First Gamma Rays Associated With Galaxy Clusters
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453April 5, 2001
Greenbelt, Md. -- Clusters of galaxies, the largest bound structures in the universe, may admit gamma rays, a discovery that would have broad implications for the structure, evolution and mass content of the universe.
Dr. Sergio Colafrancesco of the Astronomical Observatory of Rome has found a probable association between known galaxy clusters and unidentified gamma-ray sources that have stymied astronomers for several years. He presents his findings today at the Gamma Ray 2001 conference in Baltimore, Maryland.
"If these gamma-rays are truly associated with galaxy clusters, they could be produced by cosmic-ray collisions in the intergalactic space, by the annihilation of dark matter particles inhabiting the clusters, or by unknown active galaxies, among other possibilities," said Colafrancesco. "Yet any of these possibilities would be of particular interest to the scientific community."
Gamma rays represent the most energetic form of radiation. These high-energy photons are produced in cataclysmic events, such as the flow of extremely hot gas towards a black hole or the collision of elementary particles moving at nearly the speed of light.
Colafrancesco said his analysis shows, for the first time, that galaxy clusters can emit gamma rays and that the physical component responsible for the gamma-ray emission (i.e. particles moving at relativistic speed which are accelerated by shocks or produced in the annihilation of dark matter particles) might also play a significant role in cluster evolution.
Last March, a team of astronomers at NASA Goddard Space Flight Center announced that 170 of the 271 catalogued gamma-ray sources in the universe remain unidentified. The sources were detected by the EGRET instrument aboard the Compton Gamma Ray Observatory. Fifty of these sources are likely beyond the Milky Way galaxy.
Colafrancesco said that a large fraction of these unidentified, extragalactic gamma-ray sources are spatially correlated within one degree with the position of nearby galaxy clusters. The probability that such a spatial correlation is due to a random effect is less than 0.5%.
Colafrancesco compared the X-ray brightness of a given cluster with the gamma-ray brightness of the spatially associated unidentified gamma-ray source and found further correlation. The existence of such a correlation indicates with a confidence level greater than 95% a physical connection between the content of the galaxy cluster and the gamma-ray emission of the associated EGRET source.
"This may indicate that there is a substantial amount of high-energy particles whose existence was not known directly before," Colafrancesco said. "This is important in understanding the origin of the radio halos and of the emission excesses observed in the extreme UV and hard X-ray energy ranges, which are not yet fully understood. But it could also be important for cosmological studies since the presence of relativistic particles in the intra-cluster medium may change the total mass estimates of clusters, which are used for cosmological studies."
Some of the more interesting associations, Colafrancesco said, include those between the cluster Abell 85 and the EGRET source 3EG J0038-0949; between Abell 1914 and EGRET source J1424+3734; and between Abell 1758 and EGRET source 3EG J1337 +5029. (refer to figures).
Abell 85 (at 1.2 billions light years, z = 0.056) is known to possess strong radio sources, a diffuse radio halo and hence a large injection of high-energy particles in the intra-cluster medium. Abell 1914 (at 3.6 billions light years, z = 0.171) also has strong radio sources in it and a diffuse radio halo. Abell 1758 (at 6 billions light years, z = 0.279) also has strong radio sources and a diffuse radio halo.
The presence of radio halos in clusters of galaxies indicates that there are probably relativistic electrons in the intra-cluster medium. If there are electrons, it is very likely that there are relativistic protons there as well. And protons interacting with the intra-cluster gas can produce gamma rays, beyond those possibly produced by relativistic electrons through a much less efficient mechanism.
In 1971, the first observations of X-rays from galaxy clusters led to a new era in astro-physics and cosmology. Thirty years later, Colafrancesco said, preliminary evidence for gamma-ray emission from the same galaxy clusters may open another new window for study. However, Colafrancesco cautioned, much follow-up work is needed.
Colafrancesco correlated the gamma-ray sources in the Third EGRET catalogue with X-ray data of galaxy clusters from the ROSAT and BeppoSAX satellites missions and also optical and radio observations. Data were analyzed through the ASI (Agenzia Spaziale Italiana) facilities at the ASI Science Data Center in Frascati (Rome), Italy, with the collaboration of Dr. Paolo Giommi.
Earlier theoretical work on this subject was published in 1995 by Dr. A. Dar of the Technion in Haifa, Israel; in 1997 by Dr. V. Berezinsky and collaborators of the LNGS in L'Aquila, Italy; in 1998 by Drs. S. Colafrancesco and P. Blasi; in 1999 by Drs. H. Voelk and A. Atoyan of the Max Planck Institute for Kernphysik in Heidelberg, Germany; and recently in 2000 by Drs. S. Colafrancesco and B. Mele of the Astronomical Observatory of Rome, Italy. Next generation gamma-ray space instruments -- such as AGILE (2003 launch) and the Gamma Ray Large Area Space Telescope (2006 launch) -- will probe the nature of unidentified EGRET sources.
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