Antimatter Fountain in the Galactic Center
Charles D. Dermer
Naval Research Laboratory
President Lettieri called the 2081st meeting to order at 8:15 p.m.on November 21, 1997. The Recording Secretary read the minutes of the 2080th meeting and they were approved.
The speaker for the 2081st meeting was Charles D. Dermer of the Naval Research Laboratory. The title of his talk was “Antimatter Fountain in the Galactic Center.”
Our galaxy is shaped like a pinwheel some 60,000 light years in diameter which rotates about a central axis. Our little planet and solar system are located far from the center of things, and gas and dust obstruct our view of the galactic center. The effect is akin toa small child trying to see the stage when seated at the back of acrowded auditorium. The cloudiness of the center of our galaxy is most apparent at the infrared, visual and ultraviolet wavelengths, which have received the most study. Our galaxy is, however relatively transparent to radiation with energies greater than several thousand of electron volts, and is completely transparent to gamma rays above hundreds of thousands to millions of electron volts. This is true because gamma rays are a much more penetrating form of radiation and pass nearly unimpeded through the dense clouds that surround the central regions of our Milky Way. We have had a good idea of what our galactic center might look like because we can see the centers of other galaxies from the top — or the bottom if you prefer — rather than edge. Some of these galaxies show a great deal of activity at the center, either due to intense star formation or due to matter falling into and being heated by a massive black hole. There are even indications from radio and X-ray observations that our own galactic center hosts a supermassive black hole and some smaller black holes. The Oriented Scintillation Spectrometer Experiment (OSSE) aboard NASA's Compton Gamma Ray Observatory observed galactic gamma radiation in the spectrum of 0.05 to 10 million electron volts (MeV). The observations showed intense annihilation of antimatter positrons with electrons at a characteristic energy of 0.511 MeV. The maps of this emission provided a surprise. The annihilation is taking place not only in the thick disc of our galaxy but asymmetrically above the center of the galaxy in the direction called the Galactic North Pole. The OSSE observations indicate the discovery of a new component of our galaxy: an asymmetric fountain of antimatter emerging from the center of our galaxy. The search for proton-anti proton annihilation radiation, which gives a signal that is more difficult to detect, continues.
The observed antimatter positrons could be produced by black holes, neutron stars, or gamma ray bursters. Mr. Dermer argued that the most likely scenario involves an intense phase of star formation and supernovae activity in the galactic center region which drives a fountain of hot gas, laden with radioactive debris and positrons out into the surrounding halo of our galaxy. This links our galaxy with starburst galaxies which show intense star formation in their central regions, and with active galactic nuclei, which are galaxies whose central activity is thought to be powered by supermassive black holes.
President Lettieri thanked Mr. Dermer for the society and announced the next meeting. He then made the usual parking announcement and adjourned the 2081st meeting to the social hour at 9:21 p.m.