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Medium-sized black hole seen lurking in the Milky Way

05 Sep 2017 Hamish Johnston
Artist's impression of the Milky WayArtist's impression of the Milky Way
Black-hole haven: artist's impression of the Milky Way

Astronomers in Japan say they have the best evidence yet that intermediate-mass black holes (IMBHs) exist – and what’s more, there could be one in the Milky Way. Weighing in at 100,000 solar masses, the object is on the heavy side for a IMBH and could have been created in a dwarf galaxy that then merged with the Milky Way.

There is very good evidence that supermassive black holes (SMBHs) dominate the centres of many galaxies. These behemoths have masses equivalent to 100,000 to billions of Suns and their huge gravitational fields can drive spectacular emissions of radiation. At the other end of the mass scale, there is also strong evidence for the existence of “stellar mass” black holes (several to tens of solar masses) from indirect astronomical observations and the detection of the gravitational waves that are given off when pairs of black holes merge.

Coalescing IMBHs

What is not understood, however, is how SMBHs come into being. One possibility is that IMBHs are first formed when a young compact cluster of stars undergoes a gravitational collapse. Then, a number of these IMBHs coalesce at the centre of a galaxy to form a SMBH. While astronomers have identified many objects that could be IMBHs, none of these sightings are accepted as being definitive.

Now, Tomoharu Oka and colleagues at Keio University have studied a “peculiar molecular cloud” located near the centre of the Milky Way and concluded that it harbours a black hole weighing in at about 100,000 solar masses – putting it at the more massive end of the IMBH classification.

Unusual velocities

Dubbed CO-0.40-0.22, the cloud is peculiar because of the unusual distribution of the velocities of its constituent molecules. This, say Oka and colleagues, is best explained by the presence of a IMBH near the centre of the cloud that is giving the molecules a “gravitational kick”.

Writing in Nature Astronomy, the team says that the presence of a IMBH is backed up by observations of a compact region of dense gas near the centre of the cloud. Within this compact region is a point-like source of radio emissions, which could be created by gas accelerated by an IMBH.

What is not clear, however, is how the IMBH could have formed. Current theory suggests that a 100,000 solar-mass black hole is more likely to form in a nearby dwarf galaxy – rather than in a star cluster within the Milky Way. As a result, the IMBH could have formed outside the Milky Way and was then incorporated into our galaxy along with its parent dwarf galaxy. Indeed, the team points out that there is evidence that such a merger occurred about 200 million years ago.

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