Humanity's most complex ground-based astronomy observatory, the Atacama Large Millimeter/submillimeter Array (ALMA), has officially opened for astronomers. The first released image, from a telescope still under construction, reveals a view of the Universe that cannot be seen at all by visible-light and infrared telescopes.

Thousands of scientists from around the world have competed to be among the first few researchers to explore some of the darkest, coldest, furthest, and most hidden secrets of the cosmos with this new astronomical tool.

At present, around a third of ALMA's eventual 66 radio antennas, with separations up to only 125 metres rather than the maximum 16 kilometres, make up the growing array on the Chajnantor plateau in northern Chile, at an elevation of 5000 metres. And yet, even under construction, ALMA has become the best telescope of its kind - as reflected by the extraordinary number of astronomers who requested time to observe with ALMA.

"Even in this very early phase ALMA already outperforms all other submillimetre arrays. Reaching this milestone is a tribute to the impressive efforts of the many scientists and engineers in the ALMA partner regions around the world who made it possible," said Tim de Zeeuw, Director General of ESO, the European partner in ALMA.

ALMA observes the Universe in light with millimetre and submillimetre wavelengths, roughly one thousand times longer than visible-light wavelengths. Using these longer wavelengths allows astronomers to study extremely cold objects in space - such as the dense clouds of cosmic dust and gas from which stars and planets form - as well as very distant objects in the early Universe.

ALMA is radically different from visible-light and infrared telescopes. It is an array of linked antennas acting as a single giant telescope, and it detects much longer wavelengths than those of visible light. Its images therefore look quite unlike more familiar pictures of the cosmos.

The ALMA team has been busy testing the observatory's systems over the past few months, in preparation for the first round of scientific observations, known as Early Science. One outcome of their tests is the first image published from ALMA, albeit from what is still very much a growing telescope.

Most of the observations used to create this image of the Antennae Galaxies were made using only twelve antennas working together - far fewer than will be used for the first science observations - and with the antennas much closer together as well. Both of these factors make the new image just a taster of what is to come. As the observatory grows, the sharpness, efficiency, and quality of its observations will increase dramatically as more antennas become available and the array grows in size.

The Antennae Galaxies are a pair of colliding galaxies with dramatically distorted shapes. While visible light shows us the stars in the galaxies, ALMA's view reveals something that cannot be seen in visible light: the clouds of dense cold gas from which new stars form [2]. This is the best submillimetre-wavelength image ever made of the Antennae Galaxies.

Massive concentrations of gas are found not only in the hearts of the two galaxies but also in the chaotic region where they are colliding. Here, the total amount of gas is billions of times the mass of our Sun - a rich reservoir of material for future generations of stars.

Observations like these open a new window on the submillimetre Universe and will be vital in helping us understand how galaxy collisions can trigger the birth of new stars. This is just one example of how ALMA reveals parts of the Universe that cannot be seen with visible-light and infrared telescopes.

ALMA could accept only about a hundred or so projects for this first nine-month phase of Early Science. Nevertheless, over the last few months, keen astronomers from around the world have submitted over 900 proposals for observations. This ninefold level of oversubscription is a record for a telescope. The successful projects were chosen based