Long Ago and Far Away: Astronomers find distant galaxy, early cluster

Peering ever deeper into space and further back in time, two teams of astronomers have uncovered new details about the earliest galaxies and galaxy clusters in the universe.

GOING THE DISTANCE. Composite infrared and visible-light image shows the most distant galaxy known (arrow). Nearby stars cause the green streaks. Hu, L. Cowie et al.

One team, aided by a cosmic magnifying glass, has detected the earliest, most distant galaxy yet known. The scientists first told Science News about the galaxy 2 years ago (SN: 5/27/00, p. 340: Newfound Galaxy Goes the Distance), and they have now published their findings in the April 1 Astrophysical Journal Letters.

The faint body dates from a time when the universe was less than 1 billion years old, notes study coauthor Esther M. Hu of the University of Hawaii in Honolulu. The detection may push farther back the date that scientists assign to the end of the cosmos’ Dark Ages and the emergence of the first stars and galaxies (SN: 8/11/01, p. 84: Light’s Debut: Good Morning, Starshine!).

Meanwhile, another team has detected the most distant grouping of galaxies yet found. The grouping, a so-called protocluster, resides about 1 billion light-years closer to Earth than does the galaxy found by Hu and her collaborators.

To find the most distant galaxy, Hu’s team used the Keck 2 telescope atop Hawaii’s Mauna Kea to search for a particular wavelength of light emitted by hydrogen atoms. Bombarded by radiation from young, massive stars, hydrogen atoms within galaxies emit a wavelength of ultraviolet light known as Lyman-alpha.

As observed from Earth, the radiation is shifted to longer, or redder, wavelengths because of the expansion of the universe. The more distant the body, the greater this redshift. The Lyman-alpha radiation that Hu and her colleagues detected has a redshift of 6.56, the highest ever recorded. That implies that the object lies more than 13 billion light-years from Earth.

Hu’s team relied on a filter through which only the redshifted Lyman-alpha wavelength passes (SN: 5/2/98, p. 280). But even with the light-gathering power of Keck 2, the researchers needed extra help to detect the galaxy.

By chance, light from the galaxy passes by a galaxy cluster before reaching Earth. Like any massive object, the cluster behaves as a lens, focusing light rays that come near it. This lens magnified the image of the distant galaxy.

Hu and her collaborators also imaged the galaxy in infrared light by using the Subaru Telescope on Mauna Kea.

Hu notes that hydrogen gas surrounding the distant galaxy must already have been ionized at the time the observed light near the galaxy had left the object. If there were any neutral hydrogen near the galaxy, it would have absorbed the Lyman-alpha emission, thereby preventing it from reaching Earth.

An abundance of other galaxies must have already been shining brightly enough to ionize hydrogen. Galaxy formation—at least in the region around the distant galaxy—was therefore well under way a mere 780 million years after the Big Bang, Hu says.

Using a set of filters similar to those employed by Hu’s team, Arjun Dey of the National Optical Astronomy Observatories in Tucson and his collaborators recently found three extremely distant galaxies, he told Science News.

One of the bodies has a redshift of 5.742, qualifying it as the second-most-distant galaxy ever observed, a bit more distant than the previous runner-up (SN: 11/7/98, p. 296).

With a different technique, Bram P. Venemans of the University of Leiden in the Netherlands and his colleagues searched the region around a luminous radio-emitting galaxy to find the most distant known protocluster. Their report, which appears in the April 10 Astrophysical Journal Letters, suggests that radio galaxies may serve as beacons for finding other primordial groupings of galaxies.

“What they are effectively seeing is a collection of small hills—a small grouping of galaxies—that over time will grow through gravity to become the Himalayas of today, massive galaxy clusters,” says August E. Evrard of the University of Michigan in Ann Arbor. The shape and narrow extent of the protocluster support the leading cosmological model, in which structures start out small and grow successively larger, he adds.

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