In the last few years, observers have discovered huge quantities of interstellar dust near the most distant quasars in the very young universe, only 700 million years after the cosmos was born in the Big Bang.

"And that becomes a big question," said Oliver Krause of the University of Arizona Steward Observatory in Tucson and the Max Planck Institute for Astronomy in Heidelberg. "How could all of this dust have formed so quickly?"

Astronomers know two processes that form the dust, Krause said. One, old sun-like stars near death generate dust. Two, infrared space missions have revealed the dust is produced in supernovae explosions.

"The first process takes several billion years," Krause noted. "Supernovae explosions, by contrast, produce dust in much less time, only about 10 million years."

So when astronomers reported detecting submillimeter emission from massive amounts of cold interstellar dust in the supernova remnant Cassiopeia A last year, some considered the mystery solved.

Type II supernovae like 'Cas A' likely produced the interstellar dust in the very early universe, they concluded. (Type II supernovae come from massive stars that blow apart in huge explosions after their cores collapse.)

Krause and colleagues from UA's Steward Observatory and the Max Planck institute in Heidelberg have now discovered that the detected submillimeter emission comes not from the Cas A remnant itself but from the molecular cloud complex known to exist along the line of sight between Earth and Cas A. They report the work in the Dec. 2 issue of Nature.

Cas A is the youngest known supernova remnant in our Milky Way. It is about 11,000 light years away, behind the Perseus spiral arm clouds that are roughly 9,800 light years away.

Krause suspects that the Perseus clouds explain why late 17th century astronomers didn't report observing the brilliant Cas A outburst around A.D. 1680. Cas A is so close to Earth that the supernova should have been the brightest stellar object in the sky, but dust in the Perseus clouds eclipsed the view.

The Arizona and German team mapped Cas A at 160-micron wavelengths using the ultra-heat-sensitive Multiband Imaging Photometer (MIPS) aboard the Spitzer Space Telescope.

These long wavelengths are the most sensitive to cold interstellar dust emission. They then compared the results with maps of interstellar gas previously made with radio telescopes. They found that the dust in these interstellar clouds account for virtually all the emission at 160 microns from the direction of Cas A.

Minus the emission from this dust, there is no evidence for large amounts of cold dust in Cas A, the team concludes.

"Astronomers will have to go on searching for the source of the dust in the early universe," UA Steward Observatory astronomer and Regents' Professor George Rieke said. Rieke is principal investigator for the Spitzer Space Telescope's MIPS instrument and a co-author of the Nature paper.

"Solving this riddle will show astronomers where and how the first stars formed, or perhaps indicate there is some non-stellar process that can produce large amounts of dust," Rieke said.

"Either way, (finding the source of the dust) will reveal what went on at the formative stage for stars and galaxies, an epoch that is nearly unobserved in any other way."

Authors of the Nature article, "No cold dust within the supernova remnant Cassiopeia A," are Oliver Krause, Stephan M. Birkmann, George H. Rieke, Dietrich Lemke, Ulrich Klaas, Dean C. Hines and Karl D. Gordon.

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