What Are Fermi Bubbles?

   

  In 2010, data gathered by the Fermi Gamma-Ray Space Telescope revealed a new discovery. Scientists were surprised to find two enormous, bubble-like clouds that extend 50,000 light-years across the center of our galaxy, the Milky Way.

    The two gamma-ray-emitting bubbles stretch across more than half of the visible sky and may be millions of years old. (Gamma rays are electromagnetic radiation at the highest-energy, or shortestwavelength, end of the electromagnetic spectrum.) The origin of these previously unseen structures, however, remains a truly baffling mystery.

   A research paper appearing in the Astrophysical Journal in 2014 described some features of the aptly dubbed “Fermi bubbles.” First, the outlines of the structures are very sharp and well defined, and the bubbles glow evenly across their enormous surfaces. The most distant areas of the bubbles feature extremely highenergy gamma rays, yet there is noapparent cause for them that far from the galactic center. Lastly, the parts of the Fermi bubbles nearest the nucleus of the Milky Way contain both gamma rays and microwaves, but as the bubbles extend farther out, only the gamma rays are detectable.

     Theorists have offered several explanations for the unusual structures. The two most predominant theories both suggest the bubbles were formed by a large, rapid energy release. One possibility claims that enormous streams or jets of accelerated particles originating and blasting out of the supermassive black hole at the center of the Milky Way created the Fermi bubbles. Astronomers have observed such a phenomenon in other galaxies, and while it is unknown if the Milky Way black hole has an active jet today, it may have had one millions of years ago.

      Another commonly held theory argues that the Fermi bubbles were created during star formations over a period of millions or even billions of years. The gas ejections created from bursts of star formations, similar to the ones that produced huge star clusters in the Milky Way, theoretically rode massive galactic winds out to far-off distances and are held there by powerful magnetic forces. Scientists are eager to unravel the mystery of the Fermi bubbles’ origin. “Whatever the energy source behind these huge bubbles may be,” says David N. Spergel, a theoretical astrophysicist at Princeton University, “it is connected to the many deep questions in astrophysics