By Ron Cowen
Astronomers have discovered a galaxy so remote that the light reaching Earth left the body some 13.6 billion years ago. That makes the find the most distant object ever detected.
If the universe is now 14.5 billion years old, then astronomers are seeing the galaxy as it appeared when the cosmos was just one-sixteenth of its current age, says codiscoverer Richard G. McMahon of the University of Cambridge in England. Appearing as a faint red dot, the object is slightly more distant and hails from an era slightly farther back in time—about 157 million years closer to the Big Bang—than the previous record holder, he adds. That object, reported just last month, is a brilliant quasar, the beacon that resides at the core of a galaxy.
“If you want to model the universe, you want to know when the first galaxies formed,” notes McMahon. “And so what we’re trying to do is find when to start the stopwatch.
“This is part of a progressive program to go out to greater and greater distances,” he adds. “In the same way that computers get faster every year,” new technology has allowed astronomers to probe ever deeper into space and find more distant objects, McMahon says.
Astronomers not on the discovery team mentioned the find last week in Cambridge, Mass., at a conference on the earliest cosmic structures. McMahon, who observed the galaxy with Esther M. Hu of the University of Hawaii in Honolulu, provided further details to Science News.
Even with the world’s largest visible-light telescope, the 10-meter Keck II atop Hawaii’s Mauna Kea, Hu and McMahon needed help from a cosmic mirage to record the faint galaxy. The mirage is a consequence of one of the more bizarre aspects of gravity: Massive objects bend light. A foreground object such as a cluster of galaxies can act like a zoom lens. The effect makes an object that lies behind the lens appear bigger and brighter.
Last fall, Hu and McMahon used Keck to scan a patch of sky near the celestial equator. The area contains the massive cluster Abell 370, the first galaxy cluster recognized to act as a gravitational lens. Within that patch, the researchers sought—and found—a particular wavelength of light emitted by hydrogen atoms in a galaxy long ago and far away. McMahon estimates that the cluster amplifies the galaxy’s light by a factor of 2 to 4, effectively turning Keck’s 10-m mirror into a light-gathering device at least twice as big.
Theory suggests that as the first massive stars formed, they emitted high-energy radiation that would easily excite hydrogen, the most abundant gas in a galaxy. The excited hydrogen atoms would radiate much of the absorbed energy at the ultraviolet wavelength of 121.6 nanometers, radiation known as Lyman-alpha.
The expansion of the universe causes distant objects to recede from Earth at high speed and shifts the light they emit to redder, or longer, wavelengths. The more distant the object, the greater the shift, so redshift provides a direct measurement of distance. Lyman-alpha radiation from the new object detected by Hu and McMahon has shifted to a near-infrared wavelength of 918 nm, revealing its extraordinary distance. Using the same technique, the group previously found other distant galaxies (SN: 5/2/98, p. 280).
Another team has reported several galaxies that could turn out to lie even farther away (see All Aglow in the Early Universe). For now, those findings remain uncertain.
The newly discovered galaxy has a redshift of 6.55. Last month, astronomers with the Sloan Digital Sky Survey, a 5-year census of the heavens, reported finding a quasar with a redshift of 5.8.
Both distant galaxies and quasars are essential to astronomers plumbing the depths of the universe, says McMahon. The ages of remote galaxies indicate when stars first formed. Quasars act as search lights, piercing the billions of light-years of intergalactic gas that lie between them and Earth.
A single galaxy forming so early in cosmic history doesn’t pose a problem for theories of how the universe, which began as a smooth, hot soup of particles, could have formed lumpy structures like our Milky Way. “All of our theories of structure formation are statistical and so there will always be the unusually dense region that . . . forms a galaxy earlier,” notes Michael S. Turner of the University of Chicago. However, he adds, large numbers of such galaxies “would be hard to explain.”
McMahon says he is developing an infrared filter that will automatically pick out Lyman-alpha light from even more-distant galaxies. “Eventually, we expect the galaxies to disappear because we will have [looked back] to a time when they hadn’t yet formed,” he says.
