ALMA telescope poised to peer into hidden corners of the cosmos

Sitting atop a plateau some 16,500 feet high, a growing array of upturned dishes has started to plumb the secrets of planet formation and star formation in early galaxies, and promises to take astronomers to the very brink of a supermassive black hole at the center of the Milky Way.

This marks the first week of science operations for the Atacama Large Millimeter/submillimater Array (ALMA), perched high in Chile's Atacama Desert – by all accounts the driest desert on the planet.

That may be bad news for fish, but for astronomers, the desert's extreme aridity is welcome. The array observes the universe in a region of the electromagnetic spectrum whose radiation is readily absorbed by water vapor. The radiation ranges from the extreme high end of radio frequencies to the extreme low end of infrared light. In this range, cosmic objects obscured by dust or hidden deep within the cold interstellar clouds where stars eventually form – barely visible to most telescopes – burst into brilliance.

ALMA has been a long time coming. "I attended my first ALMA planning meeting in 1983," says Al Wootten, a researcher at the National Radio Astronomy Observatory headquarters in Charlottesville, Va., and the program's project scientist.

Now, he says, the observatory has 22 dish antennas installed, each just over 39 feet across. Sixteen of the 22 are now operational, en route to 66 antennas by 2013.

Signals coming in from each are combined to build images of the objects astronomers are studying, and the antennas are mobile, allowing the facility to vary the level of detail the array can capture. In their most compact array, the antennas have the ability to capture detail comparable to a single dish 525 feet across. At their maximum spread, the 66 antennas will collectively match the resolving power of a single dish 10 miles wide.

That capability opens the way for a range of new observations, Dr. Wootten says.

Seeing beyond the dust

Hints of the potential for studying the distant cosmos at ALMA's wavelengths began to appear in 1998. Researchers were trying to determine when the universe underwent its most intense burst of star formation. Based on visible and ultraviolet images gathered by the Hubble Space Telescope, it appeared that star formation peaked between 4 billion and 6 billion years ago. Not much appeared to be happening earlier than that.

But a team using the James Clerk Maxwell submillimeter telescope on Hawaii's Mauna Kea observed galaxies producing new stars at enormous rates back to about 8 billion years ago – activity Hubble couldn't see because it was obscured by dust.

This ability of submillimeter telescopes like ALMA to see into the murky regions of the universe could lead to further discoveries. For instance, astrophysicists want to know if these locally bright, early starburst galaxies had dimmer, "normal" kin more like the Milky Way. ALMA is the first instrument sensitive enough to detect dimmer galaxies that might be present during those early times, Wootten says.

"Were there galaxies such as the Milky Way, which we would call normal galaxies, or were galaxies fundamentally different then?" he asks.

Witness to planet formation?

Another objective is to capture images of planet formation around other stars at its earliest stages, when objects are still shrouded with dust.

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