New picture of the night sky, thanks to Fermi.

Imagine a world shadowed each night by a giant cloud of mysterious, illuminated fog. From far outside our galaxy, one would observe the fog rising 25,000 light years above and below the plane of the Milky Way in two enormous plumes. In fact, this world is our own. Of course, you might be more inclined to believe me if you had the ability to see gamma rays.


The Milky Way, in visible light. Gamma rays are invisible to human eyes.
Image courtesy of erikaj121893.
The new discovery comes from a team of Harvard University scientists working with Fermi's Large Area Telescope (LAT). "This work presents a multiwavelength study of the inner Galaxy and identifies several large-scale gamma-ray features, most notably 2 large structures that we refer to as the 'Fermi bubbles'," wrote the team of astronomers in their recent paper. These Fermi bubbles could be millions of years old, but their origin is something of a mystery. Some have suggested that an ancient boom of star births may have later led to a clustered grouping of star deaths. A large number of supernovae occurring all at once could have injected enough energy into interstellar space to create the two plumes of gamma-ray emitting gas. Others, including Douglas Finkbeiner, a senior member of the team, believe that the massive black hole at the center of our galaxy gave rise to the bubbles. Many galaxies emit jets of high-energy radiation due to infalling material around a central black hole, and the Milky Way may be no exception.


The Fermi bubble structures. Image courtesy of Su et al..
It may seem odd that no one had detected these colossal structures before now. As it turns out, the Milky Way has a separate, amorphous halo of gamma-ray fog that prevented astronomers from observing the bubbles until recently. The bubbles, which are slightly more energetic than the ambient haze and have clearly-defined x-ray edges, were only apparent after subtracting this fog from LAT observations. Fermi continues to observe the universe at gamma-ray frequencies.