The Atacama Large Millimeter/submillimeter Array

The Atacama Large Millimeter/submillimeter Array Alwyn Wootten and A. Richard Thompson, Life Fellow IEEE Abstract-The Atacama Large Millimeter/submillimeter Array (ALMA) is an international radio telescope under construction in the Atacama Desert of northern Chile. ALMA is situated on a dry site at 5000 m elevation, allowing excellent atmospheric transmission over the instrument wavelength range of 0.3 to 10 mm. ALMA will consist of two arrays of high-precision antennas. One, of up to 64 12-m diameter antennas, is reconfigurable in multiple patterns ranging in size from 150 meters up to ∼15 km. A second array is comprised of a set of four 12-m and twelve 7-m antennas operating in one of two closely packed configurations ∼50 m in diameter. The instrument will provide both interferometric and total-power astronomical information on atomic, molecular and ionized gas and dust in the solar system, our Galaxy, and the nearby to high-redshift universe. In this paper we outline the scientific drivers, technical challenges and planned progress of ALMA.

Index Terms-Antennas, Radio astronomy, millimeter astronomy, submillimeter astronomy

In the total electromagnetic spectrum of the Universe, there are three major peaks. One, the biggest, is the peak from the 3 K blackbody radiation relic of the Big Bang. That peak occurs in the millimeter wavelength range of the spectrum, as expected for any black body radiating at such a low temperature. The third strongest peak occurs near one 1 micron (1 µm) wavelength: this contains the accumulated light from all of the stars and planets in the Universe. The second strongest occurs at about 1.5 THz or 200 microns wavelength. Light near this wavelength cannot penetrate the atmosphere, as it is absorbed by water and other molecules in the atmosphere: this peak was identified only recently through satellite observations. This spectral feature represents all of the cool (∼200 K) objects in the Universe, that is, clouds of dust and gas as well as radiation from warmer sources that is absorbed and reradiated. Alas, with a satellite one is limited as to the size of telescope one can observe with and hence the resolution obtained. Current spacecraft apertures are far too small to give good images of what produces this second peak.

A. Wootten and A. R. Thompson are with the National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, Virginia 22901 USA email: (awootten@nrao.edu, athompson@nrao.edu).

Manuscript last draft March 26, 2009.

ALMA 1 , with excellent sensitivity and resolution at a high dry location will allow sensitive imaging in the range 31-950 GHz (wavelength range of approximately 1 cm to 300 µm). Thus ALMA will observe within the wavelength regimes of the strongest two radiation peaks, to the extent that Earth’s atmosphere allows. ALMA consists of two parts. There is an array of 12 m diameter antennas, the scientific requirement for 64 of which will ensure full realization of the scientific goals set forth in the Bilateral Agreement; contracts in place will provide at least 50 antennas. For this array baselines extend from 15 m to ∼15 km. We refer to this antenna complement as the “12-m array”. There is also the Alma Compact Array (ACA) which consists of four 12 m antennas plus twelve 7 m antennas [1]. The smaller diameter of the 7 m antennas allows a minimum antenna spacing of 8.75 m. A summary of ALMA specifications can be found in Table I. ALMA has three primary science goals, defined in the Bilateral Agreement by which the observatory was founded (for a history of ALMA, see [2]; for a compendium of science, see [3] and [4]). The first of these goals is to detect emission from the CO molecule or C + ion towards a galaxy of Milky Way luminosity at a redshift of 3 (see discussion in [5]) in less than 24 hours integration. Although CO and C + lines 1 The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, Japan and North America in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Organization for Astronomical Research in the Southern Hemisphere, in Japan by the National Institutes of Natural Sciences (NINS) in cooperation with the Academia Sinica in Taiwan and in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC). ALMA construction and operations are led on behalf of Europe by ESO, on behalf of Japan by the National Astronomical Observatory of Japan (NAOJ) and on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI).

have been detected in more distant galaxies, those galaxies emit a much higher luminosity than the Milky Way. The sensitivity needed for this measurement sets the collecting area of ALMA, since sensitivity cannot be increased by enlarging the bandw

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