Photometric red-shift technique resolves deep universe

17 Jun 2002

Johns Hopkins University astronomers have devised a new technique called photometric-redshift astronomy that enables them to quickly measure the distances to the farthest galaxies. The process yields a large sample field with which to test cosmological theories, and to study galaxy evolution and the nature of dark matter.

Using 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.Using telescopes equipped with charge-coupled devices (CCDs), astronomers take photographs of regions of space using four separate filters, producing pictures in ultraviolet, blue, red and near-infrared light. By mathematically computing how much of each color an object emits, astronomers can tell how far away it is.

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.