Quiet black hole may be lurking in cosmic 'bullet'

A chance observation in a corner of the Milky Way galaxy could hint at a hidden black hole.

Artist’s impression of a stray black hole storming through a dense gas cloud. The gas is dragged along by the strong gravity of the black hole to form a narrow gas stream.
Keio University

A cloud travelling in a corner of our galaxy at a blistering 100 kilometres per second likely harbours a black hole, according to Japanese astronomers.

Masaya Yamada and colleagues from Keio University mapped the motion and appearance of the fast-moving feature – nicknamed the "Bullet" – and concluded that its shape and velocity can be attributed to a black hole hiding within.

The work, which was published in The Astrophysical Journal, outlines one of only a few such "quiet" black holes found in the Milky Way so far.

Black holes, which don't let any light escape their gravitational clutches – aside from Hawking radiation – are generally detected from their effects on other, visible objects.

For instance, astronomers deduced the location of the Milky Way's central supermassive black hole from the movement of a single star around the centre of our galaxy.

It's not just stars that give away a black hole's location. Clouds of gas and stellar debris can do so too – such as the Bullet, outlined by Yamada and colleagues in their paper.

They discovered the Bullet while investigating the shell of an exploded star, a supernova, nearly 10,000 light-years from Earth. They noticed a cloud, around two light-years across, moving extremely quickly against the motion its surrounds.

When they examined the cloud closely with the Atacama Submillimetre Telescope Experiment's 10-metre telescope in Chile and the Nobeyama 45-metre telescope in Japan, they picked out a couple of weird features.

The Bullet isn't in the shape of an actual bullet; rather, it splits into a Y-shape. And while most of it is expanding at 50 kilometres per second, its tip is travelling at 120 kilometres per second.

Two scenarios for the formation mechanism of the bullet: (a) explosion model and (b) irruption model. Both diagrams show a part of the shock front produced by the expansion of the supernova remnant W44. The shock wave enters into quiescent gas and compresses it to form dense gas. The bullet is located in the centre of the diagram and has completely different motion compared to the surrounding gas.
Yamada et al. (Keio University)

The energy injection needed to accelerate the Bullet to this speed is more than their supernova shell could impart. So, the researchers figured, there must be something else going on.

They thought a pulsar – a spinning, highly magnetic neutron star that emits a beam of light like a lighthouse – could do the trick. But no pulsars have been seen in that corner of the galaxy.

A completely separate supernova to the one they originally observed could account for the energy too. But the probability of that happening within that same supernova shell within 20,000 years is around 0.02%. So while that scenario can't be ruled out, a black hole is a more likely possibility, they write.

In 2002, astronomers calculated there should be up to a billion black holes in the Milky Way, but scant few candidates – only around 60 – have been found.

If a black hole was responsible for the Bullet's velocity, Yamada and colleagues suggest two theories. The first is the "explosion model" where an expanding shell of gas shrugged off by a supernova passes by a stationary black hole. The black hole drags the gas close, causing an explosion and blasting the gas on its way again.

The second is the "irruption model", where a speeding black hole whizzes through a gas cloud, part of which is dragged along with it to form a gas stream.

Both account for the Y shape of the Bullet, but with current data, it's tough to calculate which of the two scenarios is most likely, they write. Still, further observations with a radio telescope such as the Atacama Large Millimetre/submillimetre Array could provide more clues.

Belinda smith 2016 2.jpg?ixlib=rails 2.1
Belinda Smith is a science and technology journalist in Melbourne, Australia.
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