News media worldwide are reporting today on a new “farthest galaxy ever found,” courtesy of the Hubble Space Telescope, but the discovery is not quite as definite as it’s being made out. In a study characterized more by a process of elimination than positive identification, the astronomers involved estimate that there’s about an 80% chance they've got it right.
If their interpretation is correct, light from the tiny, faint galaxy has undergone a redshift of 10.3, meaning the light has been en route to Earth for 13.2 billion years. In looking at this galaxy “We’ve gone back through 96 percent of the life of the universe to 500 million years after the Big Bang,” said Garth Illingworth (University of California, Santa Cruz), who reported the discovery with Rychard Bouwens (University of Leiden) in the January 27th Nature. The object, designated UDFj-39546284 for its location in the Hubble Ultra Deep Field, seems to be just 1% of the size of the Milky Way — typical of the mini-galaxies that presumably filled the early universe. It was visible only at the longest infrared wavelength that Hubble can detect (1.7 microns). “We’re really pushing Hubble to its limits here,” Illingworth said.
With more than 40 hours of exposure time on Hubble’s recently installed Wide Field Camera 3, the team created what Bouwens called “the deepest near-infrared image ever obtained.” Still, the object was so faint that team could not get a spectrum of its light to analyze, the surest way to determine a redshift. They had to rely instead on a photometric redshift estimate, based on comparing the amount of light coming through several of the camera’s wide-spectrum color filters. From such information astronomers can piece together a very rough approximation of the object’s spectrum, though without the narrow spectral lines astronomers usually rely on to measure redshift accurately.
That’s where the process of elimination came in. Since astronomers were limited to a rough approximation of spectrum, there was a much greater risk of misinterpreting the data. With only a photometric level of detail, different types of objects may show the same spectral profile. “You have to make a leap of faith and you ask yourself, ‘What else can it be?’ ” said Rogier Windhorst, a professor at the Arizona State University’s School of Earth and Space Exploration.
According to Windhorst, the mostly likely alternative explanation in this case would be a less distant galaxy reddened by dust. “These galaxies have a spectrum somewhat similar to the source,” Bouwens said. “But only somewhat similar.”
This object showed a very unusual profile, with light only showing up in the reddest filter of the camera’s seven. That’s why both Bouwens and Windhorst are fairly confident they’re really seeing an incredibly distant galaxy; a dusty galaxy closer to home would probably show some emission at other wavelengths. Other interpretations are “not impossible, but not very likely,” Windhorst said.
Observations of ultra-high-redshift galaxies provide important constraints for theories of how the first galaxies formed. Though the Hubble image shows this object as just a tiny smudge, its miniature size itself holds important information about primeval galaxies. The team also concluded that “star birth at 500 million years [after the Big Bang] was astonishingly less than starburst at 600 million years,” Illingworth said.
“For the first time now, we can make realistic statements about how the galaxy population changed during this period,” said Bouwens.