An ancient eruption of a supermassive black hole in the Milky Way may have inflated two huge bubbles of gamma rays which were just now discovered and are considered a new type of astronomical object.
"It shows, once again, that the universe is full of surprises," says Jon Morse, director of astrophysics at NASA headquarters.
Combined, the bubbles, which are aligned at the centre of the Milky Way, span a vast distance of about 50,000 light-years. The structures are very distinct, with defined edges, and have as much energy in them as 100,000 supernovae.
They were found with NASA's Fermi Gamma-Ray Telescope, which surveys the sky every three hours for the highest-energy light.
Among the 1,500 sources of gamma rays Fermi has mapped so far, nothing resembles the bubble-shaped structures, which stretch across more than half of the visible sky, from the constellation Virgo to the constellation Grus.
"You have to ask where could energy like that come from," says astronomer Doug Finkbeiner, with the Harvard-Smithsonian Center for Astrophysics.
Hints of the bubbles appeared years earlier in X-ray surveys and in maps of the cosmic microwave background radiation stemming from the Big Bang explosion.
"We had a hypothesis before Fermi launched that there should be some gamma ray emission in this part of the sky. We were thinking something a bit more modest, maybe something within 10 or 20 or 30 degrees of the centre, not these giant structures reaching all the way up to 50 degrees," Finkbeiner says.
Several possible explanations
Scientists have two possible explanations for the Fermi bubbles. Theory one: a burst of star-formation at the centre of the galaxy generated short-lived massive stars with energetic winds that blasted high-energy particles out into space.
Finkbeiner points out however, that it would take some time to accumulate as much energy as exists inside the bubbles. He favours an alternative theory: an outburst from the supermassive black hole lurking in the centre of the galaxy.
In other galaxies, astronomers have seen evidence for jets of particles triggered by matter that is being pulled into black holes, objects that have so much gravitational pull that not even light can escape their grasp.
There's no evidence that the Milky Way's central black hole, which is about 400 million times more massive than our sun, has jets, but astronomers suspect it might have in the past.
"We know it didn't get to be that big by sitting there quietly all the time. It certainly has had big accretion events in the past, where material falls on it and then some of that material comes back out as high-energy particles blasted out in the form of a jet," Finkbeiner says.
"We've never really seen very good evidence of it. This might be the first evidence for a major outburst of the black hole at the centre of the galaxy. When it's going full-blast … it would not actually take an enormous amount of time - maybe 10,000 or 100,000 years - for it to produce enough energy to create these structures," Finkbeiner says.
"This result is very exciting," says Fermi scientist Simona Murgia, with the SLAC National Accelerator Laboratory in Menlo Park, California.
"These features could reveal unexpected and very important physical processes in our galaxy that until now we knew nothing about despite the fact that these features could possibly be almost as large as the Milky Way and might have been around for millions of years."
The discovery was unveiled on Tuesday and is the subject of an upcoming paper in The Astrophysical Journal.