Mapping the universe

By Diana Steele
News Office

When the light from the distant universe first reaches the camera of the Sloan Digital Sky Survey telescope next month, it will mark one of the most important milestones in the history of the nine-year-old project to map the structure of the universe in greater detail than ever before. The so-called "first light" will be the occasion for much rejoicing on the New Mexico mountaintop that is the survey's home.

The Sloan Survey, which will begin collecting data early next year, is the largest, most comprehensive survey of the sky ever undertaken. At the end of five years, it will have generated a catalog of at least 50 million galaxies. In addition to charting positions for these galaxies, it will provide distances to a million galaxies and 100,000 quasars, mapping the structure of the universe in a three-dimensional region of the sky 100 times larger than previous surveys.

An international team of astronomers from eight research institutions, including Chicago, are collaborating on the project, which uses a specially constructed 2.5-meter telescope at the Apache Point Observatory in New Mexico's Sacramento Mountains.

From 'soup' to 'clumps'

By charting the structure of the universe, the Sloan Survey will begin to help astronomers understand the mechanisms that converted a uniform "primordial soup" into a complex network of galaxies. The map will identify the largest structures in the universe, as well as their sizes and shapes.

The universe today is filled with sheets of galaxies curving through mostly empty space. But the best model for how the universe began-the Big Bang-paints a picture of a universe that was a hot, uniform soup of fundamental particles. Somehow, between the time the universe began and today, gravity has pulled the matter into denser regions that are traced by the visible galaxies.

"We don't know how the universe developed into its present, clumpy structure," said Michael Turner, Professor and Chairman of Astronomy & Astrophysics and project spokesman for the survey. "Our best explanations involve events that happened the tiniest fraction of a second after the Big Bang and may involve the unification of the forces of nature. Thus, the Sloan Survey may allow us to peer not only into outer space but into the inner space at the heart of the fundamental structure of matter."

Many milestones to go

Richard Kron, Professor in Astronomy & Astrophysics and Director of Yerkes Observatory, who has worked extensively within a group that is building the Sloan Survey's data processing system, housed at the Fermi National Accelerator Laboratory in Batavia, Ill., said that first light is only one of several milestones that will be achieved over the course of the next several months.

"It will be very dramatic to get the camera attached to the telescope and get light coming through," he said. "But it will be some time before we can get the telescope from that state to where we can start analyzing data."

First, Kron said, wind baffles that protect the telescope must be installed. The telescope, rather than being inside a traditional astronomical dome, sits on a pier inside a rollaway enclosure that moves back to expose the telescope for nightly viewing. The structure costs less than a conventional dome and also avoids the heating and cooling problems that can distort observing from within a dome. The wind baffles also serve as light baffles; when they are installed, the first meaningful astronomical images can be obtained.

Next, the pointing and tracking system will be completed and tested, said Kron. "Soon we should have a telescope that points and tracks properly, and then we can put the spectroscopic system together," he said.

The Sloan Survey's two spectrographs are the key to creating a three-dimensional picture of the sky.

Because the universe is uniformly expanding, the farther a galaxy is from Earth, the faster it is receding. The light coming to us from these objects is shifted toward the red end of the electromagnetic spectrum, in much the same way the sound of a train whistle changes as a train leaves the station. The faster a distant object is moving, the more it is red-shifted. Astronomers measure the amount of red shift in the spectrum of a galaxy to determine its distance from Earth.

When the "seeing" isn't quite so pristine, the telescope will stare at a region of the sky and obtain spectra. On clear, dark nights, astronomers will use the camera to image the sky. An image of the sky obtained by the camera will be used to create a plug plate with holes drilled at the positions of 640 galaxies. Each hole will be connected to the spectrograph with a fiber-optic cable, so that the spectra-and therefore the distances-of 640 galaxies can be obtained simultaneously.

Casting a wide net

In addition to the three-dimensional map, the Sloan Survey will gather information about the number of galaxies and quasars in the universe, how they are distributed and their individual properties and brightness.

Kron is interested in understanding the basic properties of galaxies in the nearby universe. The study of the color, brightness and size of stars has moved from a celestial catalogue to understanding the physical basis for the structure of stars, but no such understanding of galaxies-their shapes, luminosities and colors-has yet been developed.

"When you look at spiral galaxies, for example, and see their marvelous patterns, you would like to be able to say why it is that a galaxy has this form or that size, but we can't yet say that. We simply don't have enough galaxies in different environments and in different evolutionary states to be really able to understand what's going on," said Kron. "The Sloan Survey will give us a sufficiently powerful grasp of how galaxies really look to advance our physical understanding of them."

The Sloan Survey will also collect information about our own galaxy and even about our own solar system. The wide net cast by the Sloan telescope will capture as many stars as galaxies and as many asteroids in our solar system as quasars in the universe. Knowledge of these objects will help us learn how stars are distributed in our galaxy and where asteroids fit into the history of our solar system.

By sifting through the several hundred million objects recorded by the Sloan Survey, scientists also will be able to construct entire catalogues of the most distant quasars, the rarest stars and the most unusual galaxies. The most unusual objects in the catalogue will be about 100 times rarer than the rarest objects now known.

Donald York, the Horace B. Horton Professor in Astronomy & Astrophysics and the Sloan Survey's founding director who guided the project through the first nine years, is interested in the quasars, or quasi-stellar objects, the survey will detect. Because they are so far away, quasars can serve as probes for intergalactic matter throughout the visible universe. In particular, astronomers can identify and study galaxies by the way they block certain wavelengths of light emitted by a quasar.

Using the light from quasars, the survey will detect tens of thousands of galaxies in the initial stages of formation. These galaxies are typically too faint and too diffuse to be seen even by the largest of telescopes. Quasar probes will also allow scientists to study the evolution of the chemistry of the universe throughout its history.

As the engineering work continues on the mountaintop, scientists at Fermilab and Chicago will be testing and refining the data-processing software.

The Sloan Survey, over its projected five-year lifetime, will generate mountains of data. In fact, in each night of observing, there will be far too much data to be transmitted over the Internet to Fermilab in a timely manner. Instead, each morning, data tapes will be express-mailed from the mountaintop to the laboratory for processing and analysis.

The task of sifting through the data and determining star and galaxy positions, distinguishing types of astronomical objects and selecting candidates for spectroscopy, is monumental. Eventually, the data will be made available to other scientists and to the general public, becoming an unparalleled field guide to the universe.

For more information on the survey, see http://www-sdss.fnal.gov:8000/.