Fermi Sees Giant Gamma-Ray 'Bubbles' in the Milky Way

May 5, 2015

The Fermi Gamma-Ray Space Telescope has seen giant unexpected gamma-ray structures in the center of our Milky Way galaxy. The structures, which protrude above and below the Galactic plane in the center of the Galaxy like two opposing bubbles being blown up, are approximately 50,000 light-years tall.

An all-sky map of gamma-ray emission as seen by the Fermi LAT, showing the Galactic diffuse component and the bubbles.

Fermi's Large Area Telescope (LAT), for which KIPAC is the leading science institute, has opened a huge window onto the gamma-ray Universe.  Its sensitivity, which is much higher than the gamma-ray experiments that preceded it, is allowing us to learn about the processes that produce gamma rays throughout astrophysics.  This includes the very exciting potential to discover so far unknown phenomena, which we are now witnessing with the bubble-like structures. 
 
NASA announced the discovery of the bubble structures at a media teleconference on November 9, 2010.  KIPAC scientist Simona Murgia participated as a panelist and represented the Fermi LAT collaboration together with Fermi Project Scientist Julie McEnery of NASA's Goddard Space Flight Center.  The exciting news very quickly spread around the world and triggered the attention, curiosity, and wonder of many people.  The analysis that led to the detection of the bubbles was based on the publicly available data collected by the LAT and was performed by Meng Su, Doug Finkbeiner, and Tracey Slatyer of Harvard University.
 
A critical step in the science analysis is to disentangle these bubble structures from the already known diffuse gamma-ray emission from the Milky Way, which is understood to be largely the result of cosmic rays interacting with the interstellar gas and light.  At high Galactic latitudes - distances above and below the disk that contains most of the stars - the bubbles better stand out on top of the known Galactic emission.  At lower latitudes, however, they don't stand out quite as clearly.  Since our understanding of the Galaxy is incomplete, this makes it more difficult to see the bubble component closer to the disk well. 
 
The Fermi LAT team has been working on better understanding these structures and all of the observed diffuse gamma-ray emission from the Milky Way.  KIPAC members have been leading this work, including Markus Ackerman, Seth Digel, Igor Moskalenko, and Troy Porter on the modeling of the Galactic diffuse emission, and Simona Murgia on the identification and  characterization of the newly discovered structures. 
 
These newly found gamma-ray features could reveal unexpected and important physical processes in our Galaxy.  Although their origin is not yet understood, a few possibilities have been proposed.  One possibility is that the structures were inflated by jets of particles ejected by the supermassive black hole at the center of the Galaxy at some point in the distant past.  Another idea is that they were formed as a result of gas outflows during a burst of star formation millions of years ago.  In any case, the Galactic bubbles are just the latest in a series of fascinating and important finds by Fermi.
 
This work is described in part in a paper in the Astrophysical Journal (2010, 742, 1044).
 
Science Contact: 
Simona Murgia
KIPAC
murgia@slac.stanford.edu
 
Tidbit Author: Simona Murgia and Jack Singal