Over a billion galaxies shine brightly in a colossal map of the sky

The universe is teeming with galaxies, each with billions of stars. Though all galaxies shine brightly, many are shrouded in dust, while others are so distant that they appear as just a faint speck to observers on Earth. By creating comprehensive maps of even the darkest and most distant galaxies, astronomers are better able to study the structure of the Universe and decipher the mysterious properties of dark matter and dark energy. The largest map of its kind to date just got bigger with the tenth data release from the DOE’s Dark Energy Spectroscopic Instrument (DESI) Legacy Imaging Survey.

The DESI Legacy Imaging Survey extends the data contained in two previous companion surveys: the Dark Energy Camera (DECam) Legacy Survey and the Beijing-Arizona Sky Survey. Together, these three surveys mapped 14,000 square degrees of the sky visible from the northern hemisphere using telescopes at NSF NOIRLab’s Kitt Peak National Observatory (KPNO) and Cerro Tololo Inter-American Observatory (CTIO) in Chile.

This ambitious six-year effort involved three telescopes, a petabyte of data and 100 million CPU hours on one of the world’s most powerful computers at the US Department of Energy’s National Energy Research Scientific Computing Center.

These efforts culminated in the largest two-dimensional map of the sky ever made. Featuring collective observations from the Mosaic-3 camera on the Nicholas U. Mayall 4-meter telescope and the 90Prime camera on the University of Arizona’s Bok 2.3-meter telescope, both located at KPNO, and those built by the DOE Dark Energy Camera ( DECam) on the Víctor M. Blanco 4-meter telescope at CTIO in Chile.

One of the main purposes of this map is to identify around 40 million target galaxies for the five-year DESI Spectroscopic Survey, which aims to understand dark energy by accurately mapping the universe’s expansion history over the past 12 billion years. The DESI project has selected its targets and the spectroscopic survey is currently underway. However, the team is trying to create the most comprehensive map of the sky possible, 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, particularly in areas away from the Milky Way’s disc, which are ideal for looking far into the cosmos.

The addition of southern sky imagery in the new data release has expanded the Legacy Surveys to cover over 20,000 square degrees, nearly half the sky. In addition, the new version includes images of the sky taken with an additional color filter capable of sampling infrared light, which is 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 useful to a broader population of scientists.

“Adding near-infrared wavelength data to the Legacy Survey will allow us to better calculate the redshifts of distant galaxies, or the time it took light from these galaxies to reach Earth,” said Alfredo Zenteno, astronomer at NSF’s NOIRLab .

“This is essential for surveys at radio and X-ray wavelengths that need the full ‘optical’ view 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 speaker of the DECam eROSITA Survey (DeROSITAS).

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

Not only scientists benefit from the growing archive of astronomical data from the legacy surveys. The publicly available data allows astronomy enthusiasts and curious people to digitally explore the universe around us.

“Anyone can use the survey data to explore the sky and make discoveries,” said Arjun Dey, astronomer at NSF’s NOIRLab. “I think it’s this easy approach that made this survey so powerful. We hope that in a few years the Legacy Surveys will have the most complete map of the entire sky and provide 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 at the telescopes to the catalogs reporting the positions and other properties of stars and galaxies. Astro Data Lab, part of the Community Science and Data Center (CSDC) at NSF’s NOIRLab, also serves the catalogs as databases that astronomers can easily analyze and cross-check with other datasets using Astro Data Lab’s tools and services, what more opportunities for discovery. In addition, Astro Data Lab provides astronomers with sample science applications and tutorials to aid in their research.