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Galactic explosion betrays black hole

TWO MILLION YEARS AGO a supermassive black hole at the heart of our galaxy erupted in an explosion so immensely powerful that it lit up a cloud 200,000 light years away, a team of researchers led by the University of Sydney has revealed.

The finding is an exciting confirmation that black holes can ‘flicker’, moving from maximum power to switching off over, in cosmic terms, short periods of time.

An artist's conception of a black hole generating a jet

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

“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 Professor Joss Bland-Hawthorn from the University’s School of Physics, and lead author of an article on the research to be published in The Astrophysical Journal.

Professor Bland-Hawthorn unveiled the research at the international Galaxy Zoo science conference on 24 September 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 Martin Rees, Astronomer Royal, who was one of the first scientists to suggest that massive black holes power quasars.

Fossil record

The evidence for the findings 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 centre of our galaxy.”

The region around the galaxy’s supermassive black hole and the black hole is called Sagittarius A* (pronounced Sagittarius A-star). It emits radio, infrared, ultraviolet, x-ray and gamma ray emissions. Flickers of radiation rise up when small clouds of gas fall onto the hot cloud of matter that swirls around the black hole.

The video below show a computer simulation of a black hole in real time showing how gas falling in forms a disc that spins around the black hole. The friction causes the gas to become so hot it produces beams of UV radiation. Credit: McKinney (UMD), Tchekhovskoy (Princeton), Blandford (KIPAC), Kaehler (KIPAC)

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 centre of the galaxy, covering almost a quarter of the sky,” said Professor Bland-Hawthorn.

On-and-off black holes

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 realised that this same event would also explain the mysterious glow that we see on the Magellanic Stream,” Professor 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,” said Dr Maloney. “The Galactic Centre never formed stars at a high enough rate. There had to be another explanation.”

Professor Bland-Hawthorn said, “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.”

“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.”

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,” says Professor Bland-Hawthorn. “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!

Professor Bland-Hawthorn is a Fellow of the Australian Astronomical Observatory.

Adapted from information issued by the University of Sydney.

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Aussie tech helps telescopes “see in the dark”

NGC 300

Australian technology will soon enable astronomers to get a clearer view of distant galaxies, by reducing the effect of the natural airglow of the sky.

AUSTRALIAN SCIENTISTS have made a major breakthrough in the drive to improve their view of the night sky and greatly increase the efficiency of ground-based telescopes.

While the night sky looks dark to the naked eye, to an astronomer working at infrared wavelengths the air actually glows brightly, drowning out the view of distant astronomical bodies.

This happens because molecules in our atmosphere emit their own infrared radiation, swamping the faint infrared light coming in from deep space.

What astronomers have needed is a way to filter out the atmospheric emission while letting through the infrared waves from stars and galaxies.

Traditional filters can only remove selected wavelengths at a time. What if a system could be devised that removes many at once?

Enter the “photonic lantern” and high-tech, wavelength-suppressing optical fibres, both the brainchild of Professor Joss Bland-Hawthorn (University of Sydney) and the team he leads.

The complex system, under development since 2004, recently underwent its first real test under the night sky—at Siding Spring Observatory in New South Wales—and passed with flying colours.

The system removed the unwanted air emissionwavelengths just as planned, while letting through the infrared from deep space. In effect, it made the sky look darker and clearer.

Joss Bland-Hawthorn

Professor Joss Bland-Hawthorn leads the team that has developed the photonic lantern and wavelength-suppressing optical fibres. Photo courtesy University of Sydney.

The results of the field test were published this week in the scientific journal, Nature Communications.

Looking deeper into space

The optical fibres are specially made with internal patterns that act to filter out the unwanted wavelengths, while the photonic lantern combines the output from multiple fibres. That output can then be fed into a spectrograph, a device that splits light into separate wavelengths and enables analysis to be made of the chemical nature of the stuff (stars, galaxies, nebulae) that emitted the original infrared light.

Infrared wavelengths are very important because visible wavelength light emitted from astronomical bodies when the universe was young, has by now been redshifted into the infrared by the expansion of the universe. So in order to study the universe’s past, astronomers need to see at infrared wavelengths.

The first operational device to use the new photonics was commissioned earlier this year on the Anglo-Australian Telescope. This prototype instrument, called GNOSIS, paves the way to a more powerful instrument now under development by the AAO and the University of Sydney. Called SUNESIS, it will be operational by the end of 2012.

“This will mean we’ve gone from project inception to completion within 12 months, a remarkable effort,” says Bland-Hawthorn.

And they’re also aiming to have the technology ready soon for use on other major telescopes throughout the world.

“In particular, we’re aiming at the current 8- to 10-metre class of telescopes—the largest in the world—and then the new generation of 30-metre telescopes that are currently in the design phase,” says Bland-Hawthorn.

When installed on such large telescopes, the system will enable astronomers to see five times deeper into space in the infrared part of the spectrum, which corresponds to a 100-fold increase in the volume of space covered. And that means thousands more targets for their telescopes.

And that’s not the end of it. Space-based applications also beckon, and the University of Sydney team aims to test out other uses of the photonics technology aboard a micro-satellite to be launched in 2012, as well as with high-altitude balloon flights in collaboration with NASA.

Story by Jonathan Nally. Galaxy image courtesy ESO.

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