Cosmic Census

Astronomers find the earliest galaxies seen by man

When UA professor Brant Robertson was an MIT Hubble fellow a few years ago, he was part of a team that came up with a plan to direct the Hubble Space Telescope to peer into a portion of space known as the Ultra Deep Field. Earlier this year, Robertson and his crew got a chance to put the plan into action with the Wide Field Camera 3, an infrared camera installed on Hubble by a NASA astronaut via a 2009 spacewalk. Last week, the team unveiled the results: images of galaxies that formed more than 13 billion years ago, from 380 million to 600 million years after the Big Bang.

You were able to use the Hubble Telescope to peer back in time 13.3 billion years. What did you see?

We found a new population of galaxies that hadn't been seen before. We're always interested as astronomers to learn how the population of galaxies built up over time. One of the most important questions we can answer using something like the Hubble Space Telescope is: When did galaxies first start forming? Was there an immediate rapid rise in the galaxy population or was it a more measured increase in the number? The light from these galaxies are quite blue intrinsically; they emit a lot of ultraviolet radiation. And as that light is traveling from those distant objects to us in the present day, the universe is expanding and it stretches out the light, and it takes ultraviolet light and stretches it into the infrared. So we need infrared cameras to see these very distant galaxies.

What do you learn from looking back toward the dawn of time?

We're learning how many of these early galaxies there were. Was it a single population? Were there more than one? We found six objects in the range of what we call redshift 8.5 to 10.5, and that's roughly in the range of, say, 600 million years to 450 million years after the Big Bang. And then we found, to our surprise, the record-holder galaxy at redshift 12, which is only about 380 million years after the Big Bang. We're learning a lot about these galaxies. They're not the first generation of galaxies. We think they've been around a while. They're forming stars quite prodigiously, so that's important to understanding what happens to the universe as a whole.

What is the "cosmic dawn," and how does all this work relate to it?

After the Big Bang, the universe is very hot and dense, but it expanded and cooled to the point where the free electrons in the universe—the negatively charged particles in the universe—could recombine on some protons to form hydrogen. That was first big event in the universe, this recombination. Afterwards, the universe was full of diverse, neutral gas and there weren't any stars, there weren't any galaxies, there weren't any structures. And there wasn't any light in the universe. We call that the "dark ages." But eventually the first stars and galaxies could form and they were the first sources of light, and we call that "cosmic dawn." These galaxies we found are more like midmorning—after your first cup of coffee or something. They're just now abundant and bright enough to really have some influence on what's going on in the universe.

The UA is also involved in building the James Webb Space Telescope. What's that project all about and what do you hope to learn from that?

That's tremendously exciting. The James Webb Space Telescope is kind of Hubble's big brother. The Hubble has been around for a long time and we're really pushing it to the limits. JWST is a new telescope that's much larger than Hubble and it looks primarily in the infrared. And some of those new infrared cameras are being built here in the UA, by Marcia and George Rieke. JWST will allow us to look even further back in time. It's scheduled for launch in 2018.

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