An extremely faint speck of light in an 87-hour long exposure made by the Hubble Space Telescope may be the most remote object ever seen. Astronomers think it's a galaxy 13.2 billion light-years away. Because the object's light took this long to reach Earth, researchers say the find takes us back to an epoch when the universe was only some 480 million years old. That's more than 100 million years earlier than the previous record holder. The new find reveals that galaxies were forming and growing quite fast at this very early time in the universe.
An international team led by astronomer Rychard Bouwens of the University of California, Santa Cruz, and Leiden Observatory in the Netherlands used software to comb through the Hubble image in search of record-distant galaxies. The farther away these galaxies are, the longer their ultraviolet (UV) light has been traveling through expanding space, and the more the light waves have been stretched toward redder wavelengths. For the most distant galaxies, UV gets "redshifted" all the way into the near-infrared, and the galaxy doesn't even show up at shorter wavelengths. By observing the image through different filters, astronomers can get a rough estimate of the redshift, which tells them the distance to the galaxy.
The new Hubble Ultra Deep Field (UDF) image was obtained by combining near-infrared pictures made by the Wide Field Camera 3 (installed in May 2009) and optical photos made by Hubble's older Advanced Camera for Surveys. "With 40% more sensitivity compared to earlier exposures, we were able to push to fainter and more distant sources," says Bouwens. Additional checks with NASA's Spitzer Space Telescope provided tentative confirmation that the faint object found in the image, cataloged as UDFj-39546284 (the numbers refer to its position in the sky), is a very remote galaxy. The results were published online today in Nature.
If the blip of light is indeed a galaxy 13.2 billion light-years away, as Bouwens concludes from its color and relative brightness at various wavelengths, its faintness implies that it can't be much heftier than 1% of the mass of our own Milky Way. Big galaxies like ours grew by consecutive mergers of such smaller pieces. "This galactic building block may be one of the largest at that epoch," says Bouwens. The fact that the Hubble image shows up only one extremely remote object-while many more have been found at slightly shorter distances-implies that there was a fast baby boom of new galaxies when the universe was between approximately 400 million and 700 million years old.
However, astronomer Haojing Yan of Ohio State University in Columbus, who heads a competing group hunting for distant galaxies, has some reservations. "This is important work," he says, "since, for the first time, astronomers have the capability to see those extremely remote galaxies. But how many we have seen is still controversial. Hubble might well only be scratching the surface." According to Yan, there could be many more large building blocks at comparable or even much larger distances, too faint to be detected. Without knowing how luminous to expect those objects to be, he says, it's too early to make strong claims about their evolution.
Bouwen's co-author Marijn Franx of Leiden Observatory adds that UDFj-39546284 is just a candidate galaxy as long as its distance has not been confirmed by direct spectroscopic measurements of the object's redshift. In principle, it could be an extremely red mini-galaxy much closer to Earth. "Such measurements won't be possible until the James Webb Space Telescope is launched," he says, referring to the planned successor to Hubble. On the other hand, he stresses, if looking back to a cosmic age of 500 million years reveals just one candidate galaxy in the Hubble image, and if even that one turns out to be something else, it really means galaxies were extremely rare during that early epoch. "In fact," says Franx, "it would only strengthen our conclusion that we're looking back to a time when the very first galaxies were assembled."