Quasar hunt bags unusual quarry.
The British astronomers and their colleagues, Cyril Hazard of the University of Pittsburgh, started hunting for distant quasars in 1986, using a new tool: a computerized scanning machine that enabled them to examine some 50 million images on photographs taken by the U.K. Schmidt Telescope in Epping, Australia. The study revealed several intriguing, star-like entities that glowed brightly in red light but dimly in blue--characteristics of distant quasars, notes McMahon, of the University of Cambridge in England.
Follow-up observations with telescopes in the Canary Islands of Spain further illuminated the findings. By 1990, McMahon, Hazard and Irwin, of the Royal Greenwich Observatory in Cambridge, had confirmed that 11 of the star-like objects were indeed distant quasars, which lie many billions of light-years from Earth yet appear unusually bright on photographic images. Last year, the researchers announced that they had found additional quasars, for a total of 33 of the most distant objects ever detected in the universe (SN: 6/15/91, p.375). Since looking into deep space is the same as peering back in time, these findings indicate that ultrabright quasars formed when the universe was less than 10 percent of its current age, says Irwin.
This menagerie of unusually bright, very distant quasars poses a further problem for astronomers already struggling to understand what happened soon after the birth of the universe, says cosmologist Edwin L. Turner of Princeton (N.J.) University.
Turner and most other scientists assume that the universe got its start in a violent explosion, the Big Bang. Immediately afterward, the universe would have consisted of an extremely uniform, smooth soup of material. This scenario seems at odds with the lumpy collection of galaxies and clusters of galaxies known to reside throughout nearby regions of the universe. (Indeed, researchers have had to invoke the theoretical existence of an invisible matter, called cold dark matter, that would supply the needed gravitational tug to turn a soupy universe into a lumpy one.) But if large numbers of very bright quasars, along with their parent galaxies, were fully developed a mere billion year after the Big Bang, then even the standard theory of cold dark matter might not explain the formation of largescale structure in the universe, says Turner.
Most astronomers believed quasars arise from the activity of an unimaginably powerful energy source at the center of galaxies -- perhaps a black hole devouring its surroundings. The brighter the quasar, the more powerful the energy source and, presumably, the bigger the parent galaxy. So the existence of bright quasars early in the universe would imply the existence of large, fully grown galaxies in that epoch.
As a way out of this cosmological conundrum, some scientists have suggested that the quasars found by McMahon, Irwin and Hazard aren't as bright as believed. Rather, the researchers might have been fooled by gravity.
Albert Einstein was the first to describe such a gravitational illusion, which relies on the principle that mass bends light. Like a mischievous sprite, a galaxy or other massive object situated between the quasar and Earth can act as a gravitational lens, focusing diverging light rays from the quasar, intensifying the quasar's image and even creating multiple images of the distant object. Thus, the measured brightness of a quasar image on a photographic plate might not reflect the true brightness of the object.
Intrigued by this possibility, McMahon and Irwin decided to take a closer look at their data. Last February, using two large telescopes atop Hawaii's Mauna Kea, they examined at high resolution 12 of their sample of 33 bright and faraway quasars. Searching for lensing by looking for closely spaced, doubled images of any of these quasars, they found evidence that light from only one of the 12 had been bent by a foreground galaxy. McMahon and Irwin reported their results in April at the first annual meeting of the Royal Astronomical Society in Durham, England.
McMahon emphasizes that the other 11 quasars may undergo a small amount of lensing that the Mauna Kea telescopes couldn't detect. But the study suggests that the majority of quasars in the sample, including the two most luminous bodies, are about as bright as originally calculated, he notes. "We weren't being fooled," McMahon says. Thus, this quasar collection still presents a challenge to cosmologists.
The lone quasar found to undergo gravitational lensing merits attention in its own right, he observes. Known as BR0952-01, it lies about 12 billion light-years from Earth and has earned the title of the most distant object known to undergo lensing -- beating out a lensed quasar recently detected by the Hubble Space Telescope (SN: 2/1/92, p.79). This newly identified quasar, which emits a million billion times as much light as the sun, appears about four times as bright as it would without lensing. In particular, an unidentified foreground galaxy splits the quasar's image into two components separated in space by one arc-second -- equivalent to a separation of about 10,000 light-years at the estimated location of the foreground galaxy. The primary image generated by lensing has about three times the quasar's true surface brightness, while the secondary image roughly matches the quasar in brightness.
The split images and the quasar's great distance from Earth provide astronomers with a unique tool for examining the intergalactic medium, McMahon says. Researchers have routinely used quasars as cosmic flashlights, probing the location and composition of gas and dust that lie between these brilliant bodies and Earth. For instance, a giant cloud of hydrogen gas would make its presence known by absorbing a particular wavelength of light from the quasar, depending on the cloud's distance from Earth. The presence of many clouds at different distances creates a thicket of absorption lines, called the Lyman alpha forest, within the spectrum of light emitted by the quasar. Last year, the Hubble telescope analyzed the spectra of several quasars and discovered that the neighborhood of our galaxy contains a surprising abundance of hydrogen clouds (SN: 5/25/91, p.326).
No one knows the dimensions of any of these mysterious clouds, which could represent material that somehow failed to coalesce into galaxies. But the lensing properties of BR0952-01 may allow astronomers to gauge their size.
McMahon suggests that Hubble, and perhaps some ground-based telescopes, could analyze the spectrum of light from each quasar image. Light from each image stems from a different part of the quasar and takes a slightly different path to Earth. So, if light from both images carried the same fingerprint -- the same absorption line -- this would indicate that the hydrogen cloud was wide enough for both beams to pass through it. Similarly, if light from only one image contained a particular absorption line, it would mean that the cloud was smaller than the separation between the images. While arduous, such measurements promise to shed new light on the structure of these intriguing clouds, McMahon says.
In the meantime, he and his co-workers plan to extend the lensing survey to include their entire list of distant quasars. They expect to find that only a few additional quasars undergo lensing.
For McMahon, the cosmological implications of the study remain paramount. "We're detecting a universe that was much more lumpy [early on] than people had hypothesized," he says.
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|Title Annotation:||ultrabright quasars which lie billions of light-years from the earth|
|Date:||Jun 20, 1992|
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