DESI early data release holds nearly two million objects

By June 16, 2023June 19th, 2023IFAE

The first batch of data from the Dark Energy Spectroscopic Instrument (DESI) 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. DESI has already observed more galaxies than the combination of any other previous study, and it is just starting. The BIST community research centre IFAE is part of this large international collaboration.

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. The collaboration publicly released its first batch of data this month, 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. In this period 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.

It is impressive that, after a few months, DESI has already measured positions for more objects than all previous cosmic surveys,” said Violeta González Pérez, a researcher from the Department of Theoretical Physics at the Autonomous University of Madrid, involved in the generation of computational catalogues of galaxies to help us interpret the data collected by DESI. “The data being made public today will allow us to better understand the nature of dark energy with mathematical tools that had not been possible to use until now. And not only that, with this immense amount of data, we will also be able to explore the early moments of the universe, the mass of neutrinos, the history of our galaxy, and the formation and evolution of galaxies and quasars.”

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.

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.

During the validation phase of the survey, we visually inspected the obtained spectra to confirm the quality of the observations and their classification into stars, bright galaxies, luminous red galaxies, galaxies with emission lines, and quasars,” states Ignasi Pérez i Ràfols, a scientist from the Institute of Cosmos Sciences at the University of Barcelona, who participated in the inspection of the spectra. “We have been able to optimise the classification algorithms and confirm our observation strategy.”

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 specialises 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.

It is designed to observe regions already explored with other methods,” points out César Ramírez Pérez, predoctoral researcher from the Institute for High Energy Physics (IFAE) and member of the BIST Community, who is involved in the generation of absorption catalogues in quasars. “This will help complement previous analyses, and by publishing the data, we ensure that groups outside the collaboration can access them, expanding their impact.”

Two interesting finds have already surfaced: Evidence of a mass migration of stars into the Andromeda galaxy, and incredibly distant quasars, the extremely bright and active supermassive black holes sometimes found at the centre of galaxies.

The DESI observations are much more precise than those of previous surveys, allowing us to observe all types of extragalactic objects, including quasars formed shortly after the Big Bang. The discovery of such rare objects not only constitutes a significant breakthrough in itself but also enables us to study the mystery of supermassive black hole formation,” says Malgorzata Siudek, a former postdoctoral researcher at the BIST centre IFAE, and now working at the Institute of Space Sciences in Barcelona, who leads the identification and analysis of the physical properties of galaxies hosting active supermassive black holes. “DESI will observe millions of quasars, quadrupling the number of known objects. The discovery of very distant quasars, previously reserved for large telescopes, with a 4-meter instrument, makes us all proud and confirms the excellence of DESI.”

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

The images that come directly from the spectrographs appear as lines in a strange and blurry image. The magic happens in the processing and decoding of the data, which makes them intelligible and usable for scientific analysis,” emphasises Juan Mena Fernández, a predoctoral researcher at CIEMAT. “These data will allow us to study the expansion of the universe and its enigmatic dark sector in a more precise manner.”

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 Science and Innovation of Spain, and by the DESI member institutions.

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

DESI is funded by the following institutions: the U.S. Department of Energy’s Office of Science, the National Science Foundation of the United States, the Division of Astronomical Sciences under contract with the National Optical Astronomy Observatory, 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 for Science and Technology of Mexico, the Ministry of Science, Innovation, and Universities of Spain, and the member institutions of DESI. The scientists of DESI are honored to be allowed to conduct astronomical research at lolkam Du’ag (Kitt Peak, Arizona), a mountain of particular significance to the Tohono O’odham nation.

Participating institutions in DESI include the Center for Energy, Environmental, and Technological Research (CIEMAT), the Institute of Space Sciences (ICE-CSIC), the Institute of Space Studies of Catalonia (IEEC), the Institute of High Energy Physics (IFAE), the Institute of Theoretical Physics (IFT) at the Autonomous University of Madrid (UAM) and CSIC, the Institute of Astrophysics of Andalusia (IAA), the Institute of Astrophysics of the Canary Islands (IAC), and the Institute of Cosmos Sciences at the University of Barcelona (ICCUB).

Learn more:
IFAE press release
DESI experiment
DESI dataset