Astronomers Use New Method To Study Deep Universe

August 07, 1996

Johns Hopkins University astronomers have devised a new technique that enables them to quickly measure the distances to the farthest galaxies, at last providing a large number of examples with which to test cosmological theories, and to study galaxy evolution and the nature of dark matter.

The new technique, called photometric-redshift astronomy, promises to provide a wealth of data, resolving a serious dilemma in cosmology research: if you know the distances to only a few galaxies in a given region of deep space, how can you be certain that those galaxies represent the universe at large?

With photometry, astronomers soon will know the distances to about 20,000 galaxies that are so far away the light now reaching Earth is from a time when the universe was only one-third its current age, perhaps more than 10 billion years ago, said Johns Hopkins astronomer Alexander Szalay. He has pioneered the technique with Hopkins astronomer Andrew Connolly and graduate students Mark Subbarao, Robert Brunner and Gyula Szokoly.

Two scientific papers about the work have been published so far, and another is scheduled to appear in the September issue of the Astronomical Journal. The paper was written by Subbarao, Connolly, Szalay and University of California astronomer David Koo.

It details how the method was used to confirm that elliptical galaxies had all but stopped evolving, whereas spiral galaxies, such as the Milky Way, still were undergoing dramatic evolution when the universe was a mere one-third its current age.

Astronomers usually use spectrographs to gauge the distances of objects in space by measuring their redshifts, or the degree to which their light has been stretched into longer wavelengths as they speed away from our place in the cosmos. But spectrographs require as much as 1,000 times more exposure time per object than photometry, making it impractical to measure the distances to the farthest galaxies.

It would take a full night of observing time to take a spectrograph of a single distant galaxy. But, in the same amount of time, astronomers can measure the distances to 1,000 galaxies of comparable range with the photometric-redshift technique.

The method works like this: astronomers take photographs of regions of space using four separate filters, so that they have pictures in ultraviolet, blue, red and near-infrared light. The images are taken with telescopes equipped with charge-coupled devices (CCDs), light-sensor chips used in video cameras that enable scientists to capture an image accurately.

By mathematically computing how much of each color an object emits, astronomers can tell how far away it is.

"We have discovered a very strong relation between the distance of the object and these colors," Szalay said.

In 1995 astronomers used the Hubble Space Telescope to take four-color pictures of galaxies at the edge of the visible universe; the galaxies are 30 times fainter than those for which distance measurements have been made using the largest telescope on Earth, the 10-meter Keck Telescope atop Mauna Kea in Hawaii. Astronomers are now using the photometric-redshift technique to analyze those Hubble images.

The method cannot replace spectroscopy, which reveals fine details about the composition and velocity of objects in space. Photometry provides only a crude spectrum, but it's accurate enough to measure the general distances of objects and it's the only practical way to measure the distances of large numbers of objects in deep space, said the astronomers, who have been developing the technique for about two years.

"I think now this has really taken off," Szalay said.

Photometry will enable astronomers to study dark matter by providing new details about how clusters of galaxies have changed over time. Gravity is trying to pull the galaxies together, and in the past the galaxy clusters were not as "clumpy" as they are today. By looking back far enough in time scientists will be able to assess the degree of clustering, thought to be a direct reflection of how much dark matter is present, Szalay said.

The technique also will enable astronomers to study how the brightness of galaxies, and the distribution of that brightness in the cosmos, has changed over billions of years, yielding key information about the evolution of galaxies and the universe. Connolly has used the method to discover a huge "supercluster" of galaxies stretching 40 million light years across. Those findings have not yet been published.

Johns Hopkins University

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