With Updated Hubble Telescope, Reaching Farther Back in Time
THROUGH THE AGES Using old and new cameras, the Hubble Space Telescope recorded thousands of galaxies, some dating back more than 12 billion years.
Astronaut repairmen had hardly finished tightening the last stubborn bolts on the Hubble Space Telescope last summer when astronomers set the controls on the refurbished telescope to the dim and distant past.
The result was a new long-distance observing record. Astronomers announced in a series of papers over the fall and in a news conference last week that Hubble had recorded images of the earliest and most distant galaxies ever seen, blurry specks of light that burned brightly only 600 million to 800 million years after the Big Bang.The specks are clouds only one-twentieth the size of the Milky Way galaxy and only 1 percent of its mass, and seem to show the lingering effects of the first generation of stars to form in the universe in that they get bluer the farther back you go in time.
The new galaxies, along with other recent discoveries like the violent supernova explosion of a star only 620 million years after the Big Bang, take astronomers deep into a period of cosmic history known as the dark ages, which has been little explored. It was then that stars and galaxies were starting to light up vigorously in larger and larger numbers and that a fog of hydrogen that had enveloped space after the Big Bang fires had cooled mysteriously dissipated.
"These are the seeds of the great galaxies of today," said Garth Illingworth of the University of California, Santa Cruz, who discussed the new galaxies last week at a meeting of the American Astronomical Society in Washington. "We are pushing Hubble to the limit to find these objects."
Richard Ellis of the California Institute of Technology, one of many astronomers who have been working with the observations, said, "We're reaching the beginning where galaxies formed for the first time."
Dr. Illingworth and his colleague Richard Bouwens led a team that used Hubble's new Wide Field Camera 3, which was installed by the astronauts in May, to stare at a small patch of the southern sky over 62 orbits in what they call the Hubble Ultra Deep Field. The patch, known as the southern GOODS field, for Great Observatories Origins Deep Survey, has been observed by a variety of telescopes and satellites, including Hubble in 2004.
The release of Dr. Illingworth's observations in the fall led to a kind of gold rush in astronomy. In the last three months, several teams, using different ways to analyze the data, have produced 15 papers and articles about the new galaxies. Dr. Illingworth said in an interview that his team had identified 21 galaxies from 600 million to 800 million years after the Big Bang, and that other groups had found similar numbers.
The most distant, he said, was about 600 million years after the Big Bang. The universe is about 13.7 billion years old, cosmologists agree, meaning that the light from these galaxies has been on its way to us for 13 billion years.
In addition, some of the groups say they have identified possible galaxies as far back as 480 million years after the Big Bang, but they disagree on how many and which ones they are.
The new wide-field camera has an infrared capability, which makes it well suited for probing the early universe. As the universe expands, objects farther away from us go away faster, shifting their light to longer, redder wavelengths. The most distant galaxies appear to be emitting almost all of their light at even longer wavelengths, as invisible infrared, or heat, radiation. Indeed, the James Webb Space Telescope, being built for a 2014 launch to explore the very earliest years of creation, will be an entirely infrared telescope.
The galaxies are too far away and faint to be studied spectroscopically by even the largest telescopes on Earth, but by comparing their brightnesses in different infrared wavelength bands with optical images recorded by Hubble in 2004, astronomers could estimate how reddened the galaxies were. Some that showed up in the infrared images did not even appear in visible light.
Unlike the graceful spirals and grandly round ellipticals that populate today's universe, these baby galaxies are dumpy and irregular. Another clue that astronomers are getting close to the start of time is the blueness of the new Hubble galaxies when the effects of cosmic expansion are taken into account.
According to theoretical models, the first stars were born about 200 million years after the Big Bang, and consisted solely of hydrogen and helium. Lacking the elements to make dust, which reddens starlight, these stars would be bluer than those today. The colors of these galaxies, Dr. Illingworth said, suggested the presence of stars born only 300 million years after the Big Bang.
The new galaxies continue a recent trend in which the farther into the past astronomers look, the fewer and fainter and smaller galaxies they find, suggesting that the first billion years of history was a time in which galaxies and stars were rapidly increasing in number. The universe reached a peak in the birth rate of stars about 10 billion years ago, when it was a third of its present age.
Astronomers still do not know, however, if they will find enough galaxies and stars in that epoch when the universe was only half a billion years old to have burned off the hydrogen fog. That process is technically known as reionization, in which electrons are stripped from the hydrogen nuclei, making intergalactic space transparent.
More evidence that galaxies and massive stars were already going strong a few hundred million years after the Big Bang came last spring when NASA's Swift satellite detected gamma rays from an exploding star that was traced to a galaxy only 625 million years from the Big Bang.
Nial Tanvir of the University of Leicester and his colleagues called that blast "a glimpse of the end of the dark ages," suggesting that similar gamma ray bursts from that era could be used to measure the rate of star formation back then and figure out if stars were enough to reionize the universe.
Dr. Ellis said, "It does look as if galaxies could do the trick of causing reionization." It could be that the new Hubble galaxies were just the tip of the iceberg and that many more galaxies are lurking just below the threshold of detection. "The new camera," he said, "has revealed a bunch of little glowworms. The James Webb telescope will see the sky blazing with them."
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