The Milky Way's Violent Past
March 6, 2013

Astronomers Explain Our Galaxy’s Violent Past

[ Listen to the three-part interview with Prof. Holley-Bockelmann in RedOrbit´s Your Universe Today Podcasts ]

John P. Millis, Ph.D. for — Your Universe Online

As astronomers peer out into the distant reaches of the Universe they find that some galaxies are emitting enormous amounts of radiation from their cores. Supermassive black holes at the center of these galaxies consume the surrounding gas and dust, heat it up, and thrust it into outer space at nearly the speed of light; forming these outflows into jets of radiation and matter using their powerful magnetic fields.

But in such galaxies, the supermassive black holes that lurk in their cores are some of the most massive in the Universe, often billions of times more massive than our Sun. By comparison the supermassive black hole that occupies the core of the Milky Way is a paltry 4 million solar masses.

Therefore it would be somewhat surprising to learn that our galaxy would ever enter a state of high activity. However, in the last several years some perplexing results have led scientists to wonder if the Milky Way was, in fact, active sometime in its recent history.

Hinting At A Violent Past

Back in 2010 the Fermi Gamma-ray Satellite discovered huge gamma-ray bubbles extending some 25,000 light-years above and below the galactic plane. These represent an echo of sorts, indicating where the intergalactic medium was recently — in galactic terms, anyway — energized.

Also, scientists found that three of the youngest star clusters in our galaxy are all found near the galactic center, while, at the same time, there is a distinct lack of older stars. This flies in the face of models that suggest that over time older stars will migrate toward the galactic center. All the while clusters of young, massive stars require huge, dense gas reserves to form; reserves that we would not expect to find near the supermassive black hole core.

Based on these strange findings a team led by Dr. Kelly Holley-Bockelmann, a professor of physics and astronomy at Vanderbilt University, realized that the Fermi Bubbles and the star cluster results could both be explained by a single event. If the Milky Way consumed a nearby dwarf galaxy — a galaxy similar to ours, but much smaller — the merger of our supermassive black hole and that of the other galaxy would release a massive amount of energy.

Colliding In Slow Motion

According to Dr. Holley-Bockelmann, “The scenario starts about 13 billion years ago, when a small satellite galaxy -- and the medium-sized black hole lurking within it -- falls toward the center of the galaxy. As the satellite winds its way deeper into the galaxy, it is gravitationally stripped of its stars and dark matter until it's a mere skeleton of its former self.”

Then, about 10 million years ago, “the shredded satellite finally reached the galactic center, and by this time it was just the black hole and a shroud of stars and dark matter. Still, it was massive enough that as it plunges through the final few hundred light years, it perturbs the gas that was calmly orbiting the galactic center, compressing some of it to form a burst of new stars, and driving the rest of it to fuel the supermassive black hole, which shortly after lets out an explosive Fermi Bubble 'burp'."

“In the meantime, the middleweight black hole sinks close enough to the center to bind to the supermassive black hole as a binary black hole. Once the two black holes make a binary, they are zooming around each other in a high-speed orbital dance that flings out thousands of stars that veer near couple. This relativistic black hole dance scours out the galactic core of its old stars.”

Confirming Suspicions

This new theory has a lot going for it, but the matter is far from settled. In the near term Dr. Holley-Bockelmann and her colleagues are examining the various aspects of the merger to better model how the motions of nearby stars and gas clouds proceeded over time.

They are also analyzing their theory to see what predictions can be made based on this model. Then, naturally, researchers will pour over the available data to see if what we already know about the center of our galaxy fits with this theory.

For now, however, this is an exciting take on the galactic center, and gets us one step closer to understanding the history of the Milky Way.

The study entitled "Can a satellite galaxy merger explain the active past of the Galactic Center?" is published in the Monthly Notices of the Royal Astronomical Society.