Their distance from Earth is about 11 billion light years, or 80 percent of the way back to the Big Bang.

Virtually everything about this new class of objects is educated speculation, the researchers say, since the galaxies are invisible to ground-based optical telescopes with the deepest reach into the universe.

"We think we have an idea of what they are, but we are not necessarily correct," says Cornell senior research associate in astronomy Dan Weedman.

Among the more probable ideas are that these mysterious bodies are ultraluminous infrared galaxies, powered either by an active galactic nuclei (AGN) or by a starburst, a massive burst of star formation.

AGNs are powered by the in-fall of matter to a massive black hole, while massive starbursts often are triggered by the collision of two or more galaxies.

What makes the objects studied by the Spitzer team stand out is that previously known AGNs are "not nearly as powerful, far away, or as dust-enshrouded" as these bodies are, says Weedman.

The Cornell Spitzer team's discovery is published in the March 1 issue of the Astrophysical Journal Letters (ApJL), published by the American Astronomical Society. The Spitzer telescope, which went into an Earth-trailing orbit around the sun in August 2003, is the last of NASA's Great Observatories, the Hubble being the first.

The IRS team used data obtained by the National Science Foundation's telescopes at Kitt Peak National Observatory, for the National Optical Astronomy Observatory (NOAO) Deep Wide-Field Survey.

The team also used a catalog of infrared sources obtained in a survey in early 2004 by another of the Spitzer telescope's instruments, the Multiband Imaging Photometer for Spitzer (MIPS).

From the thousands of MIPS sources in a three-degree square patch of the sky -- about one-fourth the size of the bowl of the Big Dipper - in the constellation Bootes the Herdsman, the IRS team selected and observed 31 that are quite bright in the infrared but invisible in the NOAO survey.

"The NOAO Deep Wide-Field Survey is the best available optical survey for comparing to our data," Weedman says. "It would have been much more difficult to make this discovery without such a wide area of comparison. These NOAO data allowed us to compare the sky at infrared and optical wavelengths and find things that had never been seen before."

The Bootes area was chosen by the NOAO team because of the absence of obscuring dust in our galaxy, presenting a clear view of the distant sky. The presence of these mysterious, infrared, bright, but optically invisible, objects was first hinted at in 1983 in a paper by James Houck, Cornell's Kenneth A. Wallace Professor of Astronomy and principal investigator for the IRS.

Houck was interpreting data from another space probe he was involved with, the Infrared Astronomical Satellite (IRAS), the first astronomy mission devoted to searching the heavens for infrared sources. More than a decade later these strange objects were again recorded by the European Space Agency's Infrared Space Observatory.

"Spitzer is more than 100 times more sensitive than IRAS for detecting objects at infrared wavelengths," says Houck.

"These celestial bodies are so far from our Milky Way galaxy that we detect them as they were when the universe was just 20 percent of its current age," says Sarah Higdon, a research associate in Cornell's Department of Astronomy, who led the group that developed the software package for analyzing Spitzer data.

In addition to their incredible distance, these objects also are enshrouded by a great deal of dust, which Cornell astronomy research associate Jim Higdon describes as being "the size of smoke particles made of silicates."

Other authors of the ApJL paper are: from Cornell, Terry Herter and Vassilis Charmandaris; from the Spitzer Space Science Center, L. Armus, H.I. Teplitz and B.T. Soifer; from NOAO, M.J.I Brown (now at Princeton University), A. Dey and B.T. Jannuzi; from Steward Observatory, University of Arizona, E. Le Floc'h and M. Rieke; and from Leiden Observatory, Holland, Bernhard Brandl.

The IRS, the most sensitive infrared spectrograph to be sent into space, is a collaborative venture between Cornell and Ball Aerospace and funded by NASA through the Jet Propulsion Laboratory (JPL) and Ames Research Center. JPL manages the Spitzer Space Telescope for NASA.

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