10/05/2001 07:00:00

International team uses powerful cosmic lensto find galactic building block in early universe

Exploiting a phenomenon known as gravitational lensing, an international team of astrophysicists has detected a very small, faint stellar system in the process of its formation during the first half billion years or so of the universe's existence.

The discovery is being reported in the October 20 issue of the Astrophysical Journal. According to lead author Richard Ellis, a professor of astronomy at the California Institute of Technology, the faint object is an excellent candidate for the long sought after "building blocks" thought to be abundant at early times and which later assembled to make present-day galaxies.

The discovery was made possible by examining small areas of sky viewed through a massive intervening cluster of galaxies, Abell 2218, 2 billion light-years away. The cluster acts as a powerful gravitational lens, magnifying distant objects and allowing the scientists to probe how distant galaxies assembled at very early times.

Gravitational lensing, a dramatic feature of Einstein's theory of general relativity, means that a massive object in the foreground bends the light rays radiating from one in the background because mass curves space. As a result, an object behind a massive foreground galaxy cluster like Abell 2218 can look much brighter because the foreground object has bent additional photons toward Earth, in much the same way that glass lenses in binoculars will bend more photons toward the eyes.

In the case of the system detected by Ellis and coworkers, the effect makes the image at least 30 times brighter than would be the case if the Abell 2218 cluster were not in the foreground. Without this boost, neither the Keck 10-meter Telescopes nor the Hubble Space Telescope would have detected the object.

Ellis explains, "Without the benefit of the powerful cosmic lens, the intriguing source would not even have been detected in the Hubble Deep Fields, historic deep exposures taken in 1995 and 1998."

Using the 10-meter Keck Telescopes at Mauna Kea, the collaboration found a faint signal corresponding to a pair of feeble images later recognized in a deep Hubble Space Telescope picture.

Spectroscopic studies made possible with the superior light-gathering power of the Keck confirmed that the images arise via the magnification of a single source diagnosed to be extremely distant and in the process of formation.

"The system contains about a million or so stars at a distance of 13.4 billion light-years, assuming that the universe is 14 billion years old," claims Ellis. "While more distant galaxies and quasars have been detected with the Keck Telescopes, by virtue of the magnification afforded by the foreground cosmic lens, we are witnessing a source much smaller than a normal galaxy forming its first generation of stars." " Our work is a little like studying early American history," says team member Mike Santos, a Caltech graduate student in astronomy. "But instead of focusing on prominent individuals like George Washington, we want to know how everyday men and women lived.

"To really understand what was 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. Our study represents a beginning to that understanding."

The precise location of the pair of images in relation to the lensing cluster allowed the researchers to confirm the magnification. This work was the contribution of team member Jean-Paul Kneib of the Observatoire Midi-Pyrénées near Toulouse, France, an expert in the rapidly developing field of gravitational lensing.

The team concludes that the star system is remarkably young (by cosmic standards) and thus may represent the birth of a subcomponent of a galaxy or "building block." Such systems are expected to have been abundant in the early universe and to have later assembled to form mature large galaxies like our own Milky Way.

Santos explains, "The narrow distribution of intensity observed with the Keck demonstrates we are seeing hydrogen gas heated by newly formed stars. But, crucially, there is not yet convincing evidence for a well-established mixture of stars of different ages. This suggests we are seeing the source at a time close to its formation."

In their article, the researchers infer that the stars had been forming at a rate of one solar mass per year for not much longer than a million years. Such a structure could represent the birth of a globular cluster, stellar systems recognized today to be the oldest components of the Milky Way galaxy. The work represents part of an ongoing survey to determine the abundance of such distant star-forming sources as well as to fix the period in cosmic history when the bulk of these important objects formed.

Contact:Robert Tindol (626) 395-3631