The largest two-dimensional map of the sky ever created has grown even bigger with the tenth publication of data from the DESI Legacy Imaging Surveys – a landmark six-year survey covering nearly half of the sky. This new data release adds greater sky and wavelength coverage to the already completed surveys made with data from NSF’s NOIRLAb telescopes at Kitt Peak National Observatory in Arizona and Cerro Tololo Inter-American Observatory in Chile.

The universe is teeming with galaxies, each containing billions of stars. While all the galaxies shine brightly, many are shrouded in dust, while others are so far away that they are nothing more than faint smears to observers on Earth. By making comprehensive maps of even the faintest and most distant galaxies, astronomers are better able to study the structure of the Universe and unravel the mysterious properties of dark matter and dark energy. The largest such map to date just got even bigger, with the tenth release of data from the DOE’s Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey.

The DESI Legacy Imaging Survey builds on the data included in two previous companion studies: the Dark Energy Camera (DECam) Legacy Survey and the Beijing-Arizona Sky Survey. Collectively, these three surveys imaged 14,000 square degrees of the sky, visible from the Northern Hemisphere, using telescopes from NSF’s NOIRLAb’s Kitt Peak National Observatory (KPNO) and Cerro Tololo Inter-American Observatory (CTIO) in Chile.

This ambitious six-year effort involved three telescopes, one petabyte (1,000 trillion bytes) of data, and 100 million CPU hours on one of the world’s most powerful computers at the U.S. Department of Energy’s National Energy Research Scientific Computing Center.

This effort culminated in the largest two-dimensional map of the sky ever created. With collective observations by the Mosaic-3 camera on the Nicholas U. Mayall 4-meter telescope and the 90Prime camera on the University of Arizona Bok 2.3-meter telescope, both at KPNO, as well as the DOE-built Dark Energy Camera ( DECam) on the Víctor M. Blanco 4-meter telescope at CTIO in Chile.

One of the main goals of this map is to identify approximately 40 million target galaxies for the five-year DESI Spectroscopic Survey, which aims to understand dark energy by accurately mapping the expansion history of the Universe over the past 12 billion years. The DESI project has selected its targets and the spectroscopic investigation is currently underway. However, the team is looking to create the most comprehensive map of the sky they can, so more images and improved processing have been added to the Legacy Surveys to include data that was previously missing.

Most notably, the tenth data release focuses on integrating new images from DECam of the southern extragalactic sky, especially in regions away from the Milky Way’s disk, which are ideal for looking far into the cosmos.

With the addition of images of the southern sky in the new data release, the Legacy Surveys have expanded to cover more than 20,000 square degrees, nearly half of the sky. In addition, the new release includes images of the sky taken in an additional color filter, capable of sampling infrared light just redder than what the human eye can see. The additions to the map’s footprint and wavelength coverage will, in turn, make the data usable by a broader demographic of scientists.

“The addition of near-infrared wavelength data to the Legacy Survey allows us to better calculate the redshifts of distant galaxies, or the amount of time it took the light from those galaxies to reach Earth,” said Alfredo Zenteno, an astronomer at NSF’s NOIRLab.

“This is essential for investigations at radio and X-ray wavelengths that need the full ‘optical’ image to identify the origin of the emission, such as galaxy clusters and active supermassive black holes,” said Mara Salvato, a researcher at the Max Planck Institute for Extraterrestrial Physics (MPE) and spokesperson for the DECam eROSITA Survey (DeROSITAS).

The bulk of these additional DECam observations come from the DeROSITAS team, which includes scientists from NSF’s NOIRLab, the University of La Serena, MPE, and Ludwig Maximilians University of Munich in Germany; the DECam Local Volume Exploration Survey; and the final (sixth) year of the Dark Energy Survey. The team also searched the NSF NOIRLab data archive to use public airborne data that already existed or was collected by other researchers.

It’s not just scientists who benefit from the growing archive of astronomical data emerging from the Legacy Surveys. The publicly available data makes it possible for astronomy enthusiasts and curious individuals to digitally search the universe around us.

“Anyone can use the survey data to explore the sky and make discoveries,” said Arjun Dey, an astronomer at NSF’s NOIRLab. “In my opinion, it is this ease of access that has made this survey so impressive. We hope that in a few years the Legacy Surveys will have the most complete map of the entire sky and will be a treasure trove for scientists well into the future.”

NOIRLab will host these data products in the Astro Data Archive, from the original images taken with the telescopes to the catalogs reporting the positions and other properties of stars and galaxies. Astro Data Lab, which is part of the Community Science and Data Center (CSDC) of NSF’s NOIRLab, also serves the catalogs as databases, which astronomers can easily analyze using Astro Data Lab tools and services, and compare with other datasets, opening up more possibilities for discovery. In addition, Astro Data Lab provides astronomers with examples of scientific applications and tutorials to aid in their research.