This galaxy strip will blow your socks off
Full-spectrum study of small patch of sky yields portrait of maturing universe
| 14 March 2007
A massive project to generate an all-color map of the galaxies occupying a small area of sky, utilizing four satellite telescopes and four ground-based telescopes, is yielding new information about the universe's "pre-teen" years and the early evolution of galaxies and galaxy clusters.
The five-year project, called AEGIS (All-wavelength Extended Groth Strip International Survey), involved the cooperation of more than 50 researchers from around the world observing the same small region of sky in the radio, infrared, visible, ultraviolet, and X-ray regions of the electromagnetic spectrum. The target area, called the Extended Groth Strip, covers an area the width of two full moons that is a hop, skip, and jump from the end of the Big Dipper's handle.
"The goal was to study the universe as it was when it was about half as old as it is at present," says team leader Marc Davis, a Berkeley professor of physics and astronomy. "We've gotten such fabulous data; it just blows your socks off."
Combined with spectra of these galaxies obtained through the W. M. Keck Telescope in Hawaii as part of the DEEP2 Galaxy Redshift Survey, the dataset will be an invaluable resource for astronomers, he says. Among the discoveries so far are a giant red galaxy with two black holes at its core; several new gravitational lenses - that is, galaxies whose gravity bends the light from background galaxies into multiple images; and a rogues' gallery of weird galaxies that astronomers will be busy trying to explain for decades.
To the naked eye, the Extended Groth Strip, named for Princeton University physicist Edward Groth, is empty, but the AEGIS survey pinpointed more than 150,000 galaxies in the strip. A panoramic mosaic of Hubble Space Telescope images released last week provides detailed, color images of at least 50,000 galaxies in part of this area.
The AEGIS survey is focused on studying galaxies up to 9 billion years back in time - more than halfway back to the birth of the universe 13.7 billion years ago. It covers a period when galaxies were settling down after an early phase of rapid star formation.
"We're studying a key epoch when galaxies appear to be taking on their final mature forms," says Sandra Faber, professor of astronomy at UC Santa Cruz. "It's like seeing people at the age of 10 - they are not exactly infants, but they differ substantially from adults. We are watching galaxies grow up."
In a summary paper now posted online in Astrophysical Journal Letters, Davis and his colleagues note that AEGIS is providing a unique combination of deep, intensive observations over a wide area, yielding large samples even of rare types of galaxies. They contrast their work with the Sloan Digital Sky Survey, which has observed the local universe in great detail but, of necessity (since cosmic features closer to us in the expanding universe are younger than those farther away), focuses on only the last 2 billion years of cosmic evolution.
"We have looked at this patch of sky with every possible telescope, at wavelengths covering nine orders of magnitude - that's a wavelength range of a billion, compared to the ability of our eyes to see a range of two," said cosmologist Jeffrey Newman, a Hubble Fellow at Lawrence Berkeley National Laboratory. "Each provides a little piece of the puzzle of how galaxies evolve."
"As of this time, there is no other region this large on the sky that has been looked at so deeply in so many different wavelengths," Faber said. "This is the first truly panchromatic look at the universe more than halfway back in time."
Newman, echoing Santa Cruz's Faber, says the Hubble Space Telescope images reveal a time when galaxies were starting to reach maturity.
"We see a wide diversity of galaxies. Some are beautiful spirals or massive elliptical galaxies like those seen in the nearby universe, but others look like random assemblages of material, the leftovers from violent mergers of young galaxies. These resemble some of the most distant, youngest galaxies observed," he says.
AEGIS provides many windows on this time of transition. Ultraviolet and far-infrared light from newly born stars, observed by the GALaxy Evolution eXplorer (GALEX) and the Spitzer Space Telescope, respectively, shows that stars were being formed at a much higher rate than today. Mid- and near-infrared light measures the total mass of the stars in each galaxy, allowing astronomers to see how galaxies grow larger over time, while X-ray and radio observations by the Chandra Space Telescope and the Very Large Array in New Mexico, respectively, can reveal the presence of powerful black holes at galaxies' centers.
A linchpin for AEGIS studies, Davis says, is the DEEP2 Redshift Survey, which measures many properties of a galaxy - its mass, the rate at which it is forming stars, and more - while simultaneously determining how far away it is, and, hence, how far back in time we're observing. The large number of objects surveyed by DEEP2 in the Groth Strip - 14,000 to date, though the completed survey should include nearly 18,000 galaxies - allows astronomers to see trends as the universe ages. The data clearly show that the galaxies at the far edge of the survey, around 9 billion years ago, are noticeably different from those at the near, 7 billion-year edge of the survey, which look more like "normal" galaxies today.
Because of the relatively large cosmic volume covered, astronomers are able to find even rare objects, such as galaxies with two separate black holes at their centers, that can help reveal the hidden physics of galaxy formation and evolution.
The new survey also revealed two new lensing galaxies, an already known Einstein Cross - a case where a massive galaxy has split the light from a background galaxy into four arcs - and four possible lenses. According to Puragra "Raja" Guhathakurta of UC Santa Cruz's Lick Observatory, astronomers can apply Einstein's theory of general relativity to calculate the masses of lensing galaxies given their spectra and the observed bending of starlight.
A total of 19 papers based on the Groth Strip survey will appear in a special spring issue of The Astrophysical Journal Letters (ApJL). Groth Strip and AEGIS imagery is online at aegis.ucolick.org and hubblesite.org/news/2007/06.