For years astronomers have been puzzled as to why our Milky Way galaxy's "volcano"—a supermassive black hole (SMBH) at its core—is dormant today.
It seems the answer may simply be that we didn't catch the cosmic monster—weighing at least four million times the mass of our sun—feeding at the right time, according to a new study.
"If we had been around to see it two million years ago, the situation would have been very different," said study co-author Philip Maloney of the University of Colorado in Boulder.
"The Milky Way's black hole was maybe ten million times brighter [then]," he said. "I don't think anyone really had any expectation that SMBH might vary in luminosity by such a huge factor on such a short—relatively speaking—time scale."
Evidence: "Fossil Imprint"
Astronomers have long suspected there was an ancient outburst from the hibernating black hole, but it's only now that they believe they have found an actual "fossil imprint" of the cosmic monster's last big meal.
The international team's new theory points to a lacy filament of gas, mostly hydrogen, called the Magellanic Stream, which can be seen trailing behind our galaxy's two small companion galaxies: the Large and Small Magellanic Clouds.
Maloney believes powerful beams of energy erupting from the SMBH two million years ago hit the stream—causing its hydrogen gas to get ionized and light up, much like the glow of auroras we see here on Earth. This ionization of the Magellanic Stream has puzzled scientists since its discovery two decades ago.
"No one has been able previously to come up with a good model to explain the ionization," said Maloney.
The team now suspects that this glowing stream of extragalactic gas may be the fossil imprint of the SMBH eruption two million years ago. (See black hole photos.)
The estimated orientation and amount of energy of the original outburst, including the cooling time of the illuminated stream, fit very well with the proposed model.
Further evidence for a giant eruption sometime in the distant past has also come in the form of recently detected gamma ray and radio wave signatures of two giant, hot bubbles of gas called Fermi bubbles. Thought to have been belched out by the SMBH, the Fermi bubbles sit above and below the plane of the Milky Way.
Next Outburst Soon?
The question is not if there will be another eruption, but when, scientists say.
Infrared and x-ray satellites have been able to peer into the heart of our galaxy and detect radiation flowing out from the region around the black hole as it rips apart the small, orbiting clouds of gas falling toward it and colliding with it.
Astronomers now believe many gas clouds orbit the SMBH today, and they could trigger a future outburst—in fact, it may be just around the corner.
"They have been monitoring a cloud and predict that it will fall into the black hole at some point in the next year; however, the amount of material will be far less than the event that illuminated the stream," said co-author Greg Madsen, an astronomer at the University of Cambridge.
"It will be much fainter and will pose no threat to Earth, but several powerful telescopes will be poised and ready to watch what happens."
This research has been accepted for publication in the Astrophysical Journal.