This map serves as the starting point for a three-dimensional reconstruction to be produced by DESI (Dark Energy Spectroscopic Instrument).  It cumulates many records:  Surface of the sky covered, depth of the imaging and number of galaxies (more than one billion).

Established from 200,000 images from 1,405 nights of observation on three telescopes and several years of satellite data, it includes 2 billion objects on a surface of more than 10,000 billion pixels!

More than half of these objects are galaxies like those DESI will observe. Some of them (quasars) are extremely bright because their center hosts a supermassive black hole that expels powerful jets of matter.  Each spectral observation of one of these galaxies will make it possible to measure its distance to us (via the shift to redder "colors" of the source spectrum), thus producing the missing third dimension of the map.

Compiling the map data allowed the researchers to identify different targets for DESI:

• Bright galaxies within 4 billion light years (ly) of Earth,
• "Red" galaxies up to 8 billion ly,
• "Blue" galaxies up to 10 billion ly,
• Quasars up to 12 billion ly.

Irfu plays an important role in the effort to select targets to be observed by DESI from imaging data.  "This work is delicate because the galaxies and quasars to be selected are very faint light sources," notes Christophe Yèche, researcher at Irfu.  Since mid-December 2020, DESI has been devoting its first months of observation to validating this selection, which will be the key to the success of the program over the next 5 years. "

This work should eventually provide new details on dark energy, which is at the origin of the acceleration of the expansion of the Universe. "Over the last 7 billion years, the expansion of our Universe has gradually accelerated under the influence of a mysterious dark energy," says Nathalie Palanque-Delabrouille, one of DESI's spokespersons and a cosmologist at Irfu, " the goal of DESI is to clarify precisely this description by revealing what dark energy is. "

DESI's giant map is publicly available on the LegacySurvey website.