Cosmic fossil reveals black hole feeding frenzy
Black hole burst The black hole at the centre of the Milky Way is relatively quiet, but astronomers have found traces of a major feeding event two million years ago.
The research reported in the Astrophysical Journal, provides the best estimate for the long suspected event.
The smoking gun was found in a lacy filament of gas called the Magellanic stream, according to the study's lead author Professor Joss Bland-Hawthorn of the University of Sydney and the Australian Astronomical Observatory.
The stream of mostly comprised of hydrogen is formed by material trailling two dwarf satellite galaxies, the Large and the Small Magellanic Clouds, as they orbit around the Milky Way.
"For twenty years we've seen this odd glow from the Magellanic stream," says Bland-Hawthorn. "Some of the hydrogen gas had become ionised along the stream. Something had to explain why this cold gas was being lit up."
The galaxy's stars don't produce enough ultraviolet radiation to account for the glow.
"Nor could they have in the past," says Bland-Hawthorn. "And the galactic centre never formed stars at a high enough rate."
The first clues came in 2010 from NASA's Fermi space telescope, which discovered two gigantic billowing clouds of gamma rays, now called the Fermi bubbles, stretching for thousands of light years above and below our galaxy.
"These are enormous," says Bland-Hawthorn. "If you could see gamma rays with the human eye ... you would see that a quarter of the sky is covered by these gamma ray bubbles."
Since their discovery, astronomers have been trying to determine what generated these bubbles.
Earlier this year, astronomers showed that they were probably caused by material falling into Sagittarius A*, the supermassive black hole at the centre of the galaxy, approximately two million years ago.
"That gave me the idea to check whether the same explosion lit up the [Magellanic] stream," says Bland-Hawthorn.
Ultraviolet radiation ionises hydrogen atoms by splitting electrons from protons. When they recombine, the electrons give off specific wavelengths of light, which astronomers detect as a glow in the Magellanic stream. The brightest glow in the stream comes from the region directly above the galactic centre.
"So we went through the calculations and we got the same energetics that the [Fermi Bubble] models inferred, and we also got the same time scale that they inferred," says Bland-Hawthorn.
"They said the bubbles would have occurred two million years ago and that's exactly what we find. So what we are seeing today was something caused by the black hole exploding two million years ago."
He adds that the finding also shows that black holes are stochastic in nature, much like earthquakes and cyclones, occuring at seemingly random time intervals and intensity.
"The realisation that these black holes can switch on and off within a million years, which given the universe is 14 billion years old means very rapidly, is a significant discovery."
And it could happen again.
"Next year there's a cloud called G2 which we can actually see night now, we are tracking this little cloud about twice the mass of the Earth," says Bland-Hawthorn.
"In around March, April or May of next year we expect this cloud to fall in to the black hole and we expect the black hole to fire up again, but not as spectacular as the explosion we are inferring occurred two million years ago."