CU astronomers anticipate benefits of NASA's new orbiting telescope

The Space Infrared Telescope Facility (SIRTF), an orbiting observatory, undergoes testing in a clean room at Lockheed-Martin in Sunnyvale, Calif., recently. The planned launch date is Jan. 9, 2003. Tim Maloney/Lockheed-Martin

By David Brand

With the recent delivery of the telescope and scientific instruments for the last of NASA's four Great Observatories to Lockheed-Martin in Sunnyvale, Calif., for assembly and testing, astronomers at Cornell 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," said Cornell's James Houck, principal investigator on the infrared spectrograph (IRS), one of the three instruments to be carried aboard the Space Infrared Telescope Facility (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 (ISO). Yet, said Houck, the K.A. Wallace Professor of Astronomy at Cornell, "The amount of power coming out of the optically observed star clusters is only about one-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 that might be obscuring the visible light," he said.

The view through the optically opaque but infrared-transparent dust will be supplied 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 IRS, said 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 in recent weeks show it to be as sensitive as expected, said Houck.

SIRTF is the last of NASA's four large orbiting observatories, the first of which was the Hubble Space Telescope, but it also is 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 $500 million SIRTF mission, said 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 noted.

Houck heads the scientific team on the $39 million IRS contract with the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., which is managing the SIRTF mission. Although the mission will be largely tracked at JPL, the flood of IRS data will be mined at Cornell, where senior researcher Bernhard Brandl is readying a science analysis center in the Space Sciences Building on campus. 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 United States are involved, including Terry Herter, Cornell professor of astronomy, and former Cornell graduate students Tom Roellig and Tom Soifer.

In the first two years of SIRTF's expected life span, the Cornell team is guaranteed 875 hours of observing time, beginning three months after launch. "The sensitivity of SIRTF is such that five minutes of observing time will achieve the same sensitivity as a year of observing on any previous infrared spectrometer, ground-based or in space," said Houck.

"We will be achieving the most distant infrared view of the universe yet," recording objects at least 10 to 20 times further away than was possible with the European Space Agency's ISO, he said.