy grew by cannibalising smaller ones.