Communiqué de presse

The Atacama Large Millimeter Array (ALMA)

10 juin 1999

The Atacama Large Millimeter Array (ALMA) is the new name [2] for a giant millimeter-wavelength telescope project. As described in the accompanying joint press release by ESO and the U.S. National Science Foundation , the present design and development phase is now a Europe-U.S. collaboration, and may soon include Japan. ALMA may become the largest ground-based astronomy project of the next decade after VLT/VLTI, and one of the major new facilities for world astronomy. ALMA will make it possible to study the origins of galaxies, stars and planets.

As presently envisaged, ALMA will be comprised of up to 64 12-meter diameter antennas distributed over an area 10 km across. The first image shows an artist's concept of a portion of the array in a compact configuration. The video clip illustrates how all the antennas will move in unison to point to a single astronomical object and follow it as it traverses the sky. In this way the combined telescope will produce astronomical images of great sharpness and sensitivity [3].

An exceptional site

For such observations to be possible the atmosphere above the telescope must be transparent at millimeter and submillimeter wavelengths. This requires a site that is high and dry, and a high plateau in the Atacama desert of Chile, probably the world's driest, is ideal - the next best thing to outer space for these observations.

The second image shows the location of the chosen site at Chajnantor, at 5000 meters altitude and 60 kilometers east of the village of San Pedro de Atacama, as seen from the Space Shuttle during a servicing mission of the Hubble Space Telescope. The third and fourth image show a satellite image of the immediate vicinity and the site marked on a map of northern Chile. ALMA will be the highest continuously operated observatory in the world. The stark nature of this extreme site is well illustrated by the panoramic view in the fifth image.

High sensitivity and sharp images

ALMA will be extremely sensitive to radiation at milllimeter and submillimeter wavelengths. The large number of antennas gives a total collecting area of over 7000 square meters, larger than a football field. At the same time, the shape of the surface of each antenna must be extremely precise under all conditions; the overall accuracy over the entire 12-m diameter must be better than 0.025 millimeters (25µm), or one-third of the diameter of a human hair. The combination of large collecting area and high precision results in extremely high sensitivity to faint cosmic signals.

The telescope must also be able to resolve the fine details of the objects it detects. In order to do this at millimeter wavelengths the effective diameter of the overall telescope must be very large - about 10 km. As it is impossible to build a single antenna with this diameter, an array of antennas is used instead, with the outermost antennas being 10 km apart. By combining the signals from all antennas together in a large central computer, it is possible to synthesize the effect of a single dish 10 km across. The resulting angular resolution is about 10 milli-arcseconds, less than one-thousandth the angular size of Saturn.

Exciting research perspectives

The scientific case for this revolutionary telescope is overwhelming. ALMA will make it possible to witness the formation of the earliest and most distant galaxies. It will also look deep into the dust-obscured regions where stars are born, to examine the details of star and planet formation. But ALMA will go far beyond these main science drivers, and will have a major impact on virtually all areas of astronomy. It will be a millimeter-wave counterpart to the most powerful optical/infrared telescopes such as ESO's Very Large Telescope (VLT) and the Hubble Space Telescope, with the additional advantage of being unhindered by cosmic dust opacity.

The first galaxies in the Universe are expected to become rapidly enshrouded in the dust produced by the first stars. The dust can dim the galaxies at optical wavelengths, but the same dust radiates brightly at longer wavelengths. In addition, the expansion of the Universe causes the radiation from distant galaxies to be shifted to longer wavelengths. For both reasons, the earliest galaxies at the epoch of first light can be found with ALMA, and the subsequent evolution of galaxies can be mapped over cosmic time.

ALMA will be of great importance for our understanding of the origins of stars and planetary systems. Stellar nurseries are completely obscured at optical wavelengths by dense "cocoons" of dust and gas, but ALMA can probe deep into these regions and study the fundamental processes by which stars are assembled. Moreover, it can observe the major reservoirs of biogenic elements (carbon, oxygen, nitrogen) and follow their incorporation into new planetary systems. A particularly exciting prospect for ALMA is to use its exceptionally sharp images to obtain evidence for planet formation by the presence of gaps in dusty disks around young stars, cleared by large bodies coalescing around the stars.

Equally fundamental are observations of the dying gasps of stars at the other end of the stellar lifecycle, when they are often surrounded by shells of molecules and dust enriched in heavy elements produced by the nuclear fires now slowly dying.

ALMA will offer exciting new views of our solar system. Studies of the molecular content of planetary atmospheres with ALMA's high resolving power will provide detailed weather maps of Mars, Jupiter, and the other planets and even their satellites.

Studies of comets with ALMA will be particularly interesting. The molecular ices of these visitors from the outer reaches of the solar system have a composition that is preserved from ages when the solar system was forming. They evaporate when the comet comes close to the sun, and studies of the resulting gases with ALMA will allow accurate analysis of the chemistry of the presolar nebula.

The road ahead

The three-year design and development phase of the project is now underway as a collaboration between Europe and the U.S., and Japan may also join in this effort. Assuming the construction phase begins about two years from now, limited operations of the array may begin in 2005 and the full array may become operational by 2009.

Notes

[1] Press Releases about this event have also been issued by some of the other organisations participating in this project:

• CNRS (in French)
• MPG (in German)
• NOVA (in Dutch)
• NRAO
• NSF (ASCII and HTML versions)
• PPARC

[2] "ALMA" means "soul" in Spanish.

[3] Additional information about ALMA is available on the web:

• Articles in the ESO Messenger - "The Large Southern Array" (March 1998), "European Site Testing at Chajnantor" (December 1998) and "The ALMA Project" (June 1999), cf. http://www.eso.org/gen-fac/pubs/messenger/
• ALMA website at ESO at http://www.eso.org/projects/alma/
• ALMA website at the U.S. National Radio Astronomy Observatory (NRAO) at http://www.mma.nrao.edu/
• ALMA website in The Netherlands about the detectors at http://www.sron.rug.nl/alma/

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