June 5, 2001 A pair of young astronomers has found a bumper crop of "infant" galaxies that may help scientists develop new insights into the beginnings of galaxy formation.
Reporting at this week's meeting of the American Astronomical Society in Pasadena, Calif., postdoctoral fellows Sangeeta Malhotra and James Rhoads will present what is by far the largest finding to date of very distant and highly energetic young galaxies. They found 150 in a full-moon-sized patch of sky.
"It's great to have found such a large number of these galaxies, because it's going to be harder to shove them under the rug," says Malhotra, a Hubble fellow in The Krieger School of Arts and Sciences at the Johns Hopkins University. "We're going to have to think harder about what's happening in galaxy formation and star formation at these times, early in the history of the universe."
Among the key questions, according to Malhotra, will be what makes the young galaxies so bright: intense bursts of star formation, the activity of a massive black hole at the center of the galaxy or something else?
"In about half of these galaxies, our models of how stars behave can't explain the strength of a characteristic spectral line we use to identify the galaxies," she says.
For their survey, Malhotra and Rhoads used a new instrument known as the Mosaic CCD camera on the 4-meter Mayall Telescope at Kitt Peak National Observatory, which is located near Tucson, Ariz. The camera, one of the first such instruments on a large telescope, enabled them to image distant, faint sources of radiation over a relatively wide patch of sky. They took their data from an area in the constellation Bootes, in the direction of the star Arcturus.
This field was chosen because there is already an extensive complementary survey there, led by Buell Jannuzi and Arjun Dey of the National Optical Astronomy Observatory. "This is a location with nothing much in the nearby universe, so we have a clean line of sight to the very distant universe," says Rhoads, who is an institute fellow at the Hubble Space Telescope Science Institute in Baltimore.
Malhotra and Rhoads used specially designed filters to help screen the 40,000 objects their survey detected for likely candidates for young galaxies.
"We were looking for a spectral signature known as a Lyman-alpha line that is created by ionized hydrogen," Malhotra says. "It suggests very energetic activity, and it's a signal we don't see in present-day galaxies."
When they applied a filter designed to pick up that characteristic component of the light, and compared the resulting images, objects that appeared brighter in the filtered image were moved up on the list of candidates for distant, young galaxies.
"With the aid of follow-up spectroscopic observations at the Keck telescope in Hawaii, our collaborators Arjun Dey, Daniel Stern, and Hy Spinrad confirmed that these galaxies are about 10 billion light years away, at a redshift of 4.5," says Malhotra. "Light we see now from those galaxies left them when the universe was barely one-tenth of its present age."
Rhoads and Malhotra are planning additional surveys in the near future. "We are designing filters to go to even younger times," says Rhoads.
"We hope we're at the beginning of building up a complete picture of what's going on in the universe at this early age," says Malhotra. "As we collect more and more of these galaxies, we'll get a better sense of what's usual and what's unusual. Follow-up observations in X-rays and infrared radiation will further help us understand the nature of these objects."
This survey was funded by NASA and by Kitt Peak National Observatory. Kitt Peak National Observatory is part of the National Optical Astronomy Observatory (NOAO), which is operated by the Association of Universities for Research in Astronomy Inc. (AURA) under cooperative agreement with the National Science Foundation. The W.M. Keck Observatory is operated as a scientific partnership among the University of California, The California Institute of Technology, and NASA. The observatory was made possible by the financial support of the W.M. Keck Foundation.
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