Evidence of Milky Way 'fireworks' seen

While the core of the Milky Way galaxy is a pretty tame place, cosmically speaking, evidence suggests it wasn't always that calm, U.S. astrophysicists say. Kelly Holley-Bockelmann at Vanderbilt University and Tamara Bogdanovic at the Georgia Institute of Technology said some "forensic" clues point to all manner of celestial fireworks in the galactic center several million years ago. A single event -- a violent collision and merger between the galactic black hole at the Milky Way's center and intermediate-sized black hole in one of the small "satellite galaxies" that circle our galaxy --could have produced the features seen today that suggest a more violent past, they said. One puzzling characteristic of the galactic center, they said, is the fact that it contains the three most massive clusters of young stars in the entire galaxy, containing hundreds of young, hot stars that are much larger than the Sun. Because these stars typically burn out in "only" a few million years because of their extreme brightness, there had to have been a relatively recent burst of star formation at the center, they said. Holley-Bockelmann and her colleagues set about creating a theoretical model to explain such characteristics. In their model, about 13 billion years ago the path of one of the smaller satellite galaxies orbiting the Milky Way was diverted so that it began drifting inward toward the core, and about 10 million years ago it reached the galactic center. The smaller black hole would have circled the galactic black hole for several million years before it was ultimately consumed. As the smaller black hole circled closer and closer, it would have churned up the dust and gas in the vicinity, and the violent gravitational tides produced by the process could easily have compressed the molecular clouds in the core to the super densities required to produce the young stars that are now located on the central black hole's doorstep. "The gravitational pull of the satellite galaxy's black hole could have carved nearly 1,000 stars out of the galactic center," Bogdanovic said.