Dating our galaxy's dormant volcano

Sep 23, 2013
An artist’s conception of a black hole generating a jet. Two million years ago the supermassive black hole at the center of our galaxy was 100 million times more powerful than it is today. Credit: NASA/Dana Berry/SkyWorks Digital

(Phys.org) —A dormant volcano—a supermassive black hole—lies at the heart of our galaxy. Fresh evidence suggests that it last erupted two million years ago.

Astronomers have long suspected such an outburst occurred, but this is the first time they've been able to date it.

The evidence comes from a lacy filament of gas, mostly hydrogen, called the Magellanic Stream. This trails behind our galaxy's two small , the Large and Small Magellanic Clouds.

"For twenty years we've seen this odd glow from the Magellanic Stream," said Professor Joss Bland-Hawthorn, an ARC Federation Fellow at the University of Sydney, Australia, and a Fellow at the Australian Astronomical Observatory, who led a team studying this problem.

"We didn't understand the cause. Then suddenly we realized it must be the mark, the fossil record, of a huge outburst of energy from the center of our galaxy."

"It's been long suspected that our 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 generate the power seen coming from quasars and galaxies with 'active' centers.

The team gives its arguments in a paper accepted for publication in The Astrophysical Journal. Professor Bland-Hawthorn will speak about the work at the Galaxy Zoo meeting in Sydney, Australia, on 24 September.

The galaxy's has been known for decades. It's orbited by a swarm of stars whose paths let us measure the black hole's mass: four million times the mass of the Sun.

The region around the black hole, called Sagittarius A* ["A-star"], pours out , infrared, X-rays and gamma rays.

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Simulation of a black hole’s jet. Credit: Simulation: McKinney (UMD), Tchekhovskoy (Princeton) and Blandford (KIPAC); Visualization: Kaehler (KIPAC)

Flickers of radiation rise up when small clouds of gas fall onto the hot disk of matter that swirls around the black hole.

But evidence has been building of a real cataclysm in the past. 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 center, seen in and radio waves.

"All this points to a huge explosion at the center of our galaxy," said team member Dr. Philip Maloney of the University of Colorado in Boulder, USA. "What astronomers call a Seyfert flare."

Scientists studying the galactic center came together at a workshop at Stanford University in California earlier this year.

While at the workshop, Professor Bland-Hawthorn realized the Stream could be holding the memory of the galactic center'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 colored glows of the aurorae—the Northern and Southern Lights.

In the Stream, ultraviolet light splits hydrogen atoms into protons and electrons. When those components recombine, the electrons give off 'H-alpha' emission—a specific wavelength of light.

The brightest glow in the Stream comes from the region nearest the galactic center.

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," says team member Dr. Greg Madsen of the University of Cambridge in the UK.

The galaxy's stars don't produce enough ultraviolet to account for the glow. Nor could they have in the past, says Professor Bland-Hawthorn. "The galactic center never formed stars at a high enough rate."

Will such an explosion happen again?

"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 we think will fall in next year. It's small, but we're looking forward to the fireworks!"

Explore further: Powerful jets blowing material out of galaxy: Process limits growth of central black hole and rate of star formation

More information: J. Bland-Hawthorn, Philip R. Maloney, Ralph S. Sutherland, G. J. Madsen. "Fossil Imprint of a Powerful Flare at the Galactic Center Along the Magellanic Stream." Accepted for publication in The Astrophysical Journal. www.physics.usyd.edu.au/~jbh/s… GC_AGN_MagStr_13.pdf

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c0y0te
1.6 / 5 (12) Sep 23, 2013
Paul LaViolette, Galactic superwaves?
cantdrive85
1 / 5 (14) Sep 23, 2013
"For twenty years we've seen this odd glow from the Magellanic Stream,"...
"We didn't understand the cause."

That's because;
"Students using astrophysical textbooks remain essentially ignorant of even the existence of plasma concepts, despite the fact that some of them have been known for half a century. The conclusion is that astrophysics is too important to be left in the hands of astrophysicists who have gotten their main knowledge from these textbooks. Earthbound and space telescope data must be treated by scientists who are familiar with laboratory and magnetospheric physics and circuit theory, and of course with modern plasma theory." Hannes Alfven

You can't expect to understand something without first studying it and not relying on "models we know are wrong"...
Tuxford
1 / 5 (11) Sep 23, 2013
The community must avoid the pending embarrassment of having to acknowledge the validity of a theory from a scientist they have mocked. LaViolette has been working on this for decades. And now the evidence is becoming more compelling. What they are missing is a working knowledge of control systems theory, which underlies the physics which permits the origination of matter within the core stars. Instead, they are stuck with trying to explain the unexplainable with accretion mechanisms, and the standard model. Oh well....the bewilderment continues.
namrata_gawai
1 / 5 (7) Sep 24, 2013
After reading all the comments I feel these scientists are as confused as we general people are. But continue the good work, someday we will unravel all the mysteries.
reid barnes
1 / 5 (6) Sep 28, 2013
This article says: "The evidence comes from a lacy filament of gas, mostly hydrogen, called the Magellanic Stream. … 'Then suddenly we realized it must be the mark, the fossil record, of a huge outburst of energy from the center of our galaxy'. … Infrared and X-ray satellites have seen a powerful 'wind' (outflow) of material from this central region. … While at the workshop, Professor Bland-Hawthorn realized the Stream could be holding the memory of the galactic center'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 colored glows of the aurorae—the Northern and Southern Lights." So we have all the elements of plasma, but they can't bring themselves to use the term. There is even the notion of a "fossil record" of the "filament." Yet instead of considering the possibility that we may be looking at a fossil record of a plasma filament, the scientists cling to the gravitational model.