The world's largest telescope quietly transforms astronomy
The cliche that a picture is worth a thousand words may help explain the muted response that has greeted early results from the Keck telescope, the biggest optical telescope ever built. Keck cannot match the sharp views from the orbiting Hubble Space Telescope; its main strength lies instead in its tremendous light-gathering ability, a boon for studying the spectra of extremely faint, distant objects. But "spectra are a hard sell, even if you put them in pretty colors," remarks Lennox L. Cowie of the Institute for Astronomy in Honolulu. "So people just end up running a photo of Keck."
Since it began full-time operation last year on its perch atop Mauna Kea in Hawaii, Keck has created a stir in the astronomical community. The telescope's huge segmented mirror--10 meters in diameter--speeds the process of exploring the most distant, and hence youngest, parts of the cosmos. "It really blows you away, it is so fast," marvels David Tytler of the University of California at San Diego. Tytler, Cowie and various collaborators are using Keck's talents to home in on basic questions about how galaxies form and evolve and about how much ordinary matter is out there--often with unanticipated results.
One manner in which to weigh the universe is to look at the abundance of heavy hydrogen, or deuterium, in very young objects that are not yet contaminated by nuclear by-products from stars. The big bang model predicts that the amount of such primordial deuterium depends on the total density of normal, or baryonic, matter--the stuff of stars and gas clouds.
When Tytler used Keck to look for deuterium, he found a surprisingly intense spectroscopic signal. If accurate, his results imply that the baryonic density of the universe is much greater than expected. But the observed galaxies and intergalactic material account for only a small fraction of that density. Could scientists be overlooking at least 80 percent of the normal matter out there? "It is really embarrassing," Tytler says. "Astronomers should be able to see ordinary gas."
So where is the missing material? Tytler suspects the answer lurks in another set of data from the telescope, which seems to show that the space between galaxies is packed with clouds of nearly invisible, hot gas. These intergalactic wisps may outweigh all the more prominent stars and galaxies.
Cowie and Antoinette Songaila, also at the Institute for Astronomy, have come to a different conclusion--also on the basis of observations from Keck. Using the same principles but different observing techniques, the two found one tenth as much deuterium, which suggests that there is no need to search for "missing" baryonic matter. Such disagreements are not unusual as researchers try out various approaches on a new instrument. Both Tytler and Cowie are confident that Keck has the raw power to settle the question in the next few years.
In another provocative study, Cowie and his colleagues found traces of carbon in the spectra of very distant gas clouds. Cosmologists believe that, unlike deuterium, all the carbon in the universe was created in stars, so it should serve as an indicator of star formation. Cowie was surprised to find the element in young clouds that were thought to be pristine samples from the big bang.
"We're still thrashing around trying to understand this result," Cowie says. He leans toward a model promoted by Jeremiah P. Ostriker of Princeton University, in which instabilities that arose after the big bang could have triggered star formation before the first galaxies gathered. Tytler favors a simpler interpretation: the carbon was created by stars within infant galaxies but was then puffed out into intergalactic space.
If only they could spot the youngest, newborn galaxies, astronomers could learn a great deal about the evolutionary process that connects the big bang to the modern Milky Way. Here, too, Keck is yielding provocative but puzzling results. S. George Djorgovski of the California Institute of Technology has spent years trying to learn about the birthing process of galaxies. His latest observations show many of the faintest galaxies ever seen but still no sign of the expected protogalaxies.
"The universe is very, very strange," Djorgovski says as he reflects on the unseen process of galactic formation. "Nobody knows how this is going on." Perhaps infant galaxies are cloaked in dust that obscures them, or perhaps astronomers need to use a different approach to look for them. Djorgovski is optimistic that Keck will provide some answers. "We're overdue for some nice discovery," he says impatiently.
Keck's start has not been totally free of glitches. Engineering adjustments still eat up a fair amount of time as operators learn the intricacies of the unusual design, and "the weather's been just god-awful for the past year," Djorgovski laments. "But when it all works, it is wonderful." Nobody seems distressed by the telescope's low public profile. Cowie thinks Keck just "operates in the classical Caltech mind-set": researchers are encouraged to work problems through before publishing. That approach seems to suit him and his colleagues just fine. "We've been having a lot of fun working with Keck," he laughs. "It truly is a spectacular beast."--Corey S. Powell
SCIENTIFIC AMERICAN August 1995 Volume 273 Number 2 Page 20
Scientific American (ISSN 0036-8733), published monthly by Scientific American, Inc., 415 Madison Avenue, New York, N.Y. 10017-1111. Copyright 1995 by Scientific American, Inc. All rights reserved. Except for one-time personal use, no part of any issue may be reproduced by any mechanical, photographic or electronic process, or in the form of a phonographic recording, nor may it be stored in a retrieval system, transmitted or otherwise copied for public or private use without written permission of the publisher. For information regarding back issues, reprints or permissions, E-mail SCAinquiry@aol.com.
