Sky Prospecting: Surveying the universe’s middle-aged galaxies

With a year’s worth of data now in hand from a telescope survey of thousands of galaxies 6 to 8 billion light-years away, astronomers are filling in details about the midlife years of the nearly 14-billion-year-old universe.

“We’re looking back in time,” says Alison Coil of the University of California, Berkeley. “The light we’re seeing traveled for [at least] 6 billion years before it reached our telescope.”

The new results come from the first year of a 3-year sky survey, which is the second phase of a project known as the Deep Extragalactic Evolution Probe. In DEEP2, Coil and her colleagues are mapping galaxies within four distant, cone-shaped sections of the universe that are each about 2.5 billion light-years deep. Viewed from Earth, the base at the far end of each section is about the size of the full moon. The researchers presented their first findings last week at the annual meeting of the International Astronomical Union in Sydney, Australia.

“This survey provides the first detailed picture of what galaxies looked like and how they were clustered when the universe was about half its present age,” comments Martin Rees, an astronomer at the University of Cambridge in England. In another major project, the Sloan Digital Sky Survey (SN: 5/31/03, p. 341: Red Team, Blue Team: Galaxy survey shows that color matters), astronomers are scanning more-local neighborhoods of galaxies and the universe’s outer limits–but not the vast stretches in between.

In the DEEP2 survey, Coil and her colleagues are examining how galaxies evolve by measuring their mass, motion, and chemical makeup and the ages of their stars. Clues to all these specifications can be gleaned from the different wavelengths of light that a galaxy emits. Using the telescope’s Deep Imaging Multi-Object Spectrograph, or DEIMOS, the astronomers split the faint light from distant galaxies into separate wavelengths, just as a prism separates white light into distinct colored bands.

“Spectra are the Rosetta stone for understanding celestial objects,” says Sandra Faber, an astrophysicist at the University of California, Santa Cruz and a member of the DEEP2 team. For example, a bluish tinge is the signature of a galaxy crowded with young stars, while galaxies with an older stellar population radiate more red light.

The DEEP2 team used red and blue light emissions to compare the clustering behavior of galaxies that are 6 to 8 billion light-years away to that of nearer galaxies. In both groups, galaxies containing mostly young stars don’t clump as much as galaxies with older stars do. Overall, however, galaxies from the universe’s midlife epoch show more clumping than do younger galaxies–supporting the theory that gravity increases clustering over time.

By the end of the survey, the DEEP2 scientists plan to have mapped 50,000 galaxies from the universe’s middle years. Rees says that this cache of data should help refine theories about how the universe has evolved.

Also, Coil notes, knowing the masses of that many galaxies could enable astronomers to better discern the distribution of so-called dark matter–one of the driving forces behind the overall geometry and motion of galaxies.


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