Astronomers Anticipate Clues to Galactic Mystery

ITHACA, N.Y., April 11 -- With the recent delivery of the telescope and scientific instruments for the Space Infrared Telescope Facility (SIRTF) -- the last of NASA's four great observatories -- to Lockheed Martin Space Systems in Sunnyvale, Calif., for assembly and testing, astronomers at Cornell University are eagerly anticipating the almost certain discoveries that the new telescope will make beginning early next year.

Some of the most exciting discoveries are likely to come from the first, close-up infrared look at galaxies formed in the early universe -- relatively nearby blue compact dwarf galaxies. Ranging in distance from 3 to 30 million light years and composed largely of helium and hydrogen, these dwarf galaxies are so deficient in heavy elements that they contain as little as 2 percent of the solar system's share of heavy elements.

"This will be an opportunity to measure the infrared properties of galaxies that might be similar to the galaxies formed soon after the Big Bang," says James Houck, principal investigator on the infrared spectrograph (IRS), one of the three instruments to be carried aboard the SIRTF when it is launched Jan. 9, 2003.

In the optical part of the spectrum, these dwarf galaxies are characterized by blue colors, a high surface brightness and narrow emission lines -- all the hallmarks of recent star formation but only a hint of what infrared observation might reveal.

A burst of star formation in one of the galaxies has been observed by the European Space Agency's Infrared Space Observatory. Yet, says Houck, an astronomy professor at Cornell, "The amount of power coming out of the optically observed star clusters is only about a third of what is required to account for the total infrared-plus-optical radiation we see from the galaxy."

Thus, he speculates, there must be at least two or three times as many stars forming as can be seen with optical telescopes.

"There is a strong indication that many of these galaxies contain dust which might be obscuring the visible light," he says.

The view through the optically opaque but infrared-transparent dust will be provided by SIRTF, and in particular the IRS, which is used to detect infrared radiation that lies between the visible and microwave portions of the electromagnetic spectrum. All objects with temperatures significantly above 0 degrees Kelvin, or absolute zero (-459.67 degrees Fahrenheit or -273.15 degrees Celsius), radiate in the infrared. Objects with temperatures between 100 and 1,000 Kelvin (-173 to 727 degrees Celsius) emit the bulk of their radiation in the infrared, while much hotter objects, such as the sun, emit mostly in the optical.

The IR spectrograph, says Houck, "will be the best instrument on board the observatory for the job of providing detailed physical information about distant targets."

A spectrometer spreads light -- or infrared radiation, in the case of the IRS -- into its constituent wavelengths, creating spectra within which emission and absorption lines can be studied. For astronomers, these lines are the fingerprints of atoms and molecules. The two other instruments on SIRTF, a multiband imaging photometer and an infrared array camera, will be concerned mainly with discovering new targets.

The three instruments, encased in the observatory's cryogenic telescope assembly -- also containing the telescope and the liquid helium cooling tank -- are being integrated with the spacecraft and tested at Lockheed Martin. The initial tests of the IRS recently show it to be as sensitive as expected, says Houck. Ball Aerospace built the cryogenic telescope assembly and two of the three instruments, the IRS and the multiband imaging photometer.

SIRTF is the last of NASA's four large orbiting observatories, the first of which was the Hubble Space Telescope, but is also the first mission of the space agency's Origins Program, aimed at studying the formation of galaxies, as well as stars and planets, to provide basic information about the origins of life in the universe.

During the five-year life of the approximately half-billion-dollar SIRTF mission, says Houck, a new view of the heavens is likely to be opened up. "This telescope will be 100 to 1,000 times more sensitive than any previous infrared instrument," he notes.

Houck heads the scientific team on the $39 million IRS contract with NASA's Jet Propulsion Laboratory in Pasadena, Calif., which is managing the SIRTF mission. Although the mission will be largely tracked at the laboratory, the flood of IRS data will be mined at a new science analysis center at Cornell.

The IRS team has 34 different observing projects, from quasars to the giant planets and their satellites. Two dozen researchers from Cornell and around the US are involved.