The five-color image of the sky has a resolution of one terapixel (one thousand billion pixels). Like the prestigious Palomar Sky Survey in the 1950s, it should become the reference for observers for many years to come. It will enable astronomers to find objects in the sky that will allow them to get to grips with a large number of astronomical questions that have remained unanswered until now. This survey was carried out thanks to the installation, in 1998, of the world's most powerful digital camera on the 2.5 m telescope of the Apache Point Observatory in New Mexico (United States). Over the last decade, this camera has made it possible to build an image of more than one third of the whole sky. It is not possible to cover a larger fraction of the sky under good conditions from a single site (in this case, New Mexico). The survey is thus complete. The image is now the starting point for ambitious spectroscopic studies, aimed at characterizing extragalactic sources, selected on account of their colors, and measuring their spectra using a spectrograph capable of observing more than one thousand objects at a time.

One of these studies, the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS), began over a year ago and will last five years. Its aim is to measure the distance of millions of galaxies and quasars in order to highlight the large-scale structure of the Universe. It will make it possible to obtain new data to better understand the nature of dark energy, one of the greatest mysteries of current physics. Other information from the image will also help to complete the accumulated scientific data on the structure of our Galaxy, thereby expanding the star catalogue of the SEGUE-2 (Sloan Extension for Galactic Understanding and Evolution) survey, which contains the spectroscopic measurements of 118,000 stars at different latitudes. It includes measurements of both star velocities and intrinsic properties (temperature, luminosity and chemical composition) that will make it possible to trace the structure and the dynamics of our Galaxy and to characterize the evolution of its chemical composition. With this survey, researchers hope to obtain new information on the formation and the evolution of our Galaxy, which will give a better understanding of the formation of galaxies in the Universe.

The French scientific community is closely involved in these studies, with six French laboratories at the forefront: the Laboratoire Astroparticule et Cosmologie (CEA/CNRS/Université Paris Diderot– Paris 7/Observatoire de Paris), the Institut d’Astrophysique de Paris (CNRS/UPMC), the Institut de Recherche sur les Lois Fondamentales de l’Univers (IRFU, CEA), the Laboratoire d’Astrophysique de Marseille (CNRS/Université de Provence), the Centre de Physique des Particules de Marseille (CNRS/Université de la Méditerranée) and the Observatoire Terre-Homme-Environnement-Temps-Astronomie de Franche-Comté (CNRS/Université de Besançon). These laboratories are taking part in the BOSS and SEGUE-2 surveys.

Bibliography

Aihara et al., The Eighth Data Release of the Sloan Digital Sky Survey: First Data from SDSS-III, arxiv/astro-ph 1101.1559. A description of the SDSS-III survey is being published at the same time: Eisenstein et al., SDSS-III: Massive Spectroscopic Surveys of the Distant Universe, the Milky Way Galaxy, and Extra-Solar Planetary Systems, arxiv/astro-ph 1101.1529.

The data is available on http://www.sdss3.org/dr8.