DESI Early Data Release Holds Nearly Two Million Objects

Tuesday, June 13, 2023

The first batch of data from the Dark Energy Spectroscopic Instrument is now available for researchers to mine. Taken during the experiment’s “survey validation” phase, the data include distant galaxies and quasars as well as stars in our own Milky Way 

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 more than 40 million galaxies, quasars, and stars. Today, the collaboration publicly released its first batch of data (see press release).   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 and contains nearly 2 million objects for researchers to explore. 

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Part of DESI’s survey validation included the “One-Percent Survey” visualized in this flythrough. Researchers took detailed images in 20 different directions on the sky, creating a 3D map of 700,000 objects and covering roughly 1% of the total volume DESI will study. With the instrument and survey plan successfully tested, the main DESI survey is now filling in the gaps between those observations. Credit: David Kirkby/DESI collaboration

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.

WCA Postdoctoral Fellow, Enrique Paillas, is involved in the Galaxy & Quasar Clustering group in DESI, where he is helping to assess the robustness of the methods that DESI uses to measure the expansion history of the Universe, through the detection of baryon acoustic oscillations (BAO). These correspond to ripples that were imprinted in the distribution of galaxies from the very early Universe.  He also co-leads a subgroup within DESI, which focuses on studying new statistical methods to extract information from the galaxy maps that the survey will observe.

Enrique is a co-author on the early data release paper, and says:

I’m really thrilled that these papers will finally see the light! I think this early data release is exciting, as it will be the first sneak peek to all the amazing science DESI will have to offer.

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 3D cosmic map. 

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DESI uses 5,000 fiber-optic “eyes” to rapidly collect light from distant galaxies. In good observing conditions, the experiment can image a new set of 5,000 objects every 20 minutes. Credit: Marilyn Sargent/Berkeley Lab

Perimeter Institute Computational Scientist and Associate WCA member, Dustin Lang, was heavily involved in the imaging surveys; the 2-d maps of stars and galaxies that DESI picked its targets from, and then determined 3-d distances for.  Dustin is responsible for the interactive sky viewer, that will allow people around the world to zoom around the sky and investigate the newly released DESI spectra themselves.

We have been working on DESI for more than 10 years, and we have been really excited to see the beautiful quality of the data, and the sheer bulk of data that DESI can collect, so it is amazing to finally get to share the first slice of this exciting data set with the world!

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.  WCA ATMD Postdoctoral Fellow, Alex Krolewski, is involved in mitigating redshift systematics for DESI clustering catalogs, and co-leads the topical group studying the combination of DESI data with the cosmic microwave background (CMB), in particular, its lensing and scattering by large-scale structure. He has been studying the large-scale clustering of DESI quasar targets using their gravitational lensing of the CMB.

The early DESI data is critical for this project. It has allowed me to identify and remove interlopers (stars and galaxies, which are ~20% of the quasar targets), and it has allowed me to securely measure the redshift distribution of the quasars, which directly determines how much they lens the CMB. So, in addition to the promising early Baryon Acoustic Oscillation results, I'm quite excited about how the early release has enabled this analysis and a similar analysis with Luminous Red Galaxies. Without spectra, the errors in the redshift distribution (and high rate of interlopers) would swamp the cosmological signal that we're looking for.

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This small slice of DESI data has Earth on the left and looks back in time to the right. Every dot represents a galaxy (blue) or quasar (red). The farther to the right an object is, the farther away it is. The upper wedge includes objects all the way back to about 12 billion years ago. The bottom wedge zooms in on the closer galaxies in more detail. The clumps, strands, and blank spots are real structures in the universe, showing how galaxies group together or leave voids on gigantic scales. Credit: Eleanor Downing/DESI collaboration

PhD student, Tristan Fraser, works with DESI data to look for voids (empty regions) in the large-scale structure of the universe.  He says:

The DESI data is incredibly exciting since the sheer number of galaxies surveyed (in the millions) tells us a lot about how the universe is evolving today, and how dark energy is driving its evolution - understanding voids is one small part of that larger effort!

WCA Postdoctoral Fellow, Shahab Joudaki, says, “DESI is the most powerful spectroscopic survey in the world.”  Shahab works on the combined analysis of DESI and the most powerful imaging surveys, which are complementary to one another and will together allow us to provide the most robust constraints on the underlying cosmology of the Universe. He is specifically focused on how we can account for their joint measurement uncertainties and the cosmological parameter inference.

Shahab also directly contributes to the observing of DESI data.

I am excited about how the early data release can be used to calibrate redshift uncertainties in imaging surveys, and the path that this paves towards the full data release that will be used in the forthcoming cosmological analyses.

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 catalogued more than 26 million astronomical objects in its science run, and is adding more than a million per month. 

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 Economy of Spain, and by the DESI member institutions.

Kitt Peak National Observatory is a program of NSF’s NOIRLab.

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.

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Adapted from DESI early data release by Lauren Biron, Berkley Lab