GRAVITATIONAL LENSING: Astronomers have spotted what may be the oldest galaxies yet [red circles] by training a powerful telescope on light passing near closer galaxies [dark spots], which focus the light along certain contours in the sky [black lines]. Image: STARK ET AL., 2007 APJ: 663, 10
Astronomers say they have discovered a handful of relatively small galaxies that date to 500 million years after the big bang, or several hundred million years earlier than the previous oldest galaxies. The ancient objects may have initiated a key event called reionization that led to the clumping of small galaxies into larger ones.
Confirming the finding will take additional work, but if true, "it's scratching the edge of the universe," says astronomer Richard Ellis of the California Institute of Technology in Pasadena, who took part in the research. "We're really pushing out our telescopes to new limits to find these objects."
Ellis, his PhD student Dan Stark and their colleagues trained one of the world's biggest telescopes, the Keck 2 atop Hawaii's Mauna Kea, to scan light grazing massive clusters of closer galaxies [see image above], which focused the light coming from more ancient galaxies behind them and magnified it 20 times in a process called gravitational lensing.
These natural magnifying glasses revealed six points in the sky glowing feebly at telltale wavelengths of infrared light, according to the group's study published online in The Astrophysical Journal. Because the universe is expanding, light from distant sources is stretched from visible wavelengths to infrared, called redshifting, with the exact wavelength corresponding to the age of the object.
Ellis says the intensity of the light suggests that the galaxies are one hundredth or one thousandth the Milky Way's size, with a mass of up to 10 million suns, compared with our galaxy's 10 billion solar masses.
"This is the first credible example of redshift-10 galaxies"—corresponding to 500 million years after the big bang—"but it's not conclusive," says astrophysicist Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. Nailing a galaxy's age requires precisely measuring its infrared intensity along a range of wavelengths, and for now the measurements are relatively crude, he adds.
Loeb says "cosmic dawn," when the first stars in the universe lit up, probably dates to 200 million years or sooner after the big bang. But the newly discovered objects may have kicked off the subsequent phase of reionization, in which ultraviolet starlight stripped electrons from hydrogen gas. Galaxies grew in size as a result, Loeb says, because reionized hydrogen would have been too hot to clump together unless present in more massive quantities.
The Caltech team estimates that the newly discovered galaxies' combined radiation would have been sufficient to break apart hydrogen atoms.