Like a ship approaching a distant beacon, astronomers are getting closer and closer to the cosmic dawn, the time in the universe's history when the first stars formed. A team of researchers today announced that the Hubble Space Telescope has allowed them to see as far back in time as a mere 380 million years after the big bang—more than 13.3 billion years ago. That's within striking distance of the first stars, which researchers think were born 200 million years after the universe began.
More remarkable is the fact that the researchers, led by astrophysicist Richard Ellis of the California Institute of Technology in Pasadena, have imaged not one but seven galaxies from that early cosmic period, dating between 380 million and 600 million years after the big bang. The discovery provides the first census of galaxies from what's known as the epoch of reionization. During this period, which extended from the cosmic dawn to about 1 billion years after the big bang, ultraviolet light was breaking down hydrogen in the universe into a soup of electrons and protons, making the universe more transparent.
How reionization occurred is a fundamental question in astrophysics. The findings described by Ellis and his colleagues in a telephone press conference organized by NASA this afternoon suggest that these first galaxies provided the ultraviolet radiation required to reionize the universe. The population of galaxies appears to have steadily increased from the beginning of the reionization epoch.
The researchers conclude that "reionization is an extended process associated with gradual galaxy growth," Ellis said. "Cosmic dawn was likely not a dramatic event."
Until a few years ago, astronomers did not expect to be able to see this far with the Hubble Space Telescope, but the observatory's new Wide Field Camera 3—installed during a servicing mission in 2009—has stunned researchers with its capabilities. Ellis and his colleagues used the camera to look at a tiny sliver of space for more than 100 hours. The images derived from this deep exposure were good enough for the researchers to spot the seven galaxies.
"This dataset represents the biggest archeological dig of the universe to date," says Abraham Loeb, an astrophysicist at Harvard University who was not involved in the study. Until recently, Loeb says, "most of the research on the first generation of galaxies was theoretical, but the next decade promises a flood of data." Much of that data will come from the James Webb Space Telescope—scheduled for launch in 2018—which is expected to see all the way to the cosmic dawn.