Scientists from Stanford and the Department of Energy’s SLAC National Accelerator Laboratory have analyzed more than four years of data from NASA’s Fermi Gamma-ray Space Telescope, along with data from other experiments, to create the most detailed portrait yet of two towering bubbles that stretch tens of thousands of light-years above and below our galaxy.
The bubbles, which shine most brightly in energetic gamma rays, were discovered almost four years ago by a team of Harvard astrophysicists led by Douglas Finkbeiner who combed through data from Fermi’s main instrument, the Large Area Telescope.
The new portrait, described in a paper that has been accepted for publication in The Astrophysical Journal, reveals several puzzling features, said Dmitry Malyshev, a postdoctoral researcher at the Kavli Institute for Particle Astrophysics and Cosmology who co-led on the analysis.
For example, the outlines of the bubbles are quite sharp, and the bubbles themselves glow in nearly uniform gamma rays over their colossal surfaces, like two 30,000-light-year-tall incandescent bulbs screwed into the center of the galaxy.
Their size is another puzzle. The farthest reaches of the Fermi bubbles boast some of the highest energy gamma rays, but there’s no discernable cause for them that far from the galaxy.
Finally, although the parts of the bubbles closest to the galactic plane shine in microwaves as well as gamma rays, about two-thirds of the way out the microwaves fade and only gamma rays are detectable. Not only is this different from other galactic bubbles, but it makes the researchers’ work that much more challenging, said Malyshev’s co-lead, KIPAC postdoctoral researcher Anna Franckowiak.
“Since the Fermi bubbles have no known counterparts in other wavelengths in areas high above the galactic plane, all we have to go on for clues are the gamma rays themselves,” she said.