[meteorite-list] NASA's Spitzer Space Telescope Exposes Dusty Galactic Hideouts

[Ron Baalke]

Dolores Beasley
Headquarters, Washington                       March 1, 2005
(Phone: 202/358-1743) 

Whitney Clavin
Jet Propulsion Laboratory, 
Pasadena, Calif.
(Phone: 818/354-4673)

RELEASE: 05-060

NASA'S SPITZER SPACE TELESCOPE EXPOSES DUSTY GALACTIC HIDEOUTS

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 the cosmic 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. It will be 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," 
he said.

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 a new 
breed of unusually dusty quasars, the most luminous objects in the 
universe, may be lurking inside. Quasars, like giant light bulbs at 
the centers of galaxies, are powered by huge black holes.

Another question astronomers would like to address is whether 
dusty, bright galaxies like these eventually evolve 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 onboard Spitzer called the 
multiband imaging photometer. The team 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 the 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. This particular 
dust grain is significant, because it is a planetary building block, and 
it also helped astronomers determine how far away the galaxies are from 
Earth. Silicates are sand-like planetary building blocks.

"This is the furthest back in time 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 
and director of the Spitzer Science Center, Pasadena, Calif. "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, or one-quarter of its present age of 
13.5 billion years. Galaxies similar to these in dustiness, but much 
closer to Earth, were first alluded to 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. It took Spitzer's improved sensitivity, 
100 times greater than past missions, to finally seek out the dusty 
galaxies at great distances.

The National Optical Astronomy Observatory Deep Wide-Field 
Survey used the National Science Foundation's 13-foot telescope at Kitt 
Peak National Observatory southwest of Tucson, Ariz.

NASA's Jet Propulsion Laboratory (JPL), Pasadena, Calif., manages the 
Spitzer Space Telescope mission for NASA's Science Mission Directorate, 
Washington. Science operations are conducted at the Spitzer Science 
Center. JPL is a division of Caltech.

Artist's conceptions, images and additional information about the Spitzer 
Space Telescope are available at:

http://www.spitzer.caltech.edu.

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