PULLMAN, Wash. – Hundreds of galaxies dating back to the time of the Big Bang have been discovered through an analysis of the two deepest views of the cosmos ever taken by NASA’s Hubble Space Telescope. The research was performed by a team of four astronomers that included John Blakeslee, an assistant professor with the Department of Physics and Astronomy at Washington State University.
The researchers report finding some 500 galaxies that existed less than a billion years after the Big Bang – a time when the cosmos was less than seven percent of its present age of 13.7 billion years. Their findings constitute the most comprehensive compilation of galaxies in the early universe.
The discovery is considered a significant leap forward in developing an understanding of the origin of galaxies, given that little was known of early galaxy formation just a decade ago, when astronomers had not seen even one galaxy dating back to the first billion years of the history of
the universe.
The early universe galaxies are smaller than today’s giant galaxies and quite bluish in color, indicating they are ablaze with star birth. They appear red in the Hubble images because of their tremendous distance from Earth. The blue light from the galaxies’ young stars took nearly 13 billion years to reach Earth. During the long journey, their shorter wavelength blue light shifted to longer wavelength red light due to the expansion of space.
“Finding so many of these dwarf galaxies, but so few bright ones, is evidence for galaxies building up from small pieces – merging together as predicted by the hierarchical theory of galaxy formation,” said Rychard Bouwens, an astronomer with the University of California, Santa Cruz, who led the Hubble study.
The researchers discovered the early galaxies in an analysis of the Hubble Ultra Deep Field, a patch of sky observed in unprecedented depth by Hubble in 2004, and the Great Observatories Origins Deep Survey, begun in 2003.
Their results are scheduled for publication in the Astrophysical Journal.
The astronomical data used in the study came from an instrument called the Advanced Camera for Surveys (or ACS) on board the Hubble Space Telescope. Blakeslee was part of a large team of scientists and engineers that developed the ACS and he wrote much of the software used to process the images used in identifying the early galaxies.
“Since its installation on Hubble in March 2002, the ACS instrument has been giving us spectacular views of the universe, from the most distant galaxies to nearby stars forming in our own Milky Way galaxy, and even the familiar planets within our solar system,” Blakeslee said. “Finding so many primordial galaxies in one study demonstrates the combined power of Hubble and the ACS. Nothing else in space or on Earth compares to it.”
The researchers’ findings show that early dwarf galaxies were producing stars at a furious rate, about ten times faster than is happening now in nearby galaxies. Astronomers have long debated whether the hottest stars in early star-forming galaxies, such as those in this study, may have provided enough radiation to reheat the cold hydrogen gas that existed between galaxies in the early universe. The gas had been cooling since the Big Bang.
“Seeing all of these starburst galaxies provides evidence that there were enough galaxies one billion years after the Big Bang to finish reheating the universe,” said team member Garth Illingworth of the University of California, Santa Cruz. “It highlights a period of fundamental change in the universe, and we are seeing the galaxy population that brought about that change.”
Because the evolution of galaxies and stars occurs over billions of years, astronomers rarely witness dramatic, relatively brief transitions that changed the universe. One such event was the universe’s “reheating.”
Driven by the galaxies’ ultraviolet starlight, the reheating transformed the gas between galaxies from a cold, dark hydrogen soup to a hot, transparent plasma over only a few hundred million years. With the aid of Hubble and the ACS, astronomers are now beginning to see the kinds of galaxies that brought about that reheating.
“This research provides some answers to questions about the earliest stages of galaxy formation, but it also hints at how much more we have to learn,” Blakeslee said. “Five hundred primordial galaxies may seem like a lot, but those were found over a tiny fraction of the sky. There are likely billions more out there, even at these great distances and early cosmic times. We need bigger, more representative, samples to truly understand the formation and evolution of galaxies in the universe.”
For electronic images and additional information about this research on the Web, visit http://hubblesite.org/news/2006/12 or http://www.spacetelescope.org/news/html/heic0603.html
