A new discovery could rewrite the history books on the Milky Way. According to a new study published in the Monthly Notices of the Royal Astronomical Society, our galaxy absorbed a smaller satellite galaxy several million years ago. But more than that, it was a celestial event that culminated in the meeting of each galaxy's central black holes. The ensuing collision resulted in a cataclysmic event that blasted a swath of old stars straight out of the core region at hypervelocity speeds.
The theory, put forth by Kelly Holley-Bockelmann of Vanderbilt and Tamara Bogdanović of Georgia Tech suggests that our galaxy has had a violent past — and it may not have been the only encounter like it over the course of its history.
A Fermi problem
Holley-Bockelmann and Bogdanović started to suspect something after studying Fermi Bubbles — two giant lobes of extremely energetic diffuse gamma-ray light that are bursting from the center of the galaxy straight above and below the galactic plane.
"Right now, each lobe is about 25,000 light years across," Holley-Bockelmann told io9. "Scientists know that gamma-ray light comes from the most energetic physics known, but we still don't completely understand what made these bubbles."
The astronomers suspect that particles erupted from the galactic center at nearly the speed of light, and then slammed into the ambient interstellar gas — a high-speed collision energetic enough to make just this kind of gamma ray.
But two clues in particular helped the team figure out what drove the Fermi Bubbles.
"The first clue was that the edges of these bubbles are super sharp, which made us think that whatever made the Fermi Bubbles was a very abrupt event," said Holley-Bockelmann. "By taking the current size and running the clock backwards, we could figure out that the Fermi Bubbles were made a few million years ago.
This was the second clue, she noted, because it matched the ages of the new stars formed at the galactic center — which made the researchers believe that both were caused by the same culprit.
"Ultimately, we suspected that gas was disturbed by a passing satellite galaxy, and some of the gas made stars, while the rest funneled into the supermassive black hole," she said. "The Fermi Bubbles can be thought of as the explosive 'burp' of the supermassive black hole after a gas-rich meal."
The galactic satellite infall event
And what a meal it must have been. After collaborating with Meagan Langto, Pau Amaro-Seoane, Alberto Sesana, and Manodeep Sinha, the researchers were able to piece together a chronology of events.
Things started to unravel for the smaller satellite galaxy about 13 billion years ago when it — and the medium-sized black hole lurking within it — fell towards the center of the Milky Way galaxy. As it wound its way deeper into our galaxy and towards its galactic core, it was gravitationally stripped of its stars and dark matter until it became a mere skeleton of its former self.
Then, a few million years ago, the shredded satellite finally reached the galactic center.
"But by this time it was just the black hole and a shroud of stars and dark matter," Holley-Bockelmann told us. "Still, it was massive enough that as it plunged through the final few hundred light years, it perturbed 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 let out an explosive Fermi Bubble 'burp'."
And that's when things started to get even more interesting.
The middleweight black hole sunk so close to the Milky Way's center that it became intricately bound to its supermassive black hole, thus becoming a binary black hole.
"Once the two black holes made a binary, they started zooming around each other in a high speed orbital dance that flung out thousands of stars that veered near couple," she noted.
And it was this relativistic black hole dance that caused our galaxy to fling out scores of its old stars from the core's vicinity. The forces involved must have been incredibly intense for this to happen; the Milky Way's supermassive black hole weighs about four million solar masses and is about 40 light seconds in diameter — only nine times the size of our sun.
The researchers say that those stars should still be racing through space, about 10,000 light years from their original orbits.
Out with the old, in with the new
And interestingly, this satellite infall event may help to explain not just the Fermi Bubbles, but also why the galactic core region is relatively devoid of old stars. Models predict that the density of old stars should increase with proximity to the center. But our galaxy has very few old stars within several light years of the black hole.
In addition, the galactic center contains the three most massive clusters of young stars in the galaxy (including the Central, Arches, and Quintuplet clusters). These are stars that should burn out relatively quickly on account of their extreme brightness. The only reasonable explanation, say the researchers, is that there must have been a relatively recent burst of star formation in the region.
And indeed, Holley-Bockelmann and Bogdanović are suggesting that the galactic merger triggered a brief epoch of strong star formation. In their paper, the authors admit that the theory is "not beyond reproach," but that it does a good job explaining all these galactic anomalies.
Moreover, these infall events may not be rare across cosmological timescales. Their simulations indicate that infall events could happen once every one billion years.
"This implies that there may have been other bursts of hypervelocity star ejections, which can seed a population of 'intragroup stars' farther out in the halo of the Galaxy," they noted in the study.
Looking ahead, Bogdanović and Holley-Bockelmann plan to publish a paper about these hypervelocity stars in particular.
Their paper, "Can a Satellite Galaxy Merger Explain the Active Past of the Galactic Centre" has been published in Monthly Notices of the Royal Astronomical Society.
Images: Top: an artist's illustration of a satellite galaxy falling into the submassive black hole at the center of the Milky Way galaxy (Julie Turner, Vanderbilt University). Interior images NASA; JPL via Harvard-Smithsonian Center for Astrophysics; Chandra X-Ray Telescope.