This is a true-color image from the part of the Hubble Ultra-Deep Field. It has allowed researchers to identify a new population of the most-distant known galaxies (white circles) present when the universe was less than 800 million years old. These galaxies may be responsible for ionizing intergalactic gas early in the universe's history.
Credit: R. McLure/J. Dunlop/R. Ellis/B.Robertson/D. Stark [Full Story]
Researchers are building a better picture of one of the most important changes in the early universe a process that lifted the cosmic fog from the dark, early universe.
A new study of observations by the Hubble Space Telescope has revealed that the earliest known galaxies, present some 800 million years after the Big Bang 13.7 billion years ago, may have emitted enough ultraviolet light to strip electrons from (or ionize) the hydrogen gas between galaxies.
Before this so-called re-ionization of the universe, hydrogen gas absorbed light just like fog on a dark road, limiting the distance over which the earliest galaxies could have been observed. [Hubble photo of the ancient galaxies]
Early universe unveiled
For years, researchers have believed that as these galaxies coalesced into larger and larger structures. Their subsequent ultraviolet glows ionized hydrogen in bubbles around them, which grew and eventually overlapped until essentially all the intergalactic medium was ionized and transparent.
But researchers were less sure about the timing.
"That has been the outstanding question," Brant Robertson of the California Institute of Technology in Pasadena, Calif., told SPACE.com. "At what point in time do galaxies produce enough of these ionizing photons to keep the electrons and protons in the universe unbound?"
Dark ages of the universe
Although the universe actually began in an ionized state following the Big Bang, this first period of ionization lasted for only about 300,000 years, until electrons and protons cooled enough to form hydrogen atoms. (This hydrogen formation generated photons that we know today as the cosmic microwave background.)
What followed was a period known as the cosmic Dark Ages, during which the universe's vast clouds of cloud hydrogen slowly condensed to form the first galaxies. From studies of neutral hydrogen detected in the light from distant quasars, astronomers knew that re-ionization must have been complete by about a billion years after the Big Bang.
"The thing that's changed recently is the installation of the new camera on Hubble," Robertson said.
Hubble's Wide Field Camera 3, in operation since May 2009, has allowed astronomers to pinpoint the infrared signatures of more than 50 galaxies that date to 800 million years after the Big Bang.
The research is detailed in the Nov. 4 issue of the journal Nature.
Explaining the cosmic fog's lifting
By determining whether these ancient galaxies released sufficient ultraviolet light to re-ionize the universe's intergalactic hydrogen, researchers could rule out other possible explanations for re-ionization, such as radiation from matter falling into massive galactic black holes, or the annihilation of hypothetical dark matter particles.
In their new analysis, Robertson and his colleagues first used the recent Hubble data to estimate the total number of ancient galaxies. Then they took information about the color of light from those galaxies to estimate the sizes and intensities of the stars contained in them, which yielded the number of ultraviolet photons generated by each galaxy.
Finally, based on data for closer galaxies, the researchers estimated that some 10 to 20 percent of those ultraviolet photons would have made it out of their galaxies to re-ionize intergalactic hydrogen.
The results suggest there were likely enough ultraviolet photons to re-ionize the universe in the first 800 million years of cosmic time, although uncertainties remain.
Robertson said one of the major uncertainties in their calculation was the number of early galaxies, which currently depends on Hubble's surveys of a small section of sky.
But as Hubble collects data over a wider portion of the sky, that uncertainty will decrease.
"It's a very exciting time," Robertson said. "In the next couple of years we'll be able to know if there are enough galaxies to re-ionize the universe."
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