By pushing the refurbished Hubble Space Telescope to its very limits as a cosmic time machine, astronomers have identified three galaxies that may hail from an era only a few hundred million years after the Big Bang. The faint galaxies may be the most distant starlit bodies known, each lying some 13.2 billion light-years from Earth.
Detecting galaxies at such a distance is at the very edge of what current technology can accomplish, comments Richard Ellis of Caltech, who was not part of the new study. It’s uncharted territory, he says.
If the researchers are correct in the preliminary determination, then Hubble is seeing light that reveals the galaxies as they first appeared just 480 million years after the birth of the universe. (That light traveled for billions of years to reach Earth.) The radiation from such early galaxies played a crucial role, theorists believe, in reionizing the universe. That process breaks apart neutral atoms into electrons and ions, which enabled light from the first generation of stars to stream freely into space.
The astronomers caution that because the galaxies they found with Hubble are seen at only one wavelength, it’s not certain that the bodies are extremely distant; they could just be red and faint. “We certainly don’t have smoking gun evidence,” says study coleader Rychard Bouwens of the University of California, Santa Cruz. “We just have tantalizing evidence that suggests we may be identifying a few [extremely distant] galaxies.”
Bouwens and Garth Illingworth, also of UC Santa Cruz , along with several collaborators, posted their findings online December 23 at the physics arXiv.org site (http://arxiv.org/abs/0912.4263). The team, like several others, went hunting for distant galaxies using Hubble’s newly installed Wide Field Camera 3, which in August took a long look in infrared wavelengths at a patch of sky known as the Hubble Ultra Deep Field. Another Hubble camera had examined that field five years earlier in visible light, revealing many faint, faraway galaxies, but not the most remote galaxies, which can only be seen in infrared.
Ultraviolet and visible light emitted by the youthful stars in the earliest, most distant galaxies is shifted to much longer wavelengths — the infrared part of the spectrum — by the expansion of the universe. The more remote the galaxy, the greater the redshift.
In September, two teams, including Bouwens’, reported finding galaxies with redshift values of seven to eight, corresponding to an era about 700 million years after the Big Bang. Now, the researchers estimate that another three galaxies imaged by the camera have a redshift of about 10, which if confirmed would be the largest redshift ever measured. Bouwens says that several tests, including observations with the infrared Spitzer Space Telescope, indicate that the galaxies they spotted are likely to be truly remote, reducing the possibility that his team is being fooled by intrinsically faint, infrared-emitting galaxies that lie much closer to Earth.
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Other teams, notably a group that includes Rogier Windhorst of Arizona State University in Tempe and Haojing Yan of Ohio State University in Columbus, reporting earlier on arXiv.org (http://arxiv.org/abs/0910.0077), claimed to have found 20 galaxies at that same high redshift using the same data from the refurbished Hubble.
Bouwens and Illingworth note that most of the candidate distant galaxies identified by the Windhorst team lie near known, bright galaxies. They suggest that the team may have been confused by stray light from these bright galaxies. Other astronomers say it would be surprising if all 20 galaxies were from the same early era, since the Ultra Deep Field encompasses a narrow strip of sky. That would indicate that the early universe had a surprisingly high density of such galaxies.
Although the race is on to find more-convincing examples of distant galaxies, “redshift-10 galaxies are about the very edge that our current technology can push to,” notes Yan. It’s likely that none of the distant galaxy candidates can be confirmed until the launch of Hubble’s powerful infrared successor, the James Webb Space Telescope, around 2014, astronomers agree.