Sometimes the whole really is greater than the sum of its parts.
As the 20th century drew to a close, radio astronomers were eager to jump-start research at submillimeter and millimeter wavelengths, a largely unexplored spectral window where they hoped to explore some of the coldest, darkest, and most distant objects in the universe. But to do that they'd need to build big arrays of massive, high-precision receiving dishes — with paraboloidal surfaces accurate to within 20 microns. It was a very expensive proposition, and astronomers in Europe, North America, and Japan separately fussed over plans with costs that seemed hopelessly out of reach.So, instead, they joined forces. In 1999, the U.S. National Science Foundation (representing North American interests) and the European Southern Observatory agreed on a cooperative plan to build a single, giant antenna farm; in 2004 Japan's Institutes for Natural Sciences became a partner.
The result is the Atacama Large Millimeter/submillimeter Array, or ALMA, an eventual complex of 66 dishes located at an altitude of 16,400 feet (5,000 m) atop Chajnantor Plateau in Chile's Atacama desert. There's hardly any atmospheric water vapor at that altitude — let alone rain. In fact, the air over Chajnantor has roughly half the pressure at sea level, so ALMA's control room and construction site are 17 miles away at a tolerable altitude of 9,500 feet (2,900 m).The first of the massive 100-ton antennas, 40 feet (12 m) across, reached the plateau two years ago, and since then 15 more have joined it. For now they can be separated up to a quarter mile (400 m), and by combining all their outputs observers can effectively mimic the capability of a single dish with that aperture.
Enough hardware is now in place to do real science, and regular observations got under way on September 30th. Last week the ALMA partnership released the first of its "early science" results: a revealing look at NGC 4038 and 4039, a pair of colliding galaxies nicknamed the Antennae that lie about 70 million light-years away in the constellation Corvus.
This intergalactic mash-up, which began about 600 million years ago, has triggered an intense burst of star formation 20 times greater than is occurring in the Milky Way. ALMA's observations, tuned to radio emissions from carbon monoxide (CO) molecules, have mapped the spots in the galaxies' dark, opaque hydrogen clouds where new stars are forming.Even in its unfinished state, ALMA is providing a "new and unique window on this otherwise invisible material," notes Crystal Brogan (National Radio Astronomy Observatory), a key U.S. participant in the ALMA effort. "We can see the Doppler shift of CO gas in southern nucleus, and this new third dimension allows us to see how this gas is moving."
"These are very exciting times right now," comments Lars-Åke Nyman, who heads ALMA's science operations. He notes that the array is the world's highest observatory.
When completed in 2013, ALMA's full complement will include 50 12-m dishes, with separations of up to 10 miles (16 km), and a separate cluster (Japan's contribution) comprising four 12-m antennas and twelve 7-m dishes dubbed the Atacama Compact Array. ALMA will be "10 to 100 times more powerful than any existing scope of this type," Nyman stresses, with an angular resolution of just 0.01 arcsecond when the widest spacing is used.
(For now, the Very Large Array in New Mexico remains the largest single-site radio array. It has 27 25-m dishes that can be spaced in varying interferometric configurations with spans of up to 22 miles, or 36 km.)
Even though ALMA is only partially complete, radio observers are already lining up in droves to use it. According to Leonardo Testi, who serves as the ESO's ALMA project scientist, the first call for proposals netted nearly 1,000 requests — totaling nine times the available observing time.
In the end, 112 proposals were accepted. The array detects radiation at wavelengths of 0.3 to 10 mm, long-wavelength emissions from some of the coldest matter in the universe. (The VLA's receivers cover wavelengths from 6 mm to 4 m.) Observers hope to use ALMA to see highly redshifted radiation from very distant galaxies, to determine how much star formation was occurring just a few hundred million years after the Big Bang. Other studies will search for planetary building blocks in emerging solar systems and never-before-seen details in the heart of the Milky Way.
All this radio prowess does not come at a bargain price. The project's total development will cost about $1.3 billion. Still, the team firmly believes the investment will pay big dividends in the discoveries that lie ahead. "Work does not stop for ALMA," says Testi, "but science finally begins."