Other meanings of Atacama Large Millimeter Array
RADIO ASTRONOMY
The Atacama Large Millimeter Array (ALMA) is a radio telescope array in Chile, ALMA, designed to observe millimeter and submillimeter radiation from cold gas, dust, planets, and distant galaxies. By combining signals from many antennas as an interferometer, it achieves the resolving power of a much larger telescope while operating at one of the driest and highest astronomical sites on Earth.
ALMA is a 66-antenna observatory for studying the cold universe at millimeter and submillimeter wavelengths.1 These wavelengths reveal rotational emission from molecules and thermal radiation from dust, complementing optical and infrared telescopes. The facility stands on the Chajnantor plateau in the Atacama Desert, where extreme dryness limits atmospheric absorption of submillimeter radiation.2
ALMA was built through a partnership among Europe, North America, and East Asia in cooperation with the Republic of Chile. Its principal institutional partners are the European Southern Observatory, the U.S. National Radio Astronomy Observatory, and the National Astronomical Observatory of Japan.1 Construction and commissioning proceeded in stages, with early science observations beginning before the full array was completed; the observatory was formally inaugurated in 2013.3
ALMA produces high angular resolution by correlating the signals received by separate antennas, a technique called radio interferometry. The array includes fifty 12-meter antennas in its main array and the Atacama Compact Array, consisting of twelve 7-meter antennas and four additional 12-meter antennas. Mobile transporters relocate the dishes between configurations, trading a wider field of view for finer detail.
Each antenna receives a narrow range of millimeter and submillimeter frequencies, which are divided into observing bands by sensitive receivers. A correlator combines the measurements into interferometric data that astronomers calibrate and image with specialized software.1 The longest antenna separations reach roughly 16 kilometers, allowing ALMA to resolve structures on scales comparable to the Solar System in nearby star-forming regions under favorable conditions.4
ALMA is especially powerful for tracing the formation of stars, planets, and galaxies through their cold molecular material.2 Its molecular-line observations map the composition, motion, temperature, and density of the interstellar medium, while continuum measurements reveal dust in protoplanetary disks and distant galaxies. Spectral lines from molecules such as carbon monoxide provide a widely used tracer of otherwise difficult-to-see molecular hydrogen.
The array has examined disks around young stars, complex organic molecules in star-forming regions, rapidly assembled galaxies in the early universe, and gas surrounding evolved stars. It also contributes observations to multi-observatory projects: ALMA formed part of the global network used by the Event Horizon Telescope to image the shadow of the supermassive black hole in Messier 87.5 Because atmospheric water vapor strongly affects these wavelengths, scheduling and calibration are central parts of every observation.
ALMA’s altitude is both an advantage and an engineering constraint. The array operates above most atmospheric water vapor, but personnel and equipment are commonly supported from the lower Operations Support Facility because the high site is inhospitable for prolonged work.1 Antennas must remain accurately aligned while exposed to intense sunlight, large temperature changes, dust, and thin air.
The compact array is not merely a smaller version of the main array: its shorter baselines help recover broad, diffuse emission that a widely separated interferometer can miss. ALMA also combines its data with observations from single-dish facilities to improve sensitivity to extended structures. A further subtlety is that the observatory does not detect visible pictures directly; its images are reconstructions from calibrated measurements, and spectral data can encode a three-dimensional view of gas through Doppler shifts.
ALMA is commonly expanded as the Atacama Large Millimeter/submillimeter Array; the title here follows the requested form “Atacama Large Millimeter Array.”
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