Galaxy's Light Pushes Back Dark Ages of the Universe

University of Hawaiʻi
Esther M. Hu, (808) 956-7190
Institute for Astronomy
Lennox L. Cowie, (808) 956-8134
Institute for Astronomy
Posted: Mar 6, 2002

An international team led by astronomers from the University of Hawaii reported discovery of a galaxy which gives the earliest glimpse of the epoch when galaxies and stars formed in the universe.

"This galaxy is forming stars at a time speculated to be in the 'Dark Ages' of the universe when galaxies begin to 'turn on'," said University of Hawaii professor Esther Hu, who led the team.

According to the generally accepted picture, the universe started with the Big Bang some 14-16 billion years ago. As the universe expanded and cooled over the next half billion years, the glowing plasma of which it was composed recombined into atoms of neutral gas -- mostly hydrogen and some helium. The glow from this era of recombination has been observed as the cosmic microwave background radiation, and used to study the large-scale geometry of the universe. Over the next nearly half billion years, termed the Dark Ages, the cold gas began to assemble into the first galaxies. The Dark Ages ended as the light from the newly formed galaxies and quasars reionized and changed the character of the surrounding neutral gas.

Till now the earliest probes of the Universe have been quasars, which are extremely luminous distant objects, believed to be powered by black holes. To capture an early snapshot of galaxies, which are typically a thousand times fainter, researchers concentrated on a bright hydrogen emission line dubbed "Lyman alpha" that is strongly excited during star formation. Since a large fraction of the light from early forming galaxies emerges in this line, distant galaxies can look prominent viewed through filters which only pass wavelengths near the Lyman alpha emission but appear faint or undetected when seen through other filters.

The method of discovering distant galaxies by searching for objects identified as "Lyman alpha emitters" by the sharp increase in their detectability in narrow bandpass filters has been very successful, and team members had previously used one of the giant Keck 10-m telescopes to find the most distant galaxy previously known, an object whose light took some 15.3 billion years to reach us.

In order to reach fainter and yet more distant galaxies in the present work, Hu and her colleagues used a gravitational lens, in the form of a massive cluster galaxies to further amplify the light. According to Einstein's theory of general relativity, very massive objects can bend andfocus light in much the same way as a magnifying glass. The astronomers used the cluster Abell 370, which is 6 million light years away and whose core contains the mass of several hundred galaxies, to magnify light from a galaxy behind the cluster that is 15.5 billion light years distant.

The discovery images made with the 10-m Keck I telescope were confirmed with spectra obtained later on the same telescope which showed there was indeed a strong Lyman alpha emission line.

"It's significant that you can see the line," said Peter Capak, a University of Hawaii graduate student and team member. "If only a few galaxies had turned on by this point the emission would have been smothered by the surrounding hydrogen gas and the light would never have made it out to us."

Len Cowie, another Hawaii astronomer and team member added, "The fact that this is a galaxy, and not a quasar, is also important. When the first galaxies form, it's like turning on lights to clear out a fog bank. Quasars are really bright though rare, so they can make large clear cavities around themselves, but the fact that light from the fainter but much more numerous galaxies is getting out means that a significant amount of early star formation has already taken place and much of the general fog has already dissipated."

The newly discovered galaxy has a redshift of 6.56, and samples the universe when it is about 780 million years old. This is about 50 million years earlier than the view supplied by the most distant quasar (redshift = 6.28), and 80 million years earlier than the speculated period of reionization (redshift ~6.1).

Since most of the light from these galaxies has been redshifted to infrared wavelengths, the team followed up their discovery with infrared images on the Subaru 8.3-m Telescope, also on Mauna Kea, to estimate the star formation rate -- finding that 40 times the mass of the Sun is being turned into new stars each year.

"You want to catch galaxies in their infancy and see how they develop", commented Hu. "Scaling the age of the Universe to a person's lifetime, we're showing you baby pictures. The last galaxy snapshot showed a toddler just past his fourth birthday. This one is three and a half."

"This is good news for the Next Generation Space Telescope planned for launch in the next decade," she concluded. "It means that there should be plenty of these distant galaxies bright enough to observe, using a large telescope with good infrared detectors, above the strong airglow of our atmosphere."

Images and additional information about distant galaxy searches are available at:

The paper will appear in the April 1 issue of the Astrophysical Journal Letters. The paper has been posted to the public archive and should appear this Friday at:

The Institute for Astronomy at the University of Hawaii conducts research into galaxies, cosmology, stars, planets, and the Sun. Its faculty and staff are also involved in astronomy education, and in the development and management of the observatories on Haleakala and Mauna Kea. Refer to for more information.

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