Dec. 26, 2000 AUSTIN, Texas -- Two astronomers at The University of Texas at Austin, working with an international team of collaborators, have shown that they can provide reliable measurements of black hole masses for active galactic nuclei such as quasars even at great distances.
The UT Austin astronomers have demonstrated that two techniques used for measuring nearby black hole masses are just as suitable for measuring the most distant quasars. They said use of these techniques should enable astronomers to learn more about black hole growth and the way galaxies are formed. The astronomers, Dr. Karl Gebhardt and Dr. John Kormendy, discussed their research at the 20th Texas Symposium on Relativistic Astrophysics in Austin this week. Gebhardt is an assistant professor and Kormendy holds the Curtis T. Vaughan, Jr. Centennial Chair in Astronomy at UT Austin's department of astronomy.
Black holes are celestial bodies that are so compressed their surface gravity is strong enough to keep even light from escaping. Some 38 black holes have been discovered in the universe, 13 of them by Gebhardt and six by Kormendy. Supermassive black holes are one million to one billion times the mass of the sun and are found at the centers of galaxies.
Because they are invisible, scientists detect and study black holes by observing the movements and velocities of the stars swirling around them. Quasars are brilliant, extremely distant celestial objects believed to be composed of supermassive black holes that are actively swallowing gas and stars.
The UT Austin astronomers said it is important to develop techniques to measure quasar masses because such measurements will increase the knowledge of their complex physical environment. More importantly, because of their great distances, measurements of quasar mass should provide direct information about the evolutionary history of the black holes and their host galaxies.
The scientists said that so far, direct measurements of supermassive black holes have been made in at least 38 galaxies, based on large rotation and random velocities of stars and gas near their centers. Such measurements require high spatial resolution such as the views provided by the Hubble Space Telescope.
Gebhardt said this type of measurement only works successfully for nearby galaxies. Since quasars are too distant to apply these direct methods, astronomers have relied on physical models of the regions near the black holes to measure their masses. These techniques suffer from large and unknown uncertainties because of the complex nature of quasar physics, he explained.
While investigating samples of nearby objects, astronomers recently discovered a relationship between black hole mass and galaxy mass. The relationship allows astronomers to develop better models of quasars. Gebhardt said this promises eventually to provide a large increase in measured black hole masses from the presently small sample of 38 galaxies to thousands of galaxies out to extreme distances.
Two techniques for measuring black hole masses in quasars are available, called reverberation mapping and photo-ionization models, both of which involve uncertainties.
Reverberation mapping relies on the variability of quasars, and the fact that there are numerous clouds of gas that orbit near each supermassive black hole. As the black hole varies its energy output, the brightness of the radiation from the orbiting clouds varies as well.
Because light travels at a finite speed, the brightness variations in the orbiting clouds are seen later than those in the central engine source. The time difference tells astronomers how far the orbiting gas clouds are from the black hole. The speed of the orbits of the clouds also can be measured. Taken together, the results reveal the mass of the black hole. However, there has been no way to test the results, and some of the properties of the gas clouds are uncertain, Gebhardt said.
Photo-ionization models are even more uncertain, depending on an empirical relationship that astronomers are still trying to understand. That is, the amount of radiation that an orbiting cloud emits depends on how far away it is from the black hole. Thus, by simply measuring how "bright" the cloud appears astronomers can deduce its radius from the center. Knowing both the radius and the speed, astronomers then can measure the black hole masses.
Until now, astronomers have been reluctant to trust measurements of masses made from either of these techniques. The UT Austin scientists have shown that both techniques provide the same relationship between black hole mass and galaxy mass, as compared to the nearby sample of black hole masses determined from detailed dynamic modeling.
The astronomers said they have determined that both reverberation mapping and photo-ionization models can be applied even to the most distant quasars. As a result, large surveys are under way that should provide evidence for thousands of new black hole masses, making it possible to probe the growth of black holes in the early universe. Gebhardt said the calibration of the quasar measurement techniques should provide the detailed information needed to probe the structure of the central engine driving the quasar, allowing astronomers to explore both galaxy formation and black hole growth in exquisite detail.
For more information, see the Web site: http://chandra.as.utexas.edu/~kormendy/stardate.html
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