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Why galactic black hole fireworks were a flop

Giant 'G2' cloud at centre of Milky Way could be falling in a continuous stream, without the spectacle astronomers were hoping for.


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Simulation of G2's passage by Sagittarius A*

This computer model predicted how the gas cloud known as G2 would approach and orbit around the Milky Way's central black hole, while part of it fell in, releasing intense radiation. But no such flashes have been observed.

ESO/S. Gillessen/MPE/Marc Schartmann/L. Calçada

Nothing. Nada. Zilch. That’s what astronomers saw this spring when they trained their telescopes on the giant black hole at the Milky Way’s centre, hoping to see galactic fireworks as a huge gas cloud smashed into it.

A team of astronomers now says that it has an explanation for the lack of fireworks around the black hole, called Sagittarius A*, which is four million times more massive than the Sun. The researchers think that the cloud, rather than being isolated, is a dense clump within a continuous but thinner stream of matter. This would act as a constant breeze on the disk of matter orbiting the black hole, rather than as sudden gusts that would have caused fireworks as they hit, as originally expected.

Matching orbits

In 2012, astronomer Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, and his collaborators reported that a large gas cloud known as G2 was approaching the centre of the Milky Way, and being torn apart by the central black hole's gravity1.

The team also predicted that while much of G2 would trace a parabola around the black hole, parts of it would be pulled in and smash into the accretion disk, with a violent collision that would release an intense flash of radiation.

In the new study, Gillessen and his co-authors used images taken by the European Southern Observatory’s Very Large Telescope in Chile. An analysis of infrared light emitted by G2, as well as by G1 — another cloud that passed near the black hole 13 years ago — reveal that the two clouds have almost identical orbits.

This suggests that both clouds, along with a gas tail that the team says is trailing G2, are all part of a large single gas stream, possibly torn from a star that came close to Sagittarius A* some 100 to 200 years ago, Gillessen and colleagues suggest. They report their findings in an article posted on 17 July on the arXiv preprint repository2.

James Guillochon and Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, and their colleagues proposed a similar explanation in April in an Astrophysical Journal Letters article3.

Gillessen notes that although the Very Large Telescope could not resolve structures within the tail of gas behind G2, it seems to be clumpy, indicating that there may be other gas clouds following G2 for the black hole to feed on in the near future.

However, astronomer Andrea Ghez of the University of California, Los Angeles, who leads another team examining the Galactic Centre, does not find that scenario convincing. Her team’s images, which trace the dust associated with G2 rather than the gas, suggest that there is an unseen star in the middle of G2. The gravitational pull of the star would keeps the cloud intact, preventing parts of it from falling into the black hole's disk. The team reported its findings on 2 May in the Astronomer's Telegram. If that is the case, the stream of gas to which G2 belongs might not be as steady as suggested by Gillessen and his colleagues.

A major light show could still happen in a few years to a few decades from now, when G2 spirals through the disk of gas and dust believed to surround Sagittarius A*. The time it takes for G2 to travel through the disk may speak volumes about the properties of the disk and the eating habits of supermassive black holes similar to Sagittarius A*, Loeb says

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An earlier version of this article stated that Ghez's team reporter its findings in a telegram of the International Astronomical Union. Instead, they did so in the Astronomer's Telegram.


  1. Gillessen, S. et al. Nature 481, 5154 (2012).

  2. Pfuhl, O. et al. Preprint at (2014).

  3. Guillochon, J., Loeb, A., MacLeod, M. & Ramirez-Ruiz, E. Astrophys. J. Lett. 786, L12 (2014).

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