There is growing evidence that several million years ago the galactic center was the site of violent cosmic events. A pair of assistant professors – Kelly Holley-Bockelmann at Vanderbilt and Tamara Bogdanović at Georgia Institute of Technology – have come up with an explanation that fits these “forensic” clues, suggesting how a single event – a violent collision and merger between the galactic black hole and an intermediate-sized black hole in one of the small “satellite galaxies” that circle the Milky Way – could have produced the features that point to a more violent past for the galactic core.
The structure spans more than half of the visible sky, from the constellation Virgo to the constellation Grus, and it may be millions of years old. "We don't fully understand their nature or origin," said Doug Finkbeiner, an astronomer at the Harvard-Smithsonian Center for Astrophysics, who first recognized the feature by processing publicly available data from Fermi's Large Area Telescope (LAT). The LAT is the most sensitive and highest-resolution gamma-ray detector ever launched. Gamma rays are the highest-energy form of light.
"In other galaxies, we see that starbursts can drive enormous gas outflows," said David Spergel, a scientist at Princeton University. "Whatever the energy source behind these huge bubbles may be, it is connected to many deep questions in astrophysics."
One possibility beyond the Holley-Bockelmann and Tamara Bogdanović theory includes a particle jet from the supermassive black hole at the galactic center. In many other galaxies, astronomers see fast particle jets powered by matter falling toward a central black hole. While there is no evidence the Milky Way's black hole has such a jet today, it may have in the past. The bubbles also may have formed as a result of gas outflows from a burst of star formation, perhaps the one that produced many massive star clusters in the Milky Way's center several million years ago.
The scenario began about 13 billion years ago, when the path of one of the smaller satellite galaxies orbiting the Milky Way is diverted so that it began drifting inward toward the core. According to a recent study, this may have happened dozens of times in the lifetime of the Milky Way. As the satellite galaxy – a collection of stars and gas with an intermediate-sized black hole with a mass equal to about 10,000 suns – spiraled in, most of its mass was gradually stripped away, finally leaving the black hole and a handful of gravitationally bound stars.
About 10 million years ago, the stripped down core of the satellite galaxy finally reached the galactic center. When two black holes merge, they first go through an elaborate dance. So 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 pushed enough material into the galactic black hole in the process to produce the Fermi bubbles.
The violent gravitational tidesproduced 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.
In addition, the vigorous churning would have swept out the existing stars from the area surrounding the massive central black hole. In fact, the astronomer’s model predicts that the black holes’ merger dance should have flung a large number of the missing old stars out into the galaxy at hyper velocities, thus explaining the absence of old stars immediately around the super-massive black hole.
“The gravitational pull of the satellite galaxy’s black hole could have carved nearly 1,000 stars out of the galactic center,” said Bogdanović. “Those stars should still be racing through space, about 10,000 light years away from their original orbits.”
It should be possible to detect these stars with large surveys like the Sloan Digital Sky Survey because these stars would be traveling at much higher velocities than stars that have not undergone this type of interaction. So discovery of a large number of “high velocity stars” racing outward through the galaxy would strongly support the proposed scenario of the Milky Way and satellite galaxy merger.
The 400 by 900 light-year mosaic of several Chandra images at the top of the page shows the central region of our Milky Way galaxy, only about 25,000 light years from Earth, revealing hundreds of white dwarf stars, neutron stars, and black holes bathed in an incandescent fog of multimillion-degree gas. The supermassive black hole at the center of the Galaxy is located inside the bright white patch in the center of the image. The colors indicate X-ray energy bands - red (low), green (medium), and blue (high).
The mosaic gives a new perspective on how the turbulent Galactic Center region affects the evolution of the Galaxy as a whole. This hot gas appears to be escaping from the center into the rest of the Galaxy. The outflow of gas, chemically enriched from the frequent destruction of stars, will distribute these elements into the galactic suburbs.
Image credits: Artist's conception of satellite galaxy being pulled into the galactic black hole. (Julie Turner / Vanderbilt University) and NASA/UMass/D. Wang et al.