Celebrating 50 Years of Infrared Astronomy

Fifty years ago, a group of Caltech physicists brought infrared light—then an underappreciated region of the electromagnetic spectrum—to the forefront of astrophysics. Infrared astronomy holds the keys to our cosmic origins, revealing how planets, stars, and even the earliest galaxies formed. It may even enable us to discover Earth-like planets orbiting other stars.

The clouds of dust and gas from which stars and planets form—and the newborn solar systems themselves—are too cold to be seen in visible light. However, the heat they do emit shows up in the infrared, the shortest wavelengths of which are just slightly longer than the red light we do see.  The earliest galaxies would be visible, if they were closer to us in space and time; instead, the expanding universe has stretched their light so that the wavelengths are shifted down into the infrared. However, most of this light never reaches earthbound telescopes, as Earth's atmosphere absorbs most infrared waves very efficiently.

Two next-generation space telescopes, the James Webb Space Telescope, which is slated to launch in October 2018 to replace the Hubble Space Telescope, and the Wide-Field Infrared Survey Telescope (WFIRST), which has been named a top priority for the next decade in astronomy and is being studied for launch in the mid-2020s, will be carrying forward work begun by the Infrared Astronomical Satellite (IRAS) mission in the 1980s—work in which Caltech's self-styled "Infrared Army" played a major role.

On November 2 and 3, 2015, Caltech hosted a two-day symposium in honor of the Army's three founders—the late Gerry Neugebauer (PhD '60), Caltech's Robert Andrews Millikan Professor of Physics, Emeritus; Tom Soifer (BS '68), the Harold Brown Professor of Physics and director of the Spitzer Science Center, which operates NASA's current orbiting infrared observatory, the Spitzer Space Telescope; and Keith Matthews (BS '62), chief instrument scientist for Palomar Observatory, who by his own estimation has built "scores" of instruments for the 5-meter Hale telescope at Palomar and the twin 10-meter telescopes at the W. M. Keck Observatory atop Mauna Kea, Hawaii.

Matthews' hardware output is rivaled by the rate at which Neugebauer's and Soifer's research groups have spun off infrared programs at other institutions. Says organizer Lee Armus, who arrived at Caltech as a postdoc of Soifer's in 1992, "I've got a group of grad students from the '70s and '80s, and a group from the '90s. I'm trying to sample as many epochs as I can in two days."

Neugebauer earned his doctorate in 1960 under Caltech physics professor Robert Lee Walker, who had codesigned Caltech's synchrotron—the most powerful machine of its kind in its day, capable of revving up an electron to a billion volts of energy. In those days, experimental physicists built and operated their own equipment—and thus understood it inside and out. Neugebauer brought this hands-on approach to the U.S. Army at Caltech's Jet Propulsion Lab, where he designed and operated the infrared radiometer for Mariner 2's successful flyby of Venus.

When Neugebauer returned to campus in 1962, he and fellow physics professor Robert Leighton (BS '41, PhD '47) set about making the 62-inch-diameter mirror for the world's first purpose-built infrared telescope. (It is now in the Smithsonian.) Over the next few years, they and a team of undergraduates and graduate students used the instrument to scan the entire sky—or as much as could be seen from the summit of Mount Wilson overlooking Pasadena. The Two-Micron Sky Survey's final catalog, published in 1969, inventoried some 5,000 point-like objects, many of which were previously undiscovered cool red stars or stars enshrouded in obscuring clouds of gas and dust that the stars had ejected as they entered the later stages of their life.

Other invisible objects also cropped up. In 1965, Neugebauer's first graduate student, Eric Becklin (PhD '68) discovered something in the Orion Nebula that, in the infrared, was as bright as the brightest visible star, except it had no visible counterpart. Follow-up work with the 200-inch Hale Telescope at Palomar revealed that this point of infrared light had faint "wings," about 15 times larger than its diameter, extending to its east and west—a feature unlike any ever seen before. The Orion Nebula was known to be at most a few million years old and was presumed to be a stellar nursery. The object Becklin detected was the first protostar to be caught in its shell of potentially planet-forming dust. Becklin would later pioneer high-altitude infrared astronomy aboard specially modified jet aircraft.

In the 1970s Neugebauer and Soifer became part of the science team for IRAS, a collaboration among the United States, England, and the Netherlands. Launched in 1983, IRAS surveyed more than 95 percent of the sky. The data were made available to the entire scientific community as soon as they were processed—a first for NASA—leading to the creation of Caltech's Infrared Processing and Analysis Center to curate and distribute it.

IRAS got the field of infrared astronomy off the ground. "Astronomers could trust our catalogs," Soifer says. "Every source was real. We gained the respect of the astronomical community because they could take some other telescope and point it at our coordinates, and they'd find really interesting objects to explore with millimeter telescopes, radio telescopes, optical telescopes."

Matthews "started working in cosmic rays in 1959 with Professor of Physics [Eugene] 'Bud' Cowan [PhD '48]," and still sees himself as a physicist. "I do anything that has technique to it," he says. In addition to helping design the infrared aspects of the Keck 10-meter telescopes, he built the observatory's Near-Infrared Camera, the first instrument to be mounted on the telescope.

In the early 1990s Andrea Ghez (MS '89, PhD '93), one of Neugebauer's last graduate students, used this instrument and a technique called "speckle interferometry" to measure the positions of stars close to the galactic center. Ghez now uses the telescopes as the founder and director of UCLA's Galactic Center Group. Nearly two decades' worth of measurements, mostly using the second- generation Near-Infrared Camera for the Keck II telescope built by Matthews, have allowed her to derive radial velocities of stars as they orbit that still-elusive black hole. Thanks to her work, however, the mass of our galaxy's black hole is now precisely known, making it a little less mysterious.