Analysis of data from the National Science Foundation’s South Pole Telescope, for the first time, more precisely defines the period of cosmological evolution when the first stars and galaxies formed and gradually illuminated the universe. The data indicate that this period, called the epoch of reionization, was shorter than theorists speculated — and that it ended early.
“We find that the epoch of reionization lasted less than 500 million years and began when the universe was at least 250 million years old,” said Oliver Zahn, a postdoctoral fellow at the Berkeley Center for Cosmological Physics at the University of California, Berkeley, who led the study. “Before this measurement, scientists believed that reionization lasted 750 million years or longer, and had no evidence as to when reionization began.”
The findings by Zahn, his colleagues at UChicago’s Kavli Institute for Cosmological Physics and elsewhere have been published in a pair of papers appearing in the Sept. 1, 2012 issue of the Astrophysical Journal. Their latest results are based on a new analysis that combines measurements taken by the South Pole Telescope at three frequencies, and extends these measurements to a larger area covering approximately 2 percent of the sky. The 10-meter South Pole Telescope operates at millimeter wavelengths to make high-resolution images of the cosmic microwave background (CMB), the light left over from the big bang.
“Studying the epoch of reionization is important because it represents one of the few ways by which we can study the first stars and galaxies,” said study co-author John Carlstrom, the S. Chandrasekhar Distinguished Service Professor in Astronomy & Astrophysics.
Before the first stars formed, most matter in the universe took the form of neutral hydrogen atoms. The radiation from the first stars transformed the neutral gas into an electron-proton plasma. Observations with the Wilkinson Microwave Anisotropy Probe satellite of polarized signals in the CMB indicate that this epoch occurred nearly 13 billion years ago, but these observations give no indication of when the epoch began or how long it lasted.
The first stars that formed were probably 30 to 300 times more massive than the sun and millions of times as bright, burning for only a few million years before exploding. The energetic ultraviolet light from these stars was capable of splitting hydrogen atoms back into electrons and protons, thus ionizing them.
Scientists believe that during reionization, the first galaxies to form ionized “bubbles” in the neutral gas surrounding them. Electrons in these bubbles would scatter with light particles from the cosmic microwave background. This would create small hot and cold spots in the CMB depending on whether a bubble is moving toward or away from Earth. A longer epoch of reionization would create more bubbles, leading to a larger signal in the CMB.
The epoch’s short duration indicates that reionization was more explosive than scientists had previously thought. It suggests that massive galaxies played a key role in reionization, because smaller galaxies would have formed much earlier. Rapid reionization also argues against many proposed astrophysical phenomena that would slow the process.
This is only the beginning of what astronomers expect to learn about reionization from the South Pole Telescope. The current results are based on only the first third of the telescope’s full survey. Additional work is under way to combine South Pole Telescope maps with ones made by the Herschel satellite to further increase sensitivity to the reionization signal.
“We expect to measure the duration of reionization to within 50 million years with the current survey,” said study co-author Christian Reichardt, a Berkeley astrophysicist. “With planned upgrades to the instrument, we hope to improve this even further in the next five years.”
The 280-ton South Pole Telescope stands 75 feet tall and is the largest astronomical telescope ever built in Antarctica’s clear, dry air. Sited at the National Science Foundation’s Amundsen-Scott South Pole station at the geographic South Pole, it stands at an elevation of 9,300 feet on the polar plateau. Because of its location at the Earth’s axis, it can conduct long-term observations of a single patch of sky.
UChicago leads the South Pole Telescope collaboration, which includes research groups from Argonne National Laboratory, Cardiff University, Case Western Reserve University, Harvard University, Ludwig-Maximilians-Universitat, Smithsonian Astrophysical Observatory, McGill University, University of California at Berkeley, University of California at Davis, University of Colorado at Boulder, University of Michigan and individual scientists at several other institutions.
The South Pole Telescope is primarily funded by the NSF’s Office of Polar Programs. Partial support also is provided by the NSF-funded Physics Frontier Center of UChicago’s Kavli Institute for Cosmological Physics, the Kavli Foundation, and the Gordon and Betty Moore Foundation.
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