Palomar Survey Reveals Peak in Quasar Formation
PASADENA—Astronomers have discovered direct evidence that most quasars came into existence during the same era, when the universe was still in its infancy. This discovery will help scientists use quasars, the most luminous objects in the sky, as tools for studying the universe back to a time when it was less than a billion years old.
"This survey allows scientists to investigate for the first time the era of quasar formation," said Maarten Schmidt, a Caltech astronomer and a coauthor of the study.
Using data from the recently completed quasar search known as the Palomar Transit Grism Survey, Schmidt, Donald P. Schneider of Penn State, and James Gunn of Princeton University published their discovery in the July 1995 issue of the Astronomical Journal. (A grism, from the combination of grating and prism, is a transmission grating mounted on a clear, wedge-shaped piece of glass.)
The survey shows that the space density of quasars—the number of quasars in a given volume of space—reaches a maximum for those with redshifts between 1.7 and 2.7, and declines steeply for quasars with higher redshifts. "This maximum means there was a peak in the rate of quasar formation between 1.9 and 3.0 billion years after the Big Bang," Gunn said, "and a much lower rate earlier in the history of the universe."
A typical quasar emits 100 times more energy than our home galaxy, the Milky Way. This makes them the most luminous and also some of the most distant known objects in the universe. Because light from quasars takes billions of years to reach the earth, scientists see them as they were billions of years ago. Therefore quasars are important to astronomers as one of the best probes available for studying the conditions present in the early universe.
Astronomers first identified quasars in 1960 as starlike counterparts to strong sources of radio waves, but were initially unable to determine the nature of the objects. In February 1963, Maarten Schmidt made a breakthrough.
"I recognized that the pattern of spectral lines in one particularly bright quasar was due to hydrogen, but that the location of the lines was redshifted," Schmidt said. "This indicated that the object was moving away from the earth at a very high velocity."
Redshifting is an effect seen in rapidly receding sources of light, where the spectral lines of such sources move toward longer wavelengths, or toward the red end of the visible spectrum. The larger the redshift, the more the light is shifted toward red, and the greater the distance to the source.
The small size of quasars is as astonishing as their luminosity. Studies of the variability of quasars have shown that their brightness can change on time scales of days, or sometimes just a few hours, which implies that their physical size is not much larger than our solar system. Because of quasars' extraordinary brightness and small size, astronomers suspect that they are probably powered by matter spiraling into a supermassive black hole. But just how quasars form and whether black holes really power them remain a puzzle, one which studies such as the one reported here will help scientists solve.
The Palomar Transit Grism Survey was undertaken with the goal of finding a large number of high-redshift quasars so that scientists could study the evolution of these objects back to a time when the universe was less than a billion years old. The survey began in 1985 using a special electronic camera designed by James Gunn that was mounted on the 200-inch Hale Telescope at Palomar Observatory.
Finding a large number of quasars was like looking for needles in a haystack and required special software to separate the quasars from superficially similar foreground objects. "For every high-redshift quasar that we found, we recorded and sorted through thousands of nearby objects," Schneider said.
The Palomar Transit Grism Survey succeeded in identifying 90 quasars with redshifts between 2.75 and 4.75, with a typical luminosity more than a trillion times that of our sun. Analysis of the survey data has revealed that between redshifts of 2.7 and 4.7, the space density of luminous quasars declines by a factor of seven. That is, for quasars with redshifts greater than 2.7, the higher the redshift, the fewer quasars there are in a given volume of space.
Previous studies by other groups have shown that the space density of quasars increases dramatically—by a factor of 100 or more—in the range of redshifts between 0 and 2.0. These results, combined with other studies of quasars with intermediate redshifts, show that the space density of quasars exhibits a sharp peak at a redshift between 1.7 and 2.7, indicating that the bulk of quasar formation must have occurred around 2.5 billion years after the Big Bang. This result will help astronomers refine their theories by placing important constraints both on models of galaxy and quasar formation, and on ideas about the mechanism that supplies quasars with their tremendous energy.
This is a joint release by Caltech and the Pennsylvania State University.
Written by John Avery