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Mystery Solved: Black Hole Belch Caused Galactic Glow

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Like virtually all galaxies, the Milky Way has a monster lurking at its very core: a giant black hole, as massive as four million suns, which will swallow anything that ventures too close to its intense gravitational field. Like most, it isn’t dining at the moment. When black holes eat, they belch out intense blasts of radiation, some so blindingly bright they can be seen halfway across the universe.

But thanks to an impressive feat of cosmic detective work, an international team of astronomers has dated our galactic black hole’s last full-size meal. It happened two million years ago, they say—and while the brilliant flash of light it triggered has long since faded, the effects are still visible.

At least some of those signs have been visible for nearly 20 years, which is why the discovery, announced at a meeting in Australia on September 24, is as juicy as the last chapter of a murder mystery. Back in the 1970s, astronomers discovered a giant cloud of gas arcing over the Milky Way. Known as the Magellanic Stream, it’s made of material pulled from dwarf galaxies that orbit our own.

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In the 1990s, observers realized that the Magellanic Stream has a faint glow to it, a telltale sign that its atoms have been bathed with intense light, probably ultraviolet, at some time in the past. The mystery: where did the light come from?

“There’s loads of ultraviolet coming from massive stars in the Milky Way,” says Philip Maloney of the University of Colorado, a member of the research team, “but only a few percent of it escapes.” The rest, he says, is trapped by gas clouds inside our galaxy.

Meanwhile, other astronomers had independently noted back in 2003 that a giant wind of particles is flying outward from the center of the Milky Way, something like the solar wind that emanates from the Sun, but much more powerful. And finally, in 2010, NASA’s Fermi Gamma-ray Space Telescope found two gigantic “bubbles” of gamma rays bulging in opposite directions from the Milky Way’s core and spanning a combined 50,000 light-years—impressive, considering that the spiral arms of our galaxy reach span only about twice that distance themselves.

The wind and the bubbles pointed to a cataclysm of some sort in the Milky Way’s core, and it seemed obvious that the giant black hole was involved. But it wasn’t until a conference last spring that Joss Bland-Hawthorn, of the Australian Astronomical Observatory realized that the glow in the Magellanic Stream could be related to the particle wind and the gamma-ray bubbles. “I thought, ‘my gosh, this warm plasma [i.e., the glowing gas] sits right over the South Galactic Pole.”

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That means one of the gamma-ray bubbles is pointing right at the Stream, and so is the particle wind—and working with Maloney and Greg Madsen, of the University of Cambridge, Bland-Hawthorne worked out what must have happened: about two million years ago, a star or a cloud of gas ventured too close to the black hole, and was whipped into a madly spinning disk of material as it fell in. The gas was heated to millions of degrees in the process, spewing out ultraviolet light and X-rays that blasted into the Magellanic Stream; it also spit out a powerful particle wind, that generated bubbles of gas sizzling with gamma rays.

“These observations,” said Martin Rees, the U.K.’s Astronomer Royal, in a statement, “are a highly suggestive ‘smoking gun.’”

If astronomers’ estimates are right, the black hole burped when our ancestral species Homo habilis was making the first crude stone tools in prehistoric Africa.  But they probably didn’t notice, says Maloney. Much of the energy would have been aimed up above and down below the plane of the Milky Way, not out toward its edge where we live. “It would have been only about a tenth as bright as the full Moon,” he says, “and the gas and dust that blocks our view of the galaxy’s core would have made it even dimmer.  Still, says Maloney, “If there had been astronomers hack then, they would have been really excited.”

Modern astronomers may soon get another chance. Back in 2011, astronomers detected a gas cloud approaching the Milky Way’s black hole. It’s probably not as big as the one that fell in 2 million years ago, but it will still generate some fireworks when it gets sucked in, probably sometime next year. And this time, every telescope on Earth will be watching the action.

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