Astronomers have discovered five full-sized galaxies in the extremely distant--and therefore extremely young--universe. The galaxies, which are forming stars very rapidly, are big for their age, meaning that astronomers might have to rethink current ideas about galaxy formation.
The universe is 13.7 billion years old, and its first 500 million years or so were dark because the first stars had not yet ignited their nuclear furnaces. Then, as their light began to permeate the cosmos, the first stars began to coalesce into galaxies, which merged into bigger and bigger versions of themselves. But this process was supposed to take billions of years.
Now astronomers have spotted five galaxies at least as big as the Milky Way that existed 12 billion years ago, or just a little more than a billion years after the first stars formed. An international team used a battery of instruments--including the Hubble and Spitzer Space telescopes for optical and infrared light, and the Submillimeter Array on Mauna Kea, Hawaii, for radio astronomy--to locate the five and confirm that they were individual galaxies and not just clusters of smaller galaxies. As the team reports online this week in The Astrophysical Journal, the young galaxies are quite large and are forming new stars about 1000 times faster than does the Milky Way, but dense clouds of dust shroud their brightness.
"We have no idea why these galaxies grew so large so soon," says astronomer and co-author Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. He says the team has turned the data over to theorists, who are developing new computer models to try to explain the findings. "I think we still have a lot new to learn about what's happening in the early universe," Fazio says.
Astrophysicist Edwin Turner of Princeton University thinks the implications are "potentially quite important" for understanding how dust clouds can mask the true star-formation rate of the early universe. Piercing the dust will help astronomers study the process of galaxy formation more precisely, he says. Astronomer Chris Mihos of Case Western Reserve University in Cleveland, Ohio, agrees. Although he thinks it might be too soon to reach any general conclusions based on a sample as small as five, the newly found galaxies represent a solid contribution to the census of the early universe. "What we'd really like is to know how common galaxies of different masses were at different ages of the universe," he says. "It's a very hard thing to measure, but it's critical to understanding galaxy formation."