Dawn of galaxies now in focus
Astronomers have finally found one of the most sought-after objects in the universe - a small aggregate of hot gas and young stars that could be one of the building blocks of the very first galaxies.
The object was detected because a large mass stood directly in the path of observation. Instead of blocking the light from the object, it focused it, thanks to an effect known as gravitational lensing.
"When we realized what we had found, we literally jumped up and down," says Jean-Paul Kneib at the Observatoire Midi-Pyrenees in France - a member of the discovery team.
Cosmologists believe that after the universe emerged in the big bang - an explosion of primordial energy - matter and radiation were sorting themselves out for several hundred million years. Then the matter began to form stars that clumped into small groups, such as the one just discovered. Eventually, these building blocks formed full galaxies such as our Milky Way.
"To really understand what's going on in the early universe, we need to learn about the typical, commonplace building blocks, which hold important clues to the later assembly of normal galaxies," says Mike Santos, another team member, of the California Institute of Technology (Caltech) in Pasadena. "Our study represents a beginning to that understanding."
The proto-galaxy is so small, dim, and distant that no telescope on Earth today could detect it. Fortunately, the massive Abell 2218 cluster of galaxies lies in the direct line of sight. Einstein's general relativity theory predicts that material mass can warp space in such a way that light passing by is focused into multiple images - the gravitational lens effect. In this case, Abell 2218 bent surrounding space to form a pair of images of the distant object. Because the lensing concentrates so much light, these images are at least 30 times brighter than the unmagnified object would appear to be.
"Without the benefit of the powerful cosmic lens, the intriguing source would not even have been detected in the Hubble Deep Fields historic deep [space] exposures taken in 1995 and 1998," says Caltech Professor Richard Ellis, the lead author of a paper that will describe this research in a forthcoming issue of the Astrophysical Journal Letters.
Recording the Deep Fields images was a high priority project for the Hubble Space Telescope, examining small sections of sky to record the farthest and faintest objects it could see. Many lie near the edge of the universe. Because their light has taken many billions of years to reach us, they give us insight into the nature of the universe when it was very young.
Drs. Ellis and Kneib, Mr. Santos, and the rest of their European-American team are using the Hubble pictures to systematically search for distant star-forming galaxies that gravitational lenses may make visible. The team also has used the 10-meter (32.8-foot) Keck telescope in Hawaii to confirm that they have found one of the building blocks of such young galaxies.
The new-found object is about 500 light-years across, with a mass of only about a million times the mass of our Sun. That's tiny compared with the 100,000-light-year Milky Way galaxy, with its billions of stars.
Using a typical estimate of 14 billion years since the big bang, the team estimates the newly observed object is 13.4 billion years old. Since it has taken that long for its light to reach us, the object is 13.4 billion light-years away. The Abell 2218 galaxies, in contrast, are only 2 billion light-years distant. Discovery team member Konrad Kuijken at the Kapteyn Institute in the Netherlands explains that, with such a small, low-mass object, "we may finally be witnessing the circumstances in which [the] first generation of stars was born."
Astronomers now look forward to a new generation of space telescopes that can study such objects directly. The European Space Agency (ESA) is working with NASA and the Canadian Space Agency to design and build the Next Generation Space Telescope (NGST) as a successor to the Hubble orbiting observatory. ESA also is planning its own Herschel Space Observatory.
Peter Jakobsen, ESA project scientist for the NGST, points out that, while a gravitational lens is a powerful aid in studying objects that happen to lie behind it, it's "a bit hard to point [it] in other directions."