Scientists Find Most Distant Soft Gamma-Ray Quasar Yet
Contact:
Christopher Wanjek
wanjek@gsfc.nasa.gov
301-286-4453November 22, 1999
Huntsville, Ala. -- Scientists from Italy, England and the United States have spotted the farthest, brightest object ever to be seen with a soft gamma-ray telescope, a quasar in a galaxy eleven billion light years away likely harbouring a supermassive black hole.
The discovery was made with new technology that lets scientists use an instrument aboard NASA's Compton Gamma-Ray Observatory (CGRO) for something it wasn't designed to do.
The rich gamma-ray data from this distant object, known since 1989 as an optical light source, provide new clues to how galaxies form and how black holes manifest their power. In astronomical terms, the quasar has a redshift of 2.17, which means the quasar's light was produced when the universe was only about 30 percent of its present age.
"We have now proven that we can detect quasars of such distance in gamma rays, the most energetic form of electromagnetic radiation," said Angela Malizia, a Ph.D. candidate at the University of Southampton, UK, whose doctoral thesis includes this observation.
"This is exciting, for we have several years' worth of gamma-ray data to go through and we can hope to extract more examples of these distant objects which formed when the universe was much younger," said her supervisor, Prof. Tony Dean.
Malizia and Dean worked with a team of scientists, including Drs. Loredana Bassani and J.B. Stephen of the Istituto TeSRE/CNR Bologna, Drs. Bill Paciesas and Nan Zhang at The University of Alabama in Huntsville, and Dr. Mike McCollough of the Universities Space Research Association.
Gamma rays are the most energetic form of electromagnetic radiation, about one million times more powerful than optical light. Quasars, which look like stars yet shine with the power of over a thousand galaxies, are likely the result of a supermassive black hole in the core of a galaxy. These black holes crush the mass of a billion suns into a region no larger than our solar system. The gravitational force of the black hole emits tremendous amounts of energy, mainly observable in the narrow electromagnetic bands of infrared and optical light, and for a small number of quasars, radio waves.
"Studying quasars in a narrow band of wavelengths is analogous to watching a soccer match only at the midfield," said Dr. Bassani. "Observing such a distant quasar also in gamma ray energies, and the hope of finding more, widens our view of the field and allows a better understanding of the game."
Quasars fall within a family of objects called "Active Galactic Nuclei" (AGN), which are compact objects in the cores of galaxies emitting copious amounts of energy. This "gamma-ray" quasar, known by the technical name 4C71.07, can also be called a blazar, so named because it exhibits large variability and blazes strongly at all wavelengths. This blazar, like many, emits most of its energy as gamma rays.
Quasars, blazars and many other types of AGN are likely manifestations of the same phenomenon, a supermassive black hole dragging in hot galactic gas at nearly the speed of light. Blazars in particular are believed to contain relativistic jets beaming the high energy emission in our direction, and this observation supports this view.
The period of maximum quasar formation is around redshift 2 to 3, according to Prof. Dean. It is of great interest, therefore, to study objects at this distance, as this is likely to provide important information on the role that black holes play in forming galaxies and providing overall structure to the universe.
AGN observations at hard X-ray/soft gamma-ray energies have been elusive up until few years ago. Spectral information on high-redshift quasars are virtually absent at these energies.
The gamma-ray observation of quasar 4C71.07 provides for the first time information on the blazar spectrum and gives evidence of a correlation between changes in optical light and changes in soft gamma rays.
A fundamental question is whether quasars exhibit any spectral evolution, which has a direct impact on the quasar formation model. The team said that their results seem to suggest a lack of any spectral evolution, since this object is not significantly different from similar low-redshift quasars and blazars.
The observation also holds possible implications for the study of the cosmic diffuse gamma-ray background, high-energy radiation that blankets the universe. Blazars like 4C71.07, being so powerful in gamma-rays, are likely to make a significant contribution to the diffuse background. A better understanding of this background radiation will help determine the overall energy output in the universe and complement cosmological parameters.
The team detected the blazar with BATSE and CGRO's Oriented Scintillation Spectrometer Experiment (OSSE ). BATSE and OSSE observe lower-energy gamma rays, a band hundreds of times wider than optical light.
"BATSE wasn't designed for this kind of work, but we developed Earth occultation analysis techniques that allow us to make some interesting measurements of extra-galactic sources," Paciesas said. "Interestingly, some of the mathematical techniques we used were first developed for medical imaging."
Using this new technique, the institutions involved in this work (ITeSRE/CNR, Southampton University and UAH) are participating in a long-term project aimed at providing a map of the entire sky in the soft gamma-ray band using the BATSE database. The scientists hope this will provide a sample of AGN at redshifts greater than 1.0 (about 5 billion light years) to study some of the above-mentioned issues.
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Refer to the NASA Space Science News article for images and more information.