How do you hide something as big and bright as a galaxy? You smother it in cosmic dust. NASA's Spitzer Space Telescope saw through such dust to uncover a hidden population of monstrously bright galaxies approximately 11 billion light-years away.
These strange galaxies are among the most luminous in the universe, shining with the equivalent light of 10 trillion suns. But, they are so far away and so drenched in dust, it took Spitzer's highly sensitive infrared eyes to find them.
"We are seeing galaxies that are essentially invisible," said Dr. Dan Weedman of Cornell University, Ithaca, N.Y., co-author of the study detailing the discovery, published in today's issue of the Astrophysical Journal Letters. "Past infrared missions hinted at the presence of similarly dusty galaxies over 20 years ago, but those galaxies were closer. We had to wait for Spitzer to peer far enough into the distant universe to find these."
Where is all this dust coming from? The answer is not quite clear. Dust is churned out by stars, but it is not known how the dust wound up sprinkled all around the galaxies. Another mystery is the exceptional brightness of the galaxies. Astronomers speculate that a new breed of unusually dusty quasars, the most luminous objects in the universe, may be lurking inside. Quasars are like giant light bulbs at the centers of galaxies, powered by huge black holes.
Astronomers would also like to determine whether dusty, bright galaxies like these eventually evolved into fainter, less murky ones like our own Milky Way. "It's possible stars like our Sun grew up in dustier, brighter neighborhoods, but we really don't know. By studying these galaxies, we'll get a better idea of our own galaxy's history," said Cornell's Dr. James Houck, lead author of the study.
The Cornell-led team first scanned a portion of the night sky for signs of invisible galaxies using an instrument on Spitzer called the multiband imaging photometer. The team then compared the thousands of galaxies seen in this infrared data to the deepest available ground-based optical images of the same region, obtained by the National Optical Astronomy Observatory Deep Wide-Field Survey. This led to identification of 31 galaxies that can be seen only by Spitzer. "This large area took us many months to survey from the ground," said Dr. Buell Jannuzi, co-principal investigator for the Deep Wide-Field Survey, "so the dusty galaxies Spitzer found truly are needles in a cosmic haystack."
Further observations using Spitzer's infrared spectrograph revealed the presence of silicate dust in 17 of these 31 galaxies. Silicate dust grains are planetary building blocks like sand, only smaller. This is the furthest back in time that silicate dust has been detected around a galaxy. "Finding silicate dust at this very early epoch is important for understanding when planetary systems like our own arose in the evolution of galaxies," said Dr. Thomas Soifer, study co-author, director of the Spitzer Science Center, Pasadena, Calif., and professor of physics at the California Institute of Technology, also in Pasadena.
This silicate dust also helped astronomers determine how far away the galaxies are from Earth. "We can break apart the light from a distant galaxy using a spectrograph, but only if we see a recognizable signature from a mineral like silicate, can we figure out the distance to that galaxy," Soifer said.
In this case, the galaxies were dated back to a time when the universe was only three billion years old, less than one-quarter of its present age of 13.5 billion years. Galaxies similar to these in dustiness, but much closer to Earth, were first hinted at in 1983 via observations made by the joint NASA-European Infrared Astronomical Satellite. Later, the European Space Agency's Infrared Space Observatory faintly recorded comparable, nearby objects. Spitzer's improved sensitivity, 100 times greater than past missions, has allowed the telescope to seek out a variety of dusty galaxies at great distances, including this recent batch of exceptionally dusty and bright ones.
The National Optical Astronomy Observatory Deep Wide-Field Survey used the National Science Foundation's 4-meter (13-foot) telescope at Kitt Peak National Observatory, located southwest of Tucson, Ariz.
NASA's Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington, D.C. Science operations are conducted at the Spitzer Science Center. JPL is a division of Caltech. The infrared spectrograph was built by Ball Aerospace Corporation, Boulder, Colo., and Cornell; its development was led by Houck. The multiband imaging photometer was built by Ball Aerospace Corporation, the University of Arizona, Tucson, Ariz., and Boeing North American, Canoga Park, Calif.; its development was led by Dr. George Rieke of the University of Arizona.
The Infrared Astronomical Satellite was a joint effort between NASA, the Science and Engineering Research Council, United Kingdom and the Netherlands Agency for Aerospace Programmes, the Netherlands.
Shimmering in Infrared Light:
This artist's conception shows what a dusty and bright galaxy located billions of light-years away might look like close up if viewed in infrared light. Galaxies like these are so far away and so drenched in dust, they appear invisible to optical telescopes.
NASA's Spitzer Space Telescope uncovered a hidden population of dusty galaxies like these using its heat-seeking infrared eyes. The galaxies are among the brightest in the universe and are located 11 billion light-years away back to a time when the universe was 3 billion years old. The universe is currently believed to be 13.5 billion years old.
Astronomers are not sure what is lighting up these cosmic behemoths, but they speculate that quasars -- the most luminous objects in the universe -- may be lurking inside.
Fingerprints in the Light:
This graph, or spectrum, shows the light from a dusty, distant galaxy located 11 billion light-years away. The galaxy is invisible to optical telescopes, but NASA's Spitzer Space Telescope was able to capture the light from it and dozens of other similar galaxies using heat-seeking infrared eyes.
Spectra are created when an instrument called a spectrograph spreads light out into its basic parts, like a prism turning sunlight into a rainbow. They contain the signatures, or "fingerprints," of molecules that contribute to an object's light.
In this case, the galaxy's spectrum reveals the fingerprint for silicate dust (large dip at right), a planetary building block like sand, only smaller. This particular fingerprint is important because it helped astronomers determine how far away the galaxy lies, or more specifically, how much the galaxy's light had stretched, or "redshifted," during its journey to Spitzer's eyes. Because the universe is expanding, a galaxy's light will shift toward reddish wavelengths as it moves away from us. This galaxy was found to have a redshift of 1.95, which means that its light took about 11 billion years to get here.
The presence of the silicate fingerprint is also significant because it implies that galaxies were ripe for planetary formation 11 billion years ago -- back to a time when the universe was 3 billion years old. The universe is currently believed to be 13.5 billion years old. This is the furthest back in time that silicate dust has been detected around a galaxy.
These data were taken by Spitzer's infrared spectrograph in July, 2004.
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