A group of astrophysicists has located two massive bubbles of plasma, each extending tens of thousands of light-years, emitting high-energy radiation above and below the plane of the galaxy. The researchers found the structures in publicly released data from NASA's Fermi Gamma-Ray Space Telescope, which was launched in 2008 to investigate sources of extremely energetic photons—namely, gamma rays, which have higher frequencies than x-rays.
From its orbital perch hundreds of kilometers above Earth's surface, Fermi has charted the location of gamma-ray sources with its Large Area Telescope (LAT). But just where the gamma rays originate is not always clear; the foreground of Fermi's view is clouded with emission from events such as cosmic rays striking dust in the Milky Way's disk. To get a better picture of the gamma-ray environment, Douglas Finkbeiner of the Harvard–Smithsonian Center for Astrophysics and his colleagues carefully subtracted those sources based on maps showing locations of cosmic dust, models of the galactic disk, and known emitters of gamma rays, such as active black holes in other galaxies.
"There are many kinds of emission in the Fermi maps—there are things that we're expecting to see, like the dust-correlated emission," Finkbeiner said in an interview during the May meeting of the American Astronomical Society. Finkbeiner presented at the conference an early version of the research, which has now been finalized and readied for publication. "But then we saw some other things that we weren't expecting," Finkbeiner said in the interview. "We saw these giant bubbles reaching above and below the galactic center." The study, co-authored with graduate students Meng Su and Tracy Slatyer, will appear in The Astrophysical Journal. (Slatyer is now at the Institute for Advanced Study in Princeton, N.J.)
Finkbeiner compared the shape of the lobes of the so-called Fermi bubbles with teardrops or hot-air balloons. The two bubbles are symmetric, and each appears to originate at the Milky Way's center, where a black hole with the mass of four million suns lurks. Together they span a distance roughly half the diameter of the Milky Way. The origin of the Fermi bubbles is unknown, but a population of high-energy electrons that collides with mundane photons, boosting them to gamma-ray energies, seems to be the cause. Matter falling onto the galaxy's central black hole could give off a good deal of energy to produce electrons swept up in a hot plasma—and ultimately the gamma rays.
"When stuff falls into that black hole, as you can imagine, it makes a big mess," Finkbeiner said. "One of the things that happens is very high-energy particles get ejected, and probably shock waves, and you can get jets of material coming off of the thing." Those jets could blast into the interstellar medium above and below the plane of the galaxy and form bubbles that emit gamma rays.
An alternate possibility is a relatively recent burst of star formation in the inner galaxy, probably within the last 10 million years. "If you have many young stars all forming in the same place at the same time, they have tremendous stellar winds; some of them will blow up as supernovae—a lot of things can happen that heat gas and cause bubbles to expand," Finkbeiner said.
The researchers found flaws with both proposed explanations but noted that some combination of the two could be the cause. And indeed, David Spergel, a Princeton University astrophysicist who did not contribute to the study, explains that black hole accretion and starbursts can be related. "What we see in some external galaxies is the same inflow onto a black hole also produces a burst of star formation," Spergel says.
Spergel notes that much astrophysical work focuses on filtering out the foreground of Milky Way structures to see into the distant universe. "For most cosmologists our galaxy is an enormous nuisance," he says. "This is one of those instances where one scientist's foreground and garbage is another scientist's field of study."
Scooping new discoveries from a mission's public data sometimes causes disagreements of interpretation between the independent data miners and the mission's own scientists. To wit, Finkbeiner's 2003 detection of a haze of excess microwave emission near the galactic center in data from the Wilkinson Microwave Anisotropy Probe (WMAP) has not been fully accepted by Spergel and other members of the WMAP team.
But the Fermi team appears to be on board with the new finding. "Our view of this work is that this is an important discovery," offers Stanford University astrophysicist Peter Michelson, the principal investigator for Fermi's LAT. "Doug's work is excellent." Michelson adds that the study highlights the importance of involving a wider community of scientists through public data releases and notes that the mission team is working on its own follow-up analysis. "There remains much interesting work to do to figure out the real origin of these incredible bubbles in our galaxy," Michelson says.