Hubble sees earliest known galaxies through cosmic fog

Peering through a thinning fog of hydrogen, the Hubble Space Telescope has taken the first census of the earliest known galaxies in the universe. One is the most distant galaxy yet spotted ? so far away that its light has taken 13.4 billion years to reach us.

Hubble's survey of fledgling galaxies can help us figure out what drove a phenomenon called reionisation. It ended a mysterious time called the cosmic dark ages, when a shroud of opaque hydrogen gas dominated the universe. Then radiation, probably from the first stars and galaxies, ionised the hydrogen and made it transparent, but details of precisely how this happened are scarce.

The galaxies date from the middle of the reionisation era, when the universe was about 500 million years old. "This study represents the deepest archaeological dig of the universe so far," says Avi Loeb of Harvard University, who was not involved in the survey.

Burning off fog

Until now, only a couple of galaxies had been spotted from the reionisation era, and their light would not be enough to burn off the fog. But astronomers thought there must be more out there, because previous observations showed that the number of galaxies decreases smoothly as we look back in cosmic time.

"What we set out to do was to test whether there are a sufficient amount of galaxies deep in the heart of this reionisation epoch," says Richard Ellis of the California Institute of Technology in Pasadena.

Ellis and colleagues used Hubble to stare at a spot in the sky for 100 hours ? twice as long as in previous surveys. They also used a special filter that made the telescope more sensitive to faint, distant objects.

The survey spotted a previously known galaxy called UDFj-39546284, and found that it is even more distant than we believed ? it now holds the record. They also found six other galaxies that existed no later than 13.1 billion years ago. The team is now working to determine if the light from these galaxies would have been able to reionise the universe.

Deeper looks

Although these galaxies are not the first that formed, we probably won't be able to see any further for a while, Ellis adds.

"With Hubble, it's probably as far back as we'll ever look," he says. Deeper views will have to wait for the James Webb Space Telescope, due to launch in 2018.

"Until we did these observations, it wasn't obvious if there was anything out there," Ellis says. "Now we can feel confident that there are plenty of galaxies beyond Hubble's limit that James Webb can look at."

Journal reference: arxiv.org/abs/1211.6804

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