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Tracking the evolution of galaxies.

Using telescopes to peer back in time at galaxies as they appeared billions of years ago, astronomers are documenting how the density of these bodies changes in the distant universe. One interpretation of the findings hints that researchers have glimpsed galaxies in their infancy, soon after they "switched on" and experienced the first glimmers of starbirth.

Nigel Metcalfe and Thomas Shanks of the University of Durham in England and their colleagues used the 2.5-meter Isaac Newton and 4-meter William Herschel telescopes in the Canary Islands, Spain, to study galaxies so faint and distant that the instruments could barely detect them. The team found that the faintest -- and presumably most distant -- galaxies in their survey had higher number densities than the brighter, nearer galaxies. However, the rate at which the faint galaxies increased in density seemed to begin tapering off at about 10 billion light-years from Earth.

Metcalfe suggests two explanations for the findings. The more mundane scenario relies on the phenomenon of redshift and the fact that galaxies absorb ultraviolet light shorter than the far ultraviolet wavelength known as the Lyman-alpha limit. Thus, galaxies appear dark at shorter ultraviolet wavelengths. And because light from distant galaxies gets shifted to the redder end of the spectrum, the galaxies appear dark when viewed from Earth at some visible-light wavelengths. So although the galaxies actually exist, a typical visible-light survey would fail to count them.

Alternatively, notes Metcalfe, the density of faint galaxies may increase more slowly with distance from Earth because astronomers are finally looking back far enough to see some galaxies that are still dim and haven't yet begun forming many stars. To decide between the two explanations, Metcalfe's team plans eventually to conduct their faint, visible-light survey at bluer, shorter wavelengths. If the density increase also tapers off at shorter wavelengths, then researchers may truly be viewing an important, early epoch in galaxy evolution, he says.

Metcalfe adds that the total number of galaxies detected in his team's survey -- about 1 million galaxies per square degree -- seems too big for the volume of space available in a model of the universe favored by many cosmologists. In this model, the cosmos is poised between expanding forever and having enough mass to gravitationally collapse in on itself. Metcalfe notes that the high number count would still be consistent with such a universe if many of the more distant, younger galaxies in the survey actually represent fragments that later merged to form groups of bigger galaxies.
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Title Annotation:more density changes of galaxies in distant universe than nearer ones
Author:Cowen, Ron
Publication:Science News
Article Type:Brief Article
Date:Jan 2, 1993
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