A new study of the most distant galaxies reveals that many of them were much brighter than expected, a finding with implications for our understanding of the early history of the universe.
Researchers used NASA's Spitzer Space Telescope to observe some of the first galaxies to form in the universe, less than 1 billion years after the Big Bang (or a little more than 13 billion years ago). The data show that in a few specific wavelengths of infrared light, the galaxies are considerably brighter than anticipated. The new study is the first to confirm this observation for a large sampling of galaxies from this period, demonstrating that these were not special cases of unusual brightness, but that even average galaxies present at that time were much brighter in these wavelengths than galaxies we see today.
The results indicate that these early galaxies were releasing high amounts of ionizing radiation, which may have contributed to a major event known as the "epoch of reionization," when the universe went from being mostly opaque to mostly transparent.
Garth Illingworth, professor emeritus of astronomy and astrophysics at UC Santa Cruz and a member of the team, said the new study provides insight into a select set of galaxies that can be studied with the Spitzer and Hubble Space Telescopes, revealing just how unusual these early galaxies were.
"For over ten years, we have been studying with Hubble and Spitzer some of the earliest and most distant galaxies known," Illingworth said. "Our latest Spitzer result reveals how different these early galaxies are to those at later times and pinpoints our sample as a key set for providing insights into how galaxies so efficiently reionized the universe."
Evidence suggests that between about 100 million and 200 million years after the Big Bang, the universe was filled mostly with neutral hydrogen gas that had perhaps just begun to coalesce into stars, which then began to form the first galaxies. By about 1 billion years after the Big Bang, the universe had become a sparkling firmament. Something else had changed, too: The electrons of the omnipresent neutral hydrogen gas had been stripped away, in a process known as ionization.
Cosmic transformation
Before this cosmic transformation took place, long-wavelength forms of light, such as radio waves and visible light, traversed the universe more or less unencumbered. But shorter wavelengths of light, including ultraviolet light, x-rays and gamma rays, were stopped short by neutral hydrogen atoms. At the same time, these collisions would strip those neutral hydrogen atoms of their electrons, ionizing them.
But what could have produced enough ionizing radiation to affect all the hydrogen in the universe? Was it individual stars? Giant galaxies? If either of these were the culprit, they would have been different than most modern stars and galaxies, which typically don't release high amounts of ionizing radiation. Then again, perhaps the event was caused by something else entirely, such as quasars.
"It's one of the biggest open questions in observational cosmology," said Stephane de Barros, lead author of the study and a postdoctoral researcher at the University of Geneva in Switzerland. "We know it happened, but what caused it? These new findings could be a big clue."
To peer back in time to the era just before the epoch of reionization ended, Spitzer stared at two regions of the sky for more than 200 hours each, allowing the space telescope to collect light that had traveled for more than 13 billion years to reach us. These are some of the longest science observations ever carried out by Spitzer, and were part of an observing campaign called GREATS, short for GOODS Re-ionization Era wide-Area Treasury from Spitzer. GOODS (itself an acronym: Great Observatories Origins Deep Survey) is another campaign that did the first observations of some of the GREATS targets. The study, published in the Monthly Notices of the Royal Astronomical Society, also used archival data from NASA's Hubble Space Telescope.
Ultra-deep observations
Using these ultra-deep observations by Spitzer, the team of astronomers observed 135 of those distant galaxies and found that they were all particularly bright in two specific wavelengths of infrared light produced by ionizing radiation interacting with hydrogen and oxygen gases within the galaxies. This implies that these galaxies were dominated by young, massive stars composed mostly of hydrogen and helium. They contain very small amounts of "heavy" elements (like nitrogen, carbon, and oxygen) compared to stars found in average modern galaxies.
"We did not expect that Spitzer, with a mirror no larger than a hula hoop, would be capable of seeing galaxies so close to the dawn of time," said Michael Werner, Spitzer's project scientist at NASA's Jet Propulsion Laboratory in Pasadena, California. "But nature is full of surprises, and the unexpected brightness of these early galaxies, together with Spitzer’s superb performance, puts them within range of our small but powerful observatory."
NASA's James Webb Space Telescope will study the universe in many of the same wavelengths currently observed by Spitzer. But where Spitzer's primary mirror is only 85 centimeters (33.4 inches) in diameter, Webb's primary mirror is 6.5 meters (21 feet), or about 7.5 times larger, enabling Webb to study these galaxies in far greater detail. In fact, Webb will try to detect light from the first stars and galaxies in the universe. The new study shows that, due to their brightness in those infrared wavelengths, the galaxies observed by Spitzer will be much easier for Webb to study than previously thought.
"These results by Spitzer are certainly another step in solving the mystery of cosmic reionization," said Pascal Oesch, an assistant professor at the University of Geneva and a coauthor of the paper. "We now know that the physical conditions in these early galaxies were very different than in typical galaxies today. It will be the job of the James Webb Space Telescope to work out the detailed reasons why."
The Jet Propulsion Laboratory in Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate in Washington. Science operations are conducted at the Spitzer Science Center at Caltech in Pasadena. Space operations are based at Lockheed Martin Space Systems in Littleton, Colorado. Data are archived at the Infrared Science Archive housed at IPAC at Caltech. Caltech manages JPL for NASA.
