News

Milky Way Blows Bubbles ?!

07 May 1997

Astronomers using the NASA's Compton Gamma-Ray Observatory (CGRO) satellite to map the distribution of antimatter in the Milky Way Galaxy were surprised to find what appears to be a large antimatter bubble being blown upward from the center of our Galaxy. Matter is made up of the basic atomic building blocks of protons, neutrons and electrons. Antimatter particles are exact duplicates of particles but with opposite properties. For instance, a positron, the antiparticle of the electron, carries an equal, but opposite charge (the electron is negatively charged, the positron is positively charged). When an electron and positron encounter one another they annihilate, converting the mass of both particles completely into energy which we detect as gamma-ray emission at the specific energy of 511 keV. Since the 1970s, astronomers have seen the tell-tale sign of antimatter in the galactic center: a significant emission feature at exactly the right energy.

Matter is far more common in the observable Universe than antimatter, which is rarely observed. Antiparticles can be created from radioactive decay processes, and by matter falling into a black hole. A massive black hole is believed to exist at the center of the Milky Way Galaxy, and could be responsible for generating the positrons that give rise to the 511 keV emission when they interact with and annihilate electrons.

The surprising result from the CGRO instrument OSSE was not the detection of this annihilation radiation, but how it is distributed in our Galaxy. According to all previous theoretical models, it was expected that 511 keV emission would be seen at the galactic center and along the plane of the Galaxy. However, the OSSE results, which were released April 28, 1997 at the 4th CGRO Symposium in Williamsburg, VA, showed a large cloud of radiation emanating from the galactic center, perpendicular to the disk.

Old Model	Observations	New Model
Credit: D. D. Dixon (University of California, Riverside) and W. R. Purcell (Northwestern University)

The origin of this newly discovered cloud of antimatter is a mystery. "The antimatter cloud could have been formed by multiple star bursts occurring in the central region of the Galaxy, jets of material from a black hole near the Galactic center, the merger of two neutron stars, or it could have been produced by an entirely different source," said James D. Kurfess, head of the Gamma and Cosmic Ray Astrophysics Branch at the Naval Research Laboratory. It's a sure bet that theorists will be studying these new results, and others that will follow, to shed light on the mystery of the antimatter bubble.

Our old home Galaxy has a whole new look now, thanks to astronomers taking a look at it in gamma-rays.