Information on the Gamma-Ray Halo
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Information on the Gamma-Ray Halo
PREPRINT
Final version, accepted for publication in New Astronomy.
If you requested one via email,
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in a harddrive crash.
The new, unexpected distribution of gamma rays, mapped by NASA's Compton Gamma Ray Observatory, forms an aurora many thousands of light years thick and possibly surrounding the entire Milky Way, the galaxy containing Earth, according to Dave Dixon, an assistant research physicist at UCR. The discovery was presented today at the meeting of the High Energy Astrophysics Division of the American Astronomical Society in Estes Park, Colo.
"Looking in any other wavelength, there is nothing out there that should be obviously making gamma rays. These gamma rays are providing the first evidence that some high energy process is occurring out there," said Dixon, who made the discovery with Dieter Hartmann, an astrophysicist at Clemson University, and Eric Kolaczyk, a statistician at the University of Chicago.
The three scientists analyzed data collected by the Energetic Gamma-Ray Experiment Telescope (EGRET), one of four instruments aboard the Compton Gamma Ray Observatory, which is orbiting Earth on a satellite to measure and record invisible gamma rays which cannot be detected on the ground because Earth's atmosphere absorbs them.
The visible light from stars and galaxies seen with optical telescopes represents only a fraction of the radiated energy from celestial bodies and other space phenomenon. Gamma rays are photons, or particles of light, with the highest energies of all forms of radiation, higher even than X-rays. For instance, a single gamma-ray photon seen in the newly discovered halo has about 1 billion times the energy of a photon of ordinary, visible light. Gamma rays are of great interest to astrophysicists because they may offer clues to some of the most violent events in the universe, such as the process of a dying star becoming a supernova and the birth of a galaxy.
What is so curious about the newly discovered gamma-ray cloud, Dixon said, is that the photons do not appear to be coming from any compact sources, like other galaxies or a black hole. "The reason this is interesting is that there isn't any obvious source for these gamma rays, based on astronomical observations in other wavelengths of light," Dixon said. "That is, as far as we can tell using other telescopes, the space around our galaxy is rather empty of the kinds of things which we would expect to generate gamma rays in the observed brightness distribution."
Though no particular explanation is singled out by the current data, Dixon and Hartmann offer three possibilities -- that gamma rays are created when high-energy cosmic rays collide with photons of lower energy light, such as visible or infrared light, that they are being emitted by rapidly spinning neutron stars, or that the gamma-ray distribution is providing evidence of dark matter, the missing mass of the universe that scientists have not been able to observe directly.
The high-energy gamma rays seen in the halo could be the result of collisions of high-energy cosmic rays, in the form of electrons, traveling at near the speed of light and colliding with low energy photons they encounter is space, Dixon said. Under this scenario, the electrons transfer some of their energy to the photons of visible or infrared light, boosting them to gamma-ray energies, in a process called the "inverse Compton effect" by astrophysicists.
It has recently been reported that some other spiral galaxies that are similar to the Milky Way have a dim halo of infrared photons surrounding them, providing the "seed photons" that could be converted to gamma rays by interacting with high-energy cosmic rays.
Some galaxies are also seen to be undergoing "starbursts" -- rapid formation and destruction of massive stars in their centers. These massive stars are short-lived, and die in giant explosions called supernovae. The shock wave of energy from supernovae leads to even more star formation activity, making the center of such galaxies a caldron of violent activity. Such a starburst would generate massive amounts of cosmic rays, providing the high-energy electrons needed to generate gamma rays.
The cloud of gamma-rays detected may provide evidence that the Milky Way, too, was once a starburst galaxy, Dixon said. "That is sort of an open question right now," he said. "There seems to be unexplained evidence for such past activity in the center of the Milky Way." From Earth, the center of the Milky Way is located about 25,000 light years away, in the direction of the constellation Sagittarius.
It is also possible, he said, that neutron stars, incredibly dense objects which are left over from some supernova explosions, are emitting the gamma rays. It is known that certain pulsars -- rapidly spinning neutron stars which shoot out beams of radiation like a lighthouse -- emit almost all of their energy in gamma rays and are basically invisible otherwise.
These pulsars, however, would have to exist in great numbers to account for the gamma-ray halo seen by Dixon and his colleagues. "Though it is unlikely that neutron stars were formed in the galactic halo, their massive star progenitors could have lived in the plane of the Milky Way, with the neutron stars being shot out by the violent supernova explosions," he said.
Another, more intriguing, possibility is that the cloud is providing indirect evidence of dark matter. The visible galaxies of stars and planets account for only a small percentage of the total mass of the universe, according to scientists who have calculated that a far greater total mass is needed to hold celestial objects in their gravitational orbits. Physicists have described the missing matter as "dark matter," since it doesn't absorb or emit light.
A number of theories to explain dark matter have been advanced, one attributing the missing mass to weakly interacting massive particles, or WIMPs. These heavy particles -- theoretical entities not yet detected in earthbound experiments -- would not interact with light, according to the theory. But it is possible, Dixon said, that two WIMPs could occasionally collide with one another, generating gamma rays or other particles of matter and antimatter which subsequently annihilate into gamma rays.
"If you look at the wide distribution of where the gamma-rays are coming from, it is very suggestive of a dark matter distribution," he said.
Dixon cautioned, however, that the scientific explanation for the phenomenon is still a wide open question. The Gamma Ray Large Area Space Telescope (GLAST), planned for a future NASA mission, may help answer the puzzle, he said.
"These are just three viable options, and no doubt others will be advanced in the future," he said. "The only way to nail this thing down is to get better data with a more advanced gamma-ray telescope such as GLAST, or to find some smoking gun which can be observed in another wavelength of light."
EGRET, one of the instruments aboard the Compton Gamma Ray Observatory, was designed to study high-energy gamma-ray emission. The instrument records both the direction and energy of gamma-ray photons, but this "picture" of the sky in gamma rays is noisy, similar to a television picture with a lot of static. The source of this noise is not interference of any sort, but occurs because relatively few gamma-ray photons are detected, compared with an optical telescope, for example.
Kolaczyk and Dixon developed a technique using wavelets, a relatively new signal processing tool, to remove some of this noise and get a clearer picture of the gamma-ray sky. Dixon and Hartmann, both visiting scientists at the Max-Planck Institute in Germany last summer, used the technique to map the diffuse cloud of gamma rays that appears to form a corona surrounding the Milky Way.
Their research was supported by NASA's Compton Gamma Ray Observatory Guest Investigator Program and the Max Planck Gesellschaft, Germany, with the assistance of the Compton Observatory Science Support Center. Those involved in the project in addition to Dixon, Hartmann and Kolaczyk were: Jalal Samimi of Sharif University, Tehran; Parameswaran Sreekumar of the Laboratory for High Energy Astrophysics, Goddard Space Flight Center; and Roland Diehl, Gottfried Kanbach, Hans Mayer-Hasselwander and Andy Strong of the Max Planck Institut für Extraterrestrische Physik in Germany.
Questions or problems: Contact Dave Dixon
