Snapshots Probe the Early Universe
13 January 1992Drs. Bahcall, Dan Maoz, Donald Schneider, and Brian Yanny, all of the Institute for Astronomers are reporting surprising and interesting initial results from a survey of several hundred quasars now being carried out with NASA's Hubble Space Telescope (HST). Using HST's extremely high resolution images, this "Snapshot Survey" program has sought to detect evidence on gravitational lensing at a level of detail not obtainable with ground-based telescopes.
"The HST images provide unprecedented information about the frequency of gravitational lensing by galaxies," says Dr. John Bahcall of the Institute for Advanced Study in Princeton, NJ. He adds, "These results set new constraints that cosmological models must satisfy and already rule out some of the extreme theories."
The findings set new limits on the nature and distribution of material in the universe. The results provide information on objects (stars, galaxies) that can be detected with telescopes as well as non-luminous material ("dark matter").
Astronomers estimate that at least 90 percent of the universe consists of material that does not emit any radiation that can be detected by current instrumentation. Although dark matter cannot be seen directly, its existence has been inferred from its gravitational influence on the motions of stars in galaxies and the motions of galaxies in clusters. The phenomenon of gravitational lensing provides a powerful probe in the search for dark matter.
Albert Einstein was first to point out that light, like matter, should be deflected by gravitational fields. If a massive object, such as a galaxy, lies along the line of sight to a more distant object, the galaxy will act as a gravitational lens: the light from the background source will be amplified, and the observed position of the background source will be slightly different from its true location because the light rays passing near the galaxy have been deflected. If the galaxy is sufficiently massive, it can bend the light rays enough to form several images of the distant object. By carefully analyzing how a quasar's image is distorted, astronomers can gain insights into the properties of the intervening galaxy, and, in favorable cases, derive some information about the large scale structure of the universe.
During the past 13 years, about a dozen examples of probable gravitational lenses have been discovered by ground-based telescopes. According to conventional theoretical models, approximately half of all multiple-image gravitational lenses cannot be identified from ground- based telescopes because the separation between the images will be less than one arc second, which is below the nominal resolution limit for ground-based telescopes. Hubble Space Telescope routinely provides 0.1 arc second resolution of moderately bright point sources, making it a unique and powerful tool for searching for multiple-image lenses.
The class of objects most likely to be multiply-imaged by galaxies are distant, bright quasars. The survey team prepared a list of 354 such quasars, and they began collecting data in the summer of 1990. The program was dubbed the "Snapshot Survey" by the STScI deputy director, Peter Stockman, because it uses HST's Planetary Camera (PC) to take short, two-to-four-minute exposures. Doxsey stresses that "the snapshots are taken when there are gaps in HST's normal science observing program, times when the telescope would otherwise be idle."
To use the limited time available most effectively in the scheduling gaps, the pictures are taken using only the spacecraft gyroscopes for pointing control. This eliminates the need to acquire guide stars, saving 15 precious minutes per object. The resulting images are slightly trailed (due to spacecraft drift) yet scientifically valuable. "More HST data has been studied in this survey than any other HST science program," says Bahcall.
The Snapshot Survey was initiated on a trial basis with the support of the STScI director Dr. Riccardo Giacconi, who made observing time available out of his Directors' Discretionary Time. The mission planners at STScI developed the techniques necessary to schedule these observations without affecting the primary science projects. The success of this survey has led to the adoption of this technique for other scientific programs, several of which were proposed for future HST observations.
The one Snapshot Survey gravitational lens candidate, 1208+101, may prove to be a very valuable scientific find. The picture of 1208+101, obtained in July 1991, shows that the quasar appears to be split into two and possibly three images. This result is particularly interesting because, says Schneider "Not only does this quasar have a very large redshift, but the image separations are all less than half an arc second."
Many of the Snapshot Survey quasars have been studied previously with ground-based telescopes. These observations indicated that several quasars might contain multiple images separated by one arc second or less. To date only one such previously published candidate image has been detected in the Snapshot Survey; the HST picture does not show any evidence for a second image. "Of the approximately 300 quasars that have been analyzed, only one object appears likely to be a gravitational lens," says Maoz.
On 23 December, another PC picture of the candidate was taken, this time with precise pointing and guiding. The new data confirm that the quasar consists of at least two images.
Scientists do not know at this time if this is a case of gravitational lensing or whether the faint image is simply a star in our galaxy that just happens to be projected along the line of sight to the quasar. If the fainter image is that of the quasar, then the astronomers have made an important discovery, for this would be the most distant known example of gravitational lensing and thus an excellent probe of the matter between us and the quasar. A description of the Snapshot observation of 1208+101 will appear in the February issue of The Astrophysical Journal letters.
Bahcall says that HST observations of 1208+101 scheduled for the week of January 20 should settle the issue, and the astronomers are eagerly awaiting the outcome. Bahcall explains: "In science, truth ultimately depends upon nature, not people. We don't know if this is an important discovery or just a chance coincidence. Only observations next week will tell."
Snapshot Survey observations continue to be taken, and the results will take months more to analyze. The HST data may ultimately provide new and unexpected insights into the early universe. Bahcall cautions, however, that the Snapshot Survey will not offer insights into competing Big Bang models. "The results tell us about the denizens of the universe but not how they got there."