Faint body may be galaxy building block

Using a cosmic zoom lens, astronomers have discovered what may be one of the first baby galaxies in the universe–a clump of young stars that might have merged long ago with thousands of other infants to form one of the earliest full-grown galaxies.

Distant object, distorted by a gravitational lens into two images (circled), may be an early galactic building block. ESA, NASA, Ellis, Kneib

According to the standard theory of galaxy formation, big galaxies evolve from smaller ones. Star-bearing clumps form first and then join to make larger galaxies like the Milky Way. Astronomers have documented that ongoing process as far back as 8 billion years ago, when the cosmos was less than half its current age. But researchers don’t yet know when the first galaxies coalesced, and whether they too resulted from the merger of smaller bodies. The new findings, reported in an upcoming Astrophysical Journal Letters, provide a hint.

Richard S. Ellis of the California Institute of Technology in Pasadena and his team could observe the extremely distant, faint clump of stars because its light passes through a massive cluster of galaxies before reaching Earth. As a consequence of one of gravity’s more peculiar properties, a high concentration of mass warps surrounding space and acts as a lens. So, the massive cluster splits and bends the light from the more distant clump of stars to create a pair of images, each bigger and at least 30 times brighter than it would otherwise be.

It’s that boost in brightness that enabled the Hubble Space Telescope to spot the two images and the Keck I telescope atop Hawaii’s Mauna Kea to measure the intensity of their light. The Keck measurements reveal that the object lies 13.4 billion light-years from Earth and dates from a time when the cosmos was only 4 percent of its current age.

Although astronomers have identified galaxies and quasars that are slightly more distant, the newly found star cluster seems to be the smallest body detected at these far reaches of the cosmos. The object is only about 500 light-years across. By comparison, the Milky Way spans about 100,000 light-years.

By examining the intensity of light from the cluster’s hydrogen atoms, the researchers estimate that the body had been making stars for only about a million years–at the time the light left the object–and weighed just a million times the sun’s mass. That’s minuscule compared with a mature galaxy, but just the right weight for globular clusters, the oldest known groupings of stars in our galaxy. Moreover, models suggest that globular clusters could have been the first starlit bodies in the universe.

Ellis and his colleagues say the distant object may indeed be one of the first galactic building blocks in the universe. “But with [just] one object . . . one can never be sure,” cautions Ellis.

The starlit body could simply be the bright part of a larger, extremely dim galaxy, adds Christopher S. Kochanek of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

Alternatively, says Richard G. McMahon of the University of Cambridge in England, the body may be nothing more than debris left over from the formation of a large galaxy some distance away.

“The most significant result is that the technique [of gravitational lensing] has found such a faint galaxy,” comments McMahon. To prove that galaxy formation began with a swarm of small fry, astronomers need to count how many of these small galaxies there are and whether there were more of them in the distant past than at later times in the universe, he says. “It would be very exciting to find that the [early] universe was filled with little galaxies like this,” he notes.

Detecting many of these faint bodies requires a keen eye, such as the Next Generation Space Telescope. The proposed successor to Hubble, it won’t be launched for a decade. But by 2003, Ellis says, his team will have examined other gravitationally lensed patches of sky as it searches for additional objects that might be among the universe’s earliest galactic building blocks.

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