THE DEEP END: Images of a small patch of sky called the Hubble Ultra Deep Field have revealed several of the most distant galaxies ever seen. The newfound galaxies and their associated redshifts are labeled on the Hubble image. Image: NASA/ESA
The title of most distant known galaxy has bounced around a bit in the past few years, as various teams of researchers have laid claim to progressively more remote objects in the early universe. Now a group of astronomers using the Hubble Space Telescope has tentatively identified a new record-holder: a galaxy so distant that its light has taken most of the 13.7-billion-year history of the universe to reach humankind’s telescopes.
By peering back to a time a few hundred million years after the big bang, Hubble spotted the galaxy, known as UDFj-39546284, which appears as little more than a faint smudge of infrared light. The galaxy has already had one turn as the most distant known galaxy—it was previously identified by a different group of astronomers using Hubble imagery but was later supplanted by a more recent, seemingly more distant discovery. The new data, however, suggest that UDFj-39546284 lies at redshift 11.9, corresponding to 380 million years after the big bang. That is even more distant than had been believed, leapfrogging the ancient galaxy back into the top spot. (Redshift, which quantifies how much an object’s emitted light wavelength has been stretched toward the red end of the spectrum in an expanding universe, is the astronomer’s preferred distance measure for faraway objects.)
More important to cosmologists, however, the new researchers identified not just one but seven objects that are most likely distant galaxies, all of which lie at redshift 8.5 or greater—that is, within 600 million years of the big bang. The results of the cosmic census may help researchers determine exactly what happened in the first billion years of cosmic history, when ultraviolet light from the earliest stars and galaxies ionized the hydrogen of the once-opaque intergalactic medium. The epoch of re-ionization, as it is known, effectively lifted a pervasive cosmic fog and left the universe in the transparent state that today allows us to observe objects billions of light-years away.
“Of course the most distant object is interesting,” Richard Ellis of the California Institute of Technology, lead author of a study to be published in The Astrophysical Journal Letters, said in a December 12 teleconference about the findings. “But it’s the census, it’s the seven objects, that give us the first indication of the population of objects in the heart of this re-ionization era.”
In conjunction with studies of galaxies a few hundred million years later, the new observations offer clues to the pace of re-ionization. “In our article we describe a very smooth decline in the number of objects as we probe back into cosmic history, into this re-ionization era,” he said. “Re-ionization is an extended process. It’s associated with gradual galaxy growth.”
But re-ionization probably began some 200 million years before the earliest galaxies Hubble has discovered. So the beginning of the story may have to wait for Hubble’s successor, set to launch in 2018. “This may well be as deep as Hubble can look into the re-ionization era,” Ellis noted. “But we confidently predict that there will be many galaxies beyond this into the re-ionization era that can be explored with the James Webb Space Telescope.”
Said NASA science chief, John Grunsfeld, who as an astronaut helped repair and upgrade Hubble for the last time in 2009, “We really are pushing Hubble well beyond what it was ever imagined it would do.”