ALMA construction nearly complete

The Atacama Large Millimeter/submillimeter Array (ALMA), the world’s largest telescope, is near completion with the 65th antenna accepted and the final 66th antenna  scheduled for delivery in mid-September.  ALMA operates at mm and sub-mm wavelengths, allowing astronomers to peer through dust that obscures visible light to observe star and planet formation, study the complex chemistry of giant molecular clouds, and observe star formation activity in high redshift galaxies.

First remote-controlled aerial view over the ALMA site, from July 2013 (Credit: Arial Marinkovic, ALMA/NRAO/ESO/NAOJ)

The ALMA site is located in the Chajnantor plateau in the Chilean Atacama desert at an altitude of 5000 m.  This winter has brought very heavy snowfall at the ALMA site, slowing construction and observations. Construction also stalled during a 17-day workers strike, which was recently resolved and work is again proceeding.

Once completed, ALMA will comprise an array of fifty 12-metre diameter antennas and the Atacama Compact Array (ACA), which includes four 12-metre antennas and twelve 7-metre antennas.  The maximum baseline, or distance between antenna pairs, will be 16 km, offering extremely high resolution. The compact configuration of the antennas that make up the ACA  allow ALMA to image extended sources.  The antennas can be moved around to form arrays with different baselines.  ALMA will initially observe at wavelengths from 3 mm to 400 μm (84 to 720 GHz), with additional wavelengths possible in future developments. 

Given the scope of the project and the number of years required for the construction of ALMA, observations began with a smaller set of the antenna.  Cycle 0 Early Science observations started on September 30, 2011, using about 23 antenna and included 111 observing projects.  Cycle 1 Early Science observations began in January this year, using about 32 of the antennas.   Both Cycle 0 and 1 offered observing in four ALMA bands: Band 3 at 100 GHz (3mm), Band 6 at 230 GHz (1.3mm), Band 7 at 345 GHz (0.88mm) and Band 9 at 675 GHz 0.44mm).  About 60 of the cycle 0 datasets re now publicly available in the Science Archive.

Commissioning of ALMA Band 8 (550 GHz or 0.6mm) has already produced some great results, showing the distribution of atomic carbon in the planetary nebula NGC 6302. In the final stages of its life, optical images of NGC 6302 shows the bipolar ejection of gas from the dying star. The ALMA Band 8 observations targeted the central region of the nebula, showing the concentration of carbon atoms.

Planetary Nebula NGC 6302. The image is the composite of ALMA Band 8 (yellow) and the Hubble Space Telescope (grey). The full image includes an upper left image of NGC 6302 taken by the Hubble Space Telescope and a lower left panel showing the line profile of atomic carbon. (Credit: ALMA, ESO/NAOJ/NRAO, NASA/ESA Hubble Space Telescope)

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