Astronomers led by Prof Joss Bland-Hawthorn from the University of Sydney have found evidence that the supermassive black hole at the heart of our galaxy, known as Sagittarius A*, erupted in a colossal explosion around 2 million years ago. The explosion was so powerful that it lit up a cloud 200,000 light years away.

The findings, published in the Astrophysical Journal (arXiv.org version), confirm that black holes can ‘flicker’, moving from maximum power to switching off over, in cosmic terms, short periods of time.

“For 20 years astronomers have suspected that such a significant outburst occurred, but now we know when this sleeping dragon, four million times the mass of the sun, awoke and breathed fire with 100 million times the power it has today,” Prof Bland-Hawthorn said.

“It’s been long suspected that our Galactic Center might have sporadically flared up in the past. These observations are a highly suggestive smoking gun,” added astronomer Dr Martin Rees.

The evidence comes from a lacy filament of hydrogen gas called the Magellanic Stream. It trails behind our galaxy’s two small companion galaxies, the Large and Small Magellanic Clouds.

“Since 1996, we’ve been aware of an odd glow from the Magellanic Stream, but didn’t understand the cause. Then this year, it finally dawned on me that it must be the mark, the fossil record, of a huge outburst of energy from the supermassive black hole at the center of our galaxy.”

The region around the galaxy’s supermassive black hole emits radio, infrared, ultraviolet, X-ray and gamma ray emissions. Flickers of radiation rise up when small clouds of gas fall onto the hot disk of matter that swirls around the black hole. In stark contrast to this current inactivity, evidence is emerging that there was a cataclysmic event in the past.

“In particular, in 2010 NASA’s Fermi satellite discovered two huge bubbles of hot gas billowing out from the center of the galaxy, covering almost a quarter of the sky,” Prof Bland-Hawthorn explained.

Earlier this year, computer simulations of the Fermi bubbles made by the University of California Santa Cruz controversially suggested that they were caused by a colossal explosion from Sagittarius A* within the last few million years.

“When I saw this research I realized that this same event would also explain the mysterious glow that we see on the Magellanic Stream,” Prof Bland-Hawthorn said.

“I calculated that to explain the glow it must have happened two million years ago because the energy release shown by the Santa Cruz group perfectly matched, to our delight, that from the Magellanic Stream.”

“The galaxy’s stars don’t produce enough ultraviolet to account for the glow, nor could they have in the past. The Galactic Center never formed stars at a high enough rate. There had to be another explanation,” said study co-author Dr Philip Maloney of the University of Colorado.

“In fact the radiation from stars is one hundred times too little to account for the radiation now or at any time. The galaxy could never have produced enough UV radiation to account for it. So the only explanation was it had to be produced from our dragon, the massive black hole,” Prof Bland-Hawthorn said. “The realization 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.”

Will such a colossal explosion ever happen again? “Yes, absolutely! There are lots of stars and gas clouds that could fall onto the hot disk around the black hole. There’s a gas cloud called G2 that astronomers around the world are anticipating will fall onto the black hole early next year. It’s small, but we’re looking forward to the fireworks!” Prof Bland-Hawthorn said.

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Bibliographic information: Joss Bland-Hawthorn et al. 2013. Fossil imprint of a powerful flare at the Galactic Centre along the Magellanic Stream. Astrophysical Journal; arXiv: 1309.5455