Workers on the Atacama Large Millimeter/submillimeter Array (ALMA) project stand next to three of the telescope’s antennas. This photograph gives a real sense of the scale of the giant dishes, whose 12-metre diameters are about seven times the average human height. When completed, ALMA will consist of 66 high-precision antennas, 54 of them with 12-metre dishes as seen in this image, and 12 more compact ones with diameters of 7 metres. The yellow 28-wheel transporter vehicle, which has to be powerful enough to carry the 100-tonne antennas, is built on a similarly giant scale.

This photograph was taken at the 2900-metre-high ALMA Operations Support Facility in the foothills of the Chilean Andes, where the antennas are assembled and tested. On the left is one of the European ALMA antennas, pointing at the horizon. Behind it is one of the antennas provided to the project by Japan, while on the right, on the transporter vehicle and pointing upwards, is another European antenna. This is the first European antenna starting its journey up to the Array Operations Site on the Chajnantor plateau, photographed in July 2011 (see eso1127). Since this photograph was taken, the antennas, and others like them, have been put into operation on Chajnantor as ALMA has made its first scientific observations (see eso1137). ALMA is designed to study the cool Universe — the relic radiation of the Big Bang and the molecular gas and dust from which stars, planets and galaxies originate.

ALMA, an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

Twenty-five European ALMA antennas are being provided by ESO through a contract with the European AEM Consortium. ALMA will also have 25 antennas provided by North America, and 16 by East Asia.

The picture is part of the COMBO-17 survey (Classifying Objects by Medium-Band Observations in 17 Filters), a project dedicated to recording detailed images of small patches of the sky through filters of 17 different colours. The area covered in this image is only about the size of the full Moon, but thousands of galaxies can be identified just within this small region.

The image was taken with an exposure time of almost seven hours, which allowed the camera to capture the light from very faint and distant objects, as well as those that are closer to us. Galaxies with clear and regular structures, such as the spiral specimen viewed edge-on near the upper left corner, are only up to a few billion light-years away. The fainter, fuzzier objects are so far away that it has taken nine or ten billion years for their light to reach us.

The COMBO-17 survey is a powerful tool for studying the distribution of dark matter in galaxies. Dark matter is a mysterious substance that does not emit or absorb light and can only be detected by its gravitational pull on other objects. Some of the closer galaxies pictured act as lenses that distort the light coming from more distant galaxies placed along the same line of sight. By measuring this distortion, an effect known as gravitational lensing, astronomers are able to understand how dark matter is distributed in the objects that act as lenses.

The distortion is weak and, therefore, almost imperceptible to the human eye. However, because surveying the sky with 17 filters allows extremely precise distance measurements, it is possible to determine if two galaxies that appear to lie close to each other are actually at very different distances from the Earth. After identifying the galactic lensing systems, the distortion can be measured by averaging over thousands of galaxies. With more than 4000 galactic lenses identified, this COMBO-17 survey is an ideal method to help astronomers to understand the dark matter better.

This image was taken with three of the 17 filters from the project: B (blue), V (green), and R (red). Data through an additional near-infrared filter was also used. 

Links

• COMBO-17 at the Max Planck Institute for Astronomy

ESO turns 50 this year, and to celebrate this important anniversary, we will be showing you glimpses into our history. Once a month throughout 2012, a special “then and now” comparison Picture of the Week will show how things have changed over the decades at the La Silla and Paranal observatory sites, the ESO offices in Santiago de Chile, and the Headquarters in Garching bei München, Germany.

Our first stop on this journey through time is at La Silla, the first of ESO’s observatory sites. The historical image was taken in the late 1960s or early 1970s from the dome of the ESO 1.52-metre telescope, which had its first light in 1968. A second photograph, taken in the present day, shows how much the observatory has changed over the decades. You can examine the changes with our mouseover image comparison.

In the historical image, we can see the ESO 1-metre telescope in the foreground on the right, with the Grand Prism Objectif telescope (GPO) just peeking out from behind. The third telescope in this photo is the Schmidt 1-metre telescope, on the left. Behind it, at a higher level, are the water tanks of the observatory.

Moving through time to the present-day, we can see how much La Silla has evolved, with many more telescopes on the site. The ESO 3.6-metre telescope and the adjacent Coudé Auxiliary Telescope now stand out on the highest peak. The angular enclosure of the New Technology Telescope (NTT) is just to the left, next to the water tanks. The 15-metre-diameter dish of the Swedish–ESO Submillimetre Telescope (SEST) watches the horizon on the far right.

The new photograph was taken from a slightly different position on top of the ESO 1.52-metre telescope building, so the GPO is now hidden behind the ESO 1-metre telescope in the foreground. The white dome that is just visible behind the 1-metre is the Danish 1.54-metre telescope. In the centre of the photo we now see the silvery dome of the MPG/ESO 2.2-metre telescope.

Although some telescopes at La Silla, such as the ESO 1-metre and 1.52-metre, and the SEST, are no longer in operation, others are still doing front-line astronomy. The ESO 3.6-metre telescope hosts the HARPS instrument, the world’s leading exoplanet hunter (see eso1134 for some recent results). The NTT has been used to help explain the formation of massive stars (see eso1029). Both telescopes provided vital data which led to the discovery of the accelerating expansion of the Universe — a discovery for which the 2011 Nobel Prize in Physics was awarded. The MPG/ESO 2.2-metre telescope has also produced a treasure trove of data from breathtaking wide-field images to studies of gamma-ray bursts, the most explosive events in the Universe.

Links

• The historical image
• The present-day image
• Side-by-side composite of the historical and present-day images
• More about La Silla
• Press release on the occasion of the 40th anniversary, in 2009, of La Silla’s inauguration
• ESO timeline