An international team led by astronomers at UC Santa Cruz has discovered and characterized four surprisingly bright galaxies that are among the earliest and most distant galaxies ever observed. Based on image data gathered by NASA's Hubble Space Telescope and Spitzer Space Telescope, the results show the galaxies as they appeared more than 13 billion years ago, just 500 million years after the big bang.
Although Hubble has previously identified galaxies at this early epoch, astronomers were surprised to find objects that are about 10 to 20 times more luminous than anything seen previously at this distance. "These just stuck out like a sore thumb because they are far brighter than we anticipated," explained Garth Illingworth, professor of astronomy and astrophysics at UC Santa Cruz.
Illingworth presented the team's findings this week at the American Astronomical Society meeting in Washington, D.C. "There are strange things happening regardless of what these sources are. We're suddenly seeing luminous, massive galaxies quickly build up at such an early time. This was quite unexpected," he said.
The tiny galaxies are bursting with star formation activity, which accounts for their brilliance. The brightest one is forming stars approximately 50 times faster than the Milky Way does today. Although these fledgling galaxies are only one-twentieth the size of the Milky Way, they probably contain around a billion stars crammed together.
The astronomers were puzzled to find the four galaxies appearing in one of two deep survey fields, more than have been found in the other previously studied field that is in the opposite direction on the sky. "It was amazing to find four very luminous galaxies in one area in the early universe. They would have needed to grow very fast," said coauthor Rychard Bouwens, now at Leiden University in the Netherlands after a decade as a researcher at UC Santa Cruz.
The galaxies were first detected with Hubble, whose sharp images are crucial to finding such distant galaxies and enabled the astronomers to measure their star-formation rates and sizes. Using Spitzer, the astronomers were able to estimate the stellar masses by measuring the total stellar luminosity of the galaxies.
"This is the first time scientists were able to measure an object's mass at such a huge distance," said Pascal Oesch, who worked on the study as a Hubble Postdoctoral Fellow at UC Santa Cruz and is now at Yale University. Oesch is first author of a paper on the findings that is available online at arXiv.org. "It's a fabulous demonstration of the synergy between Hubble and Spitzer," he said.
According to Illingworth, these are the first such distant galaxies that are bright enough to be observable using powerful ground-based telescopes such as the Keck Telescopes in Hawaii, which would provide valuable spectroscopic data. "It will be a challenge, because it's at the limit of what we can do with Keck, but it is now possible for the first time ever," Illingworth said.
Coauthor Ivo Labbé of Leiden University noted that "the extreme masses and star formation rates are really mysterious, and we are eager to confirm them with future observations on our powerful telescopes."
These bright, young galaxies are thought to have grown from interactions and mergers of smaller infant galaxies that started forming stars even earlier in the universe. Since the ancient time billions of years ago when the light now reaching the telescopes started its long journey across the universe, they have probably kept growing to become similar to the largest modern galaxies. Many of the stars of these infant galaxies likely live on today in the centers of giant elliptical galaxies, much larger even than our own Milky Way.
This result is very encouraging for NASA's upcoming James Webb Space Telescope. As scientists look back to even earlier times, they should find lots of young, growing galaxies that can be studied in detail with the Webb Telescope. If galaxies like this existed a few hundred million years after the big bang, the Webb telescope should be able to see them easily.