Scientists have for the first time found that a dormant volcano - a supermassive black hole - lying at the heart of our Milky Way galaxy last erupted two million years ago.
Astronomers have long suspected such an outburst occurred, but this is the first time it has been dated.
The evidence comes from a lacy filament of gas, mostly hydrogen, called the Magellanic Stream. This trails behind our galaxy's two small companion galaxies, the Large and Small Magellanic Clouds.
"For twenty years we've seen this odd glow from the Magellanic Stream," said lead researcher Professor Joss Bland-Hawthorn at the University of Sydney, Australia, and a Fellow at the Australian Astronomical Observatory.
"We didn't understand the cause. Then suddenly we realised it must be the mark, the fossil record, of a huge outburst of energy from the centre of our galaxy," he said.
"It's been long suspected that our galactic centre might have sporadically flared up in the past. These observations are a highly suggestive 'smoking gun'," said Martin Rees, Astronomer Royal, who was one of the first people to suggest that black holes generate the power seen coming from quasars and galaxies with 'active' centres.
The galaxy's supermassive black hole is orbited by a swarm of stars whose paths help measure the black hole's mass: four million times the mass of the Sun, 'phys.Org' reported.
The region around the black hole, called Sagittarius A, pours out radio waves, infrared, X-rays and gamma rays.
Infrared and X-ray satellites have seen a powerful 'wind' (outflow) of material from this central region. Antimatter boiling out has left its signature. And there are the 'Fermi bubbles' - two huge hot bubbles of gas billowing out from the galactic centre, seen in gamma-rays and radio waves.
"All this points to a huge explosion at the centre of our galaxy. What astronomers call a Seyfert flare," said team member Dr Philip Maloney of the University of Colorado in Boulder, US.
At a workshop at Stanford University in California earlier this year, researchers realised the Stream could be holding the memory of the galactic centre's past.
Struck by the fiery breath of Sagittarius A, the Stream is emitting light, much as particles from the Sun hit our atmosphere and trigger the coloured glows of the aurorae - the Northern and Southern Lights.
The brightest glow in the Stream comes from the region nearest the galactic centre.
"Geometry, the amount of energy from the original flare from Sagittarius A, the time the flare would take to travel to the Magellanic Stream, the rate at which the Stream would have cooled over time - it all fits together, it all adds up," said team member Dr Greg Madsen of the University of Cambridge in UK.
