Three Millimeter Arrays Converge in Inyo Mountains

After announcing in 2000 that two California telescope arrays would be united at a fresh site in the state’s Inyo mountains, the project’s university partners finally have the formal agreement and most of the money to go ahead with the merger (see Physics Today, January 2001, page 27 ). But before the Combined Array for Research in Millimeter-wave Astronomy is formed, a piggyback array of smaller telescopes will begin using the Sunyaev-Zel’dovich effect (SZE) to survey galaxy clusters.

Located at CARMA’s center, the subgroup of telescopes that make up the Sunyaev-Zel’dovich Array (SZA) will use the SZE to spot galaxy clusters by the distortions they cause in the cosmic microwave background. “Most of the scattering is from the intracluster medium—the soup between the galaxies,” says the University of Chicago’s John Carlstrom, SZA project leader. “It turns out [that the SZE] has this amazing property, that how well you can measure the effect is independent of how far away the cluster of galaxies is.” In combination with x-ray and optical data, he adds, “you can solve for distances to galaxy clusters. And if you get the distance, you can get the Hubble constant. You can put together a history of cluster formation.” The SZE was postulated more than 30 years ago and has been observed. The trouble, says Carlstrom, is that data collection is very slow. Starting with the new survey, he predicts that “the SZE will become mainstream.”

After a year or two of dedicated cluster searching, the SZA’s eight 3.5-meter telescopes are expected to be linked to the main CARMA—nine 6.1-meter telescopes from the Berkeley Illinois Maryland Association (BIMA) array, which will be moved from Hat Creek in northern California, and six 10.4-meter telescopes from Caltech’s nearby Owens Valley Millimeter Array. Spread out over four square kilometers, CARMA will have a flexible configuration and observe mostly at wavelengths of 1.3 mm and 3 mm; with the SZA, it would gain sensitivity in the 8–11 mm range.

CARMA“will be a nonhomogeneous array, so we will be able to do studies of wide fields of view at high resolution. For example, we can learn about the origin of galaxies, stars, and planets, and look at comets, which tell us about the origins of material in the Solar System,” says Anneila Sargent, director of the Owens Valley Radio Observatory. CARMA will be at an altitude of roughly 2400 meters, more than twice that of either BIMA or the OVRO array, and it will have an angular resolution of about 0.2 arcseconds and be sensitive to temperature differences of a fraction of a kelvin—better than its parent arrays. The same sorts of problems will be tackled very efficiently by the larger Atacama Large Millimeter Array under construction in Chile, Sargent adds. “But we will have five years before ALMA comes on line, and CARMA will observe the northern skies. This university-based array will be the place to train scientists and technicians and build equipment.”

Caltech and the BIMA group—the University of California, Berkeley; the University of Illinois at Urbana-Champaign; and the University of Maryland, College Park—have pledged $10 million to CARMA; NSF has contributed $2 million and the project still needs about $3 million. If the environmental impact studies currently under way go well, the SZA, an independent project priced at $7–8 million, will start collecting data next spring, and CARMA could be fully operational in mid 2005.