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Ancient Galaxies Come to Light

21 September 2005 (All day)
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Now you see them. About a dozen distant galaxies not recognized before are circled in this Very Large Telescope image.

Using the European Very Large Telescope (VLT), a French-Italian team of astronomers has found many more galaxies in the distant past than had been previously observed. Now, the generally accepted theories of the formation and evolution of galaxies in the early Universe needs a serious rethink, they say.

Just like archeologists who find older artifacts by digging deeper, astronomers use "deep" surveys--during which a telescope looks at one patch of sky for a long time to gather the most possible light--to look back to the distant past. Starlight from very distant galaxies takes billions of years to reach Earth, so we see these galaxies as they were billions of years ago. To know how many galaxies there were in the Universe's early youth, astronomers just have to count the number of galaxies at specific distances. However, the light from distant galaxies is very faint, making them hard to find and identify. That's why astronomers have used relatively crude techniques in the past, such as first selecting galaxies on the basis of their observed colors. The light waves from distant galaxies are stretched and reddened by the expansion of the universe, and this redshift is proportional to a galaxy's distance.

Now, Olivier Le Fèvre of the Astrophysical Laboratory in Marseille, France, and his colleagues have used a powerful spectrograph on the 8.2-meter VLT of the European Southern Observatory in Chile to rigorously measure precise redshifts of thousands of distant galaxies. In a small part of the sky in the constellation Cetus the Whale, the researchers found no less than 970 star-forming galaxies at distances that correspond to a period some 1.5 billion to 4 billion years after the Big Bang. They found six times as many of the most luminous galaxies in this epoch compared to earlier surveys, while the dimmer ones were about twice as numerous as previously thought, according to findings published 22 September in Nature.

The survey is "a very nice way of doing things," says Chuck Steidel of the California Institute of Technology in Pasadena, who pioneered the color selection survey technique. "No one has tried this brute force approach before." But Steidel believes the claims of the paper are overstated because the reported implications are based on a "misleading" statistical presentation. "No doubt we're missing something" with the color selection technique, he says, "but there just isn't any way that there's a factor of six missing." But Le Fèvre says the results are solid. "We try to avoid making prior assumptions and complex corrections. We just go there and count," he says.

Related sites:
VIMOS-detector, used in the survey
Background on the color selection technique

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