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DESI early data release holds nearly 2M objects

The universe is big, and it’s getting bigger. To study dark energy, the mysterious force behind the accelerating expansion of our universe, scientists are using the Dark Energy Spectroscopic Instrument (DESI) to map nearly 40 million galaxies, quasars and stars. Today, the collaboration publicly released its first batch of data, with nearly 2 million objects for researchers to explore.

The 80-terabyte data set comes from 2,480 exposures taken over six months during the experiment’s “survey validation” phase in 2020 and 2021. Between turning the instrument on and beginning the official science run, researchers made sure their plan for using the telescope would meet their science goals—for example, by checking how long it took to observe galaxies of different brightness, and by validating the selection of stars and galaxies to observe.

“The fact that DESI works so well, and that the amount of science-grade data it took during survey validation is comparable to previous completed sky surveys, is a monumental achievement,” said Nathalie Palanque-Delabrouille, co-spokesperson for DESI and a scientist at the Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), which manages the experiment. “This milestone shows that DESI is a unique spectroscopic factory whose data will not only allow the study of dark energy but will also be coveted by the whole scientific community to address other topics, such as dark matter, gravitational lensing and galactic morphology.”

A telescope at dusk on a hill.

PHOTO CREDIT: Marilyn Sargent/Berkeley Lab

The Dark Energy Spectroscopic Instrument sits atop the Mayall 4-Meter Telescope at Kitt Peak National Observatory. DESI’s early data gathered in 2020 and 2021 is now publicly available.

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DESI uses 5,000 robotic positioners to move optical fibers that capture light from objects millions or billions of light-years away. It is the most powerful multi-object survey spectrograph in the world, able to measure light from more than 100,000 galaxies in one night. That light tells researchers how far away an object is, building a 3-D cosmic map.

“This new sample represents the first science-quality data taken with this powerful new instrument. These survey-validation data are better quality and provide spectra and classification of a wider range of stars, galaxies and quasars than the data we expect in the main five-year program,” said Kyle Dawson. Dawson of the University of Utah was one of the two primary leads of the survey validation effort and is also DESI co-spokesperson.  “We have learned from these data how to build the most effective cosmology program.”

Key aspects of survey validation included software to automatically classify tens of millions of stellar, galaxy and quasar spectra over the next five years. “In particular, the modeling of quasar spectra requires sophisticated computational tools. By studying these new quasar spectra, we were able to upgrade the models from a previous program and demonstrate that we can do significantly better in characterizing quasar spectra,” said Allyson Brodzeller, a graduate student researcher at the University of Utah.

As the universe expands, it stretches light’s wavelength, making it redder—a characteristic known as redshift. The further away the galaxy, the bigger the redshift. DESI specializes in collecting redshifts that can then be used to solve some of astrophysics’ biggest puzzles: what dark energy is and how it has changed throughout the universe’s history.

While DESI’s primary goal is understanding dark energy, much of the data can also be used in other astronomical studies. For example, the early data release contains detailed images from some well-known areas of the sky, such as the Hubble Deep Field.

Two interesting finds have already surfaced—evidence of a mass migration of stars into the Andromeda galaxy, and incredibly distant quasars, which are extremely bright and active supermassive black holes sometimes found at the center of galaxies.

“We observed some areas at very high depth. People have looked at that data and discovered very high redshift quasars, which are still so rare that basically any discovery of them is useful,” said Anthony Kremin, a postdoctoral researcher at Berkeley Lab who led the data processing for the early data release. “Those high redshift quasars are usually found with very large telescopes, so the fact that DESI—a smaller, 4-meter survey instrument—could compete with those larger, dedicated observatories was an achievement we are pretty proud of and demonstrates the exceptional throughput of the instrument.”

Today, the collaboration also published a set of papers related to the early data release, which include early measurements of galaxy clustering, studies of rare objects and descriptions of the instrument and survey operations. The new papers build on DESI’s first measurement of the cosmological distance scale that was published in April, which used the first two months of routine survey data (not included in the early data release) and also showed DESI’s ability to accomplish its design goals.

Survey validation was also a chance to test the process of transforming raw data from DESI’s 10 spectrometers (which split a galaxy’s light into different colors) into useful information.

“Besides advancing our understanding of cosmology, the unprecedentedly large set of spectroscopic data from DESI encode a wealth of information for us to investigate the activities around the supermassive black holes in galaxies, to study galaxy formation and evolution, and to probe the circum-galactic and intergalactic medium. We cannot wait to explore the data for all the exciting research topics,” said Zheng Zheng of the University of Utah’s Department of Physics & Astronomy.

The DESI early data release is now available to access for free through NERSC.

There is plenty of data yet to come from the experiment. DESI is currently two years into its five-year run and ahead of schedule on its quest to collect more than 40 million redshifts. The survey has already classified more than 26 million astronomical objects in its science run and is adding more than a million per month.

“The early data release follows the premiere in March 2023 of a planetarium show about the DESI experiment and the diverse group of students and scientists whose collective effort makes DESI possible. As an international collaboration, we are fortunate to be able to provide translations of the planetarium show in many languages, including Spanish, French, Mandarin and German,” said Angela Berti, a postdoctoral researcher at the University of Utah who chairs the DESI education and public outreach effort.

DESI is supported by the DOE Office of Science and by the National Energy Research Scientific Computing Center, a DOE Office of Science user facility. Additional support for DESI is provided by the U.S. National Science Foundation, the Science and Technologies Facilities Council of the United Kingdom, the Gordon and Betty Moore Foundation, the Heising-Simons Foundation, the French Alternative Energies and Atomic Energy Commission (CEA), the National Council of Science and Technology of Mexico, the Ministry of Science and Innovation of Spain and by the DESI member institutions.

The DESI collaboration is honored to be permitted to conduct scientific research on Iolkam Du’ag (Kitt Peak), a mountain with particular significance to the Tohono O’odham Nation.