Distant Galaxy Helped Relight the Universe

By Ken Croswell

Published on Physics World (March 8, 2011)

The discovery of a small but distant galaxy 12.8 billion light-years from Earth is providing important clues about the earliest years of the universe's life. By measuring the age of the galaxy's stars, astronomers in Europe and the US say the galaxy began to shine when the universe was just 150 to 300 million years old. The work suggests that such galaxies were responsible for dispersing the atomic fog that once cloaked the cosmos, during a period in the history of the universe that astronomers know very little about.

Following the big bang, 13.7 billion years ago, the universe was hot and ionized. But as the universe expanded, it cooled, and 380,000 years after the big bang, protons joined electrons to make neutral hydrogen atoms, which block light. Then, stars and galaxies eventually arose whose radiation ionized the universe anew, allowing light to speed through space unimpeded--a time called the epoch of reionization.

Our understanding of this ancient era is very limited because the light from galaxies that were around at the time has travelled great distances and is therefore extremely faint when it reaches Earth. As a result the study of such galaxies can only offer tantalizing clues to what happened in the early universe.

But now Johan Richard of the University of Lyon in France and his colleagues have spotted a distant galaxy that appears much brighter. "What makes this object very special is that we can really get a very strong signal on a very faint object," says Richard.

The galaxy appears bright thanks to gravitational lensing. The galaxy happens to lie far behind a galaxy cluster in the constellation Eridanus named Abell 383. The cluster's gravity splits the distant galaxy's light into three images and boosts its brightness by a factor of eleven, making it much easier to study.

The galaxy's redshift is 6.027, which means it is so distant that the universe's expansion stretches the galaxy's light waves by 602.7 percent as they travel to Earth. Thus, all of the galaxy's visible and most of its ultraviolet light appear to us at infrared wavelengths.

To make the discovery, Richard's team used optical and infrared data from two NASA observatories, the Hubble Space Telescope and the Spitzer Space Telescope. Because the galaxy--which we see as it was when the universe was 950 million years old--looks so bright, the astronomers easily determined the age of its stars: most are older than 640 million years old.

"That means that the galaxy formed these old stars very early--like only a couple of hundred million years after the big bang," says Richard. His team estimates that the galaxy's stars began shining between redshift 14 and 22, when the universe was just 150 to 300 million years old. For comparison, last October astronomers reported the farthest known galaxy, at redshift 8.6, but that redshift is uncertain.

Richard Ellis, an astronomer at the California Institute of Technology in Pasadena who is not involved with the discovery, calls the implications exciting. "Because of the magnification, the data is of excellent quality," he says, noting that earlier claims of old stars in distant galaxies were tentative. "It gives strength to the fact that there are old galaxies at redshift 6, and that means that these galaxies have been around for a long time. The implication is that there are many galaxies out there waiting to be discovered that are at redshifts of 10 and beyond." The Hubble Space Telescope may find them, he says, if it takes deeper exposures.

The newly discovered galaxy is small, measuring only a few thousand light-years across, compared with the Milky Way's diameter of 120,000 light-years. Yet it's spawning stars as fast as the Milky Way. Richard and his colleagues estimate that the mass of the galaxy's stars is 6 billion times that of the Sun, a far cry from the Milky Way, whose stars weigh in at about 100 billion solar masses.

Small galaxies such as this one may have cleared the fog that pervaded the early universe. Says Richard, "That's why we're using gravitational lensing to look for fainter objects, because if they're very numerous, then they would have reionized the intergalactic medium."

Richard and his colleagues have submitted their work to Monthly Notices of the Royal Astronomical Society Letters.

Ken Croswell is an astronomer and the author of eight astronomy books, including The Lives of Stars.

"A stellar picture of what we know or guess about those distant lights."--Kirkus. See all reviews of The Lives of Stars here.