Milky Way's black hole catches itself a snack

The supermassive black hole at the centre of our galaxy has been spotted slurping up a cloud of hot gas that is falling into it.

A month ago, astronomers at the European Space Agency captured an event 47 million light years away: a black hole in the galaxy NGC 4845 waking from long hibernation to take a bite out of a passing brown dwarf.

Now, the supermassive black hole at the centre of our own Milky Way galaxy has been caught, in flagrante delicto, indulging in some snacking shenanigans of its own. Thanks to far-infrared imaging, NASA's Herschel observatory has seen in detail a cloud of extremely hot gas that is orbiting or falling towards the black hole — and being swallowed by it.

"The black hole appears to be devouring the gas," said Paul Goldsmith, the US project scientist for Herschel. "This will teach us about how supermassive black holes grow."

The black hole, about 26,000 light years away, in the region known as Sagittarius A*, has a mass of about four million times that of our sun. It is surrounded by a dense, toroidal cloud of gas about 15 light years in diameter, with a central cavity filled with dust and lower-density gas, which includes molecules of carbon monoxide, water vapour and hydrogen cyanide.

But the heat of the gas surprised the astronomers — in parts, it reaches up to 1000 degrees Celsius. Usually, interstellar gas clouds are only a few tens of degrees above the between minus 273 degrees Celsius of absolute zero.

While the astronomers believe that some of the heat is caused by ultraviolet radiation from a cluster of massive stars that orbit the black hole, it doesn't explain all of it. They posit that some of the heat may be caused by strong shocks in highly magnetised gas in the region, due to collisions between gas clouds or high-speed material flow from stars and protostars.

Javier Goicoechea of the Centro de Astrobiología, Spain, lead author of the paper reporting the results, said: "The observations are also consistent with streamers of hot gas speeding towards Sagittarius A*, falling towards the very centre of the Galaxy. Our Galaxy's black hole may be cooking its dinner right in front of Herschel's eyes."

A separate, smaller, more compact cloud of gas, just a few Earths in mass, has also been spotted much closer to the black hole. It's expected to be guzzled down later this year.

Via www.nasa.gov