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Our black hole’s
impending lunch

Posted: 16 December 2011

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The giant black hole at the hub of the Milky Way will soon be munching on a cloud of gas three times more massive than the Earth that is accelerating towards it, report a multinational team of astronomers who have utilised the light gathering power of the the Very Large Telescope (VLT) at the European Southern Observatory in Chile to probe the galactic centre.

The heart of our Milky Way Galaxy is a busy place, crammed with stars, gas and the imposing presence of Sagittarius A*, known in shorthand as Sgr A*, which is a supermassive black hole that has a mass 4.3 million times that of the Sun (the mass of our Sun is 2 x 1030 kilograms, so the black hole is rather hefty). Watching with the adaptive optics system and the infrared SINFONI instrument on one of the 8.2-metre telescopes of the VLT, the astronomers have for the past seven years been tracking a cloud of gas – composed mostly of hydrogen and helium – that is racing towards Sgr A* at eight million kilometres per hour, having doubled its speed over that time. It’s rather elongated orbit will be seen to take the cloud to within 40 billion kilometres of the black hole’s event horizon in 2013, where the Sgr A*’s immense gravity will create tidal forces that stretch and pull the cloud, raising its temperature from its current 280 degrees Celsius to millions of degrees, hot enough to emit powerful bursts of X-rays. The cloud may be torn in two, one chunk settling to form a spiralling disc of gas around the black hole known as an accretion disc, and the other half flung away, discarded and never to return.

A schematic of the interior of the Milky Way, around the black hole Sagittarius A-Star. Orbits of nearby stars are shown, as is the giant gas cloud that is on a collision course with the black hole in a few years time. Sgr A* is at the very centre of the looping tangle of star orbits. Image: ESO/MPE/Marc Schartmann.

The environment around Sgr A* is so crammed with stars and gas that resolving the accretion disc is beyond the ability of even the largest optical telescopes, although radio interferometry (linking disparate radio telescopes across wide distances) may have better luck, says Eliot Quataert of the University of California, Berkeley, who participated in the study, the results from which will appear in the 5 January 2012 issue of Nature. Nevertheless, the behaviour of the gas as it accelerates towards the black hole should reveal some secrets about how black holes consume matter.

“We hope to use the information about how the observed emission in X-rays and infrared varies in time to infer some of the physics of how the gas is spiralling into the black hole,” Quataert tells Astronomy Now. “In particular, the gas cloud is ploughing through an already existing accretion disc and so understanding how the gas cloud behaves will tell us in part about the accretion disc that is already there.”

In addition, it should also be possible to determine the density, rotation and the intrinsic magnetic field strength of the gas around the black hole, fully characterising the gaseous environment of an accretion disc for the first time.

“We will gain a better understanding of the physics of accretion around a dormant black hole, and this might be applied to other galaxies,” says Stefan Gillessen, one of the lead authors of the study from the Max Planck Institute for Extraterrestrial Physics. Many of the galaxies that we see in the night sky contain supermassive black holes and some are more active than others, flaring in outbursts of X-rays and gamma rays as material is consumed. The Milky Way’s upcoming event will help calibrate measurements of those distant active black holes and allow astronomers to better judge how big the accretion events in those galaxies are.

In comparison our own black hole is relatively quiet. “It is a long-standing puzzle why the black hole is so quiet,” says Gillessen. Sgr A* radiates at a one-hundred millionth of what it could were it to become fully active, and is spoon fed by the occasional tiny morsel of a gas cloud or an unlucky star that has wandered too close to it. We know there have been larger events in the past because the resultant X-rays have produced ‘light echoes’ that have caused gas clouds farther away to glow, centuries after the event.

“The cloud probably won’t release that much energy, but the event might still be similar,” says Gillessen. Already the leading edge of the cloud has been seen to begin fraying, while a trail of gas behind the cloud may prolong the event. “We don’t really know how bright this will be, but since we can’t know that precisely, I very much hope for some exciting observations in the coming years!”

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