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Black Hole Collision May Have Set Off Fireworks in the Milky Way

The Milky Way's center houses a supermassive black hole so sleepy that it probably hasn't swallowed a decent meal for years. Yet a growing body of evidence indicates that the now-dormant beast, about as massive as 4 million suns, fueled a firestorm of activity just a few million years ago, including the sustained emission of some of the highest energy radiation in the universe. A new study offers a dramatic explanation for these past fireworks: The sleeping giant woke when a smaller black hole from another galaxy smashed into it.

Kelly Holley-Bockelmann of Vanderbilt University in Nashville and her colleagues say their scenario could explain three puzzling phenomena. Last year, astronomers discovered a striking new feature in the Milky Way: a pair of gamma ray-emitting gas bubbles, each the size of a small galaxy, emanating from the Milky Way's center and apparently fueled by some kind of violent event at the core of the galaxy. The core also contains an unusually high abundance of newborn stars and a lower-than-expected number of elderly stars.

All three phenomena, Holley-Bockelmann says, could result from the same event: the dregs of a small satellite galaxy, housing an intermediate-mass black hole about as heavy as 10,000 suns, smacking into the Milky Way's center about 10 million years ago. The Milky Way's gravity would slowly have stripped the satellite galaxy of most of its mass since the body first began falling toward the Milky Way about a billion years after the big bang but would still be hefty enough to make a stir, the team's simulations show.

The collision would have churned up gas orbiting within the innermost 5000 light-years of the Milky Way, pushing the gas into the center, Holley-Bockelmann says. Some of the incoming gas would have fallen onto the Milky Way's supermassive black hole, generating the bubbles of gamma ray-emitting gas like a belch after a good meal. Other inflowing gas would provide the raw material for making the young stars observed at the center today. And interactions between the Milky Way's black hole and the smaller one from the satellite galaxy could have flung out old stars from the center as the two black holes merged, she and colleagues report in a paper posted online this month at

The astronomers conjecture that the Milky Way's supermassive black hole went back to sleep some time after this event because after it gorged itself, the beast was no longer refueled by an incoming supply of gas.

The new study "is one of the more plausible explanations I have seen and merits further study," says Harvard University's Douglas Finkbeiner, who discovered the gamma ray bubbles and was not part of the current collaboration. He notes that it remains unclear whether the bubbles were generated by a massive wave of star formation at the center, material falling onto the supermassive black hole, or some other violent event.

Two tests could assess the validity of the merger model, says study collaborator Tamara Bogdanović of the University of Maryland, College Park. If a black hole merger did fling out old stars from the Milky Way's center some 10 million years ago, they should have formed a ring or shell of high-velocity stars a few thousand light-years from the center, she notes. These stars could be detected by the Hubble Space Telescope, which has already recorded high-speed stars that lie much farther from the galactic center.

In addition, detailed simulations of the new model should be able to explain the peculiar distribution of the young stars at the center of the Milky Way. The stars appear to form two separate disks which lie nearly at right angles to each other. "How such an orbital configuration can arise is still a mystery," says Bogdanović. But the merger model might account for it by sending multiple streams of star-forming gas into the Milky Way's center at different angles.

The proposed merger may not be unique in the history of the Milky Way, the astronomers note. Simulations by other teams suggests a small galaxy may collide with the Milky Way once every few billion years. In that case, our galaxy's supermassive black hole has yet to eat its last supper.