Nowadays, the Milky Way is a pretty quiet galaxy, but that wasn’t always the case. An international team of scientists has found evidence that just a few million years ago the center of our home galaxy exploded, sending off a huge flare that extended more than 200,000 light-years from the core.
According to the study, about 3.5 million years ago two cone-shaped bursts of radiation blasted outwards in opposite directions from the center of the Milky Way. If you picture the galaxy as a flat disk, the flare extended above and below the plane, radiating out like two gargantuan lighthouse beams.
Known as a Seyfert flare, this outburst was so powerful that it disrupted the Magellanic Stream, which sits on average about 200,000 light-years away from the Milky Way. The Magellanic Stream is a trail of gas left behind by the Large and Small Magellanic Clouds, two dwarf galaxies that orbit the Milky Way.
And it was this effect on the Magellanic Stream that gave away the violent event. It gives off a strange glow that has largely gone unexplained, but in a previous study the researchers suggested that ultraviolet light from the Seyfert flare would have caused the hydrogen in the Magellanic Stream to glow at a particular wavelength.
In the new study, the researchers say they’ve used data from the Hubble Space Telescope to confirm the timing – 3.5 million years ago – and the culprit – Sagittarius A (Sgr A*), the supermassive black hole at the center of the Milky Way.
It might sound like an eternity ago, but 3.5 million years is pretty recent, cosmically-speaking. After all, the Milky Way itself is thought to be about 13.5 billion years old. Even in the 4.5-billion-year history of Earth that’s recent – human ancestors were already wandering around the African continent by then.
The event also didn’t last very long. The researchers estimate that it came and went within about 300,000 years, which is a blink of an eye on the galactic scale.
The team also says that the new study underlines Sgr A* as the prime suspect. Nuclear activity in the supermassive black hole is essentially the only thing powerful enough to create these flares.
"These results dramatically change our understanding of the Milky Way," says Magda Guglielmo, co-author of the study. "We always thought about our galaxy as an inactive galaxy, with a not so bright center. These new results instead open the possibility of a complete reinterpretation of its evolution and nature. The flare event that occurred three million years ago was so powerful that it had consequences on the surrounding of our galaxy. We are the witness to the awakening of the sleeping beauty."
This study adds more evidence to the growing pile that reveals the tumultuous past of the Milky Way. Previous research indicates that our galaxy may have gone through a quasar phase about six million years ago. Two colossal chimneys of X-ray radiation were recently discovered pumping energy from the center of the galaxy to the outskirts, which could be relics from this active past.
The Milky Way might not be done with activity, either. Earlier this year, astronomers noticed unprecedented flares of light coming from Sgr A*. But that doesn’t mean it’s kicking back into quasar levels of activity – instead, it might have just been a burp after a hearty meal of a gas cloud that wandered too close.
The study was conducted by researchers at the ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia National University, the University of Sydney, the University of North Carolina, University of Colorado and the Space Telescope Science Institute.
The research is due to be published in the Astrophysical Journal.
