26 September 2013

Sleeping Supermassive Black Holes Can Have Periods Of Intense Flare-Ups

Image Caption: An artist's conception of a black hole generating a jet. Two million years ago the supermassive black hole at the centre of our Galaxy was 100 million times more powerful than it is today. Credit: NASA/Dana Berry/SkyWorks Digital 

Around 2 million years ago, the supermassive black hole at the center of our galaxy erupted in a blaze of glory big enough to leave a residual glow that can still be seen today, according to an upcoming paper to be published in The Astrophysical Journal.

The astronomers who conducted the study said their findings support the theory that black holes can ‘awaken from their slumber’ and burst to maximum power in relatively 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,” said study author Joss Bland-Hawthorn, from the University of Sydney in Australia.

Bland-Hawthorn will be presenting the study’s findings at the international Galaxy Zoo conference today in Sydney.

“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 University of Cambridge professor emeritus Martin Rees, who also holds the UK’s top official astronomy post – Astronomer Royal.

The new study is based on observations of a thin strand of hydrogen gas known as the Magellanic Stream. The filament trails behind the Milky Way’s two small companion galaxies, the Large and Small Magellanic Clouds.

Astronomers noticed a strange glow coming from the Magellanic Stream in 1996, but didn’t know the cause. According to the study researchers, they devised a theory this year that the glow must be the remnant of a huge burst of energy from the supermassive black hole at the center of the Milky Way.

The center of our galaxy, known as Sagittarius A, emits radio, infrared, ultraviolet, x-ray and gamma ray emissions. Scientists detect flickers of radiation when clouds of gas drop onto the hot disk of matter that swirls around this central black hole.

Despite the relative inactivity of Sagittarius A* today, evidence is beginning to suggest that there was a catastrophic event in the not-too-distant 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,” Bland-Hawthorn said.

Computer models of the Fermi bubbles made by the University of California, Santa Cruz earlier this year indicated that the bubbles were caused by a massive 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,” Bland-Hawthorn said. “Together with Dr Ralph Sutherland from Mount Stromlo Observatory and Dr Phil Maloney, from the University of Colorado, 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,” explained Maloney. “The Galactic Center never formed stars at a high enough rate. There had to be another explanation.”

“In fact the radiation from stars is one hundred times too little to account for the radiation now or at any time,” Bland-Hawthorn said. 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.”

The Australian astronomer said he expects a similar event to occur relatively soon.

“There’s a gas cloud called G2 that astronomers around the world are anticipating will fall onto the black hole early next year,” he said. “It’s small, but we’re looking forward to the fireworks!”

Read more:  The giant black hole at the center of the Milky Way is murdering a huge gas cloud


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