Sen—Astronomers using the Hubble Space Telescope have discovered a population of six previously unseen galaxies that formed 13 billion years ago. They also refined the distance of a seventh galaxy, identified as UDFj-39546284, as the most distant galaxy on record, which we are seeing as it was when the universe was only 380 million years old, less than 3% of its current age. That is further back in time than any object seen before.

The survey of a part of the sky called the Ultra Deep Field (UDF) has given scientists the first robust sample of galaxies that show how abundant they were in the era when galaxies first formed, and support the theory that galaxies assembled continuously over time and could have provided enough radiation to reionize the universe just a few hundred million years after the big bang.

In the 2012 campaign, called UDF12, a team of astronomers led by Richard Ellis of the California Institute of Technology in Pasadena used Hubble's Wide Field Camera 3 (WFC 3) to peer deeper into space in near-infrared light than Hubble has ever observed before. They observe in near-infrared light as the expansion of space stretches, or "redshifts", ultraviolet and visible light from galaxies into infrared wavelengths. The more distant a galaxy, the higher its redshift. By studying the galaxies' colors through a set of four filters at specific near-infrared wavelengths the team are able to estimate the galaxy distances. The record breaking UDFj-39546284, has a redshift of 11.9.

Whether hot stars in such early galaxies could have provided enough radiation to warm the cold hydrogen that formed soon after the big bang has long been debated. This re-ionization is thought to have occurred 200 million to 1 billion years after the birth of the universe and made the universe transparent to light, allowing astronomers to look far back into time. It is in this early epoch that the galaxies in the new study are seen.

"Our data confirm re-ionization was a gradual process, occurring over several hundred million years, with galaxies slowly building up their stars and chemical elements," said Brant Robertson of the University of Arizona in Tucson. "There wasn't a single dramatic moment when galaxies formed. It was a gradual process."

The observations were made during six weeks from August to September. A major goal of the study was to determine how rapidly the number of galaxies increased in the early universe providing key evidence for how quickly galaxies build up their constituent stars.

All of the light originally emitted as visible light, and most released at near-infrared wavelengths from galaxies whose light has been shifted to infrared wavelengths, will have been absorbed by the intervening hydrogen. So these galaxies will not be detected in most of Hubble's filters. They will only be seen in Hubble's longer-wavelength infrared filters.

"Our study has taken the subject forward in two ways," Ellis explained. "First, we have used Hubble to make longer exposures. The added depth is essential to reliably probe the early period of cosmic history. Second, we have used Hubble's available color filters very effectively to more precisely measure galaxy distances."