The Atacama Large Millimeter/submillimeter Array (ALMA) has passed a key milestone. Astronomers and engineers of the European Southern Observatory (SEO) have, for the first time, successfully linked three of the observatory's antennas at the 5000-metre elevation observing site in northern Chile. Having three antennas observing in unison paves the way for precise images of the coldest objects in the Universe at unprecedented resolution.

On 20 November 2009 the third antenna for the ALMA observatory was successfully installed at the Array Operations Site, the observatory’s “high site” on the Chajnantor plateau, at an altitude of 5000 metres in the Chilean Andes. Later, after a series of technical tests, astronomers and engineers observed the first signals from an astronomical source making use of all three 12-metre diameter antennas linked together, and are now working around the clock to establish the stability and readiness of the system. Each antenna has four different receivers: band-3, 6, 7 and 9 receivers. The band-9 receiver is developed and built by a Dutch consortium which consists of the Kapteyn Institute of the Rijksuniversiteit Groningen, the Kavli Institute of the TU Delft and SRON, led by NOVA. The band-9 receiver is sensitive for radiation with a frequency of 600 to 720 Ghz. This type of radiation, in between infrared radiation and radio waves, has wavelenghts of approximately one millimetre and is therefore called (sub)millimetre radiation.

The third antenne is the missing link to correct errors that arise when only two antennas are used.'The first signal using just two ALMA antennas, observed in October, can be compared to a baby’s first babblings,' says Leonardo Testi, the European Project Scientist for ALMA at ESO. 'Observing with a third antenna represents the moment when the baby says its very first, meaningful word — not yet a full sentence, but overwhelmingly exciting! The linking of three antennas is indeed the first actual step towards our goal of achieving precise and sharp images at submillimetre wavelengths.'

Distant galaxies

The successful linking of the antenna trio was a key test of the full electronic and software system now being installed at ALMA, and its success anticipates the future capabilities of the observatory. When complete, ALMA will have at least 66 high-tech antennas operating together as an 'interferometer', working as a single, huge telescope probing the sky in the millimetre and submillimetre wavelengths of light. The combination of the signals received at the individual antennas is crucial to achieve images of astronomical sources of unprecedented quality at its designed observing wavelengths.

Radiation in these wavelenghts originates from elongated cold clouds in interstellar space, where temperatures are merely tenths of degrees above the absolute zero, and some of the earliest and most distant galaxies in the Universe. Astronomers use this radiation to determine the chemical and fysical conditions in molecular clouds – the dense areas of gas and dust where new stars are born. In visible light these areas are dark but in the (sub)millimeter area they shine brightly. The first scientific results with ALMA are expected in 2011.