[Paper Review] The Dark Energy Spectroscopic Instrument (DESI)

M. E. Levi, Allen, Lori E.+43 more|HAL (Le Centre pour la Communication Scientifique Directe)|Jul 24, 2019

Astronomy and Astrophysical Research2 references22 citations

TL;DR

DESI is a multi-fiber spectrograph installed on the Mayall 4m telescope at Kitt Peak, designed to conduct a 5-year survey of 14,000 deg², measuring 34 million redshifts of galaxies and quasars to study dark energy via baryon acoustic oscillations and redshift-space distortions. With 5000 robotically positioned fibers and high-throughput spectroscopy from 360–980 nm, it enables precision cosmology and stellar surveys, remaining a world-class facility into the 2020s with potential for continued operations beyond 2025.

ABSTRACT

We present the status of the Dark Energy Spectroscopic Instrument (DESI) and its plans and opportunities for the coming decade. DESI construction and its initial five years of operations are an approved experiment of the US Department of Energy and is summarized here as context for the Astro2020 panel. Beyond 2025, DESI will require new funding to continue operations. We expect that DESI will remain one of the world's best facilities for wide-field spectroscopy throughout the decade. More about the DESI instrument and survey can be found at https://www.desi.lbl.gov.

Motivation & Objective

• To conduct a 5-year wide-field spectroscopic survey of galaxies and quasars to probe dark energy using baryon acoustic oscillations and redshift-space distortions.
• To achieve high multiplex and throughput for efficient redshift measurement across a broad redshift range (0 < z < 3.5).
• To provide precise radial velocity measurements (~1 km/s) for stars and extragalactic targets using a bench-mounted, thermally stable spectrograph.
• To support post-2025 science by identifying viable survey extensions, including high-redshift targets, Lyman-alpha forest studies, and Milky Way surveys.
• To enable data-driven cosmological probes by releasing annual data sets with selection functions and mock catalogs for clustering analyses.

Proposed method

• DESI uses 5000 robotically positioned fibers in an 8 deg² focal plane to collect light from celestial sources and feed it into 10 triple-arm spectrographs.
• The spectrographs cover 360–980 nm with spectral resolution R > 2000 in the UV and R > 4000 in the red and infrared, enabling high-precision redshift and radial velocity measurements.
• The instrument is bench-mounted for thermal stability, ensuring velocity precision of ~1 km/s, critical for cosmological clustering studies.
• Target selection is based on deep imaging from the Legacy Survey (Blanco, Mayall, Bok) and WISE data, identifying luminous red galaxies, emission-line galaxies, and quasars.
• The survey uses a tiered approach: dark/grey time for high-redshift targets (z > 0.6), bright time for low-redshift galaxies (median z ~ 0.2), and backup programs for stars when conditions limit extragalactic observations.
• Data are released annually with full selection functions and mock catalogs to support cosmological clustering and systematics modeling.

Experimental results

Research questions

• RQ1 How can a wide-field, high-multiplex spectroscopic survey measure cosmic distance scales with sub-percent precision using baryon acoustic oscillations?
• RQ2 What is the optimal strategy for targeting emission-line galaxies and quasars to maximize large-scale structure sampling across 0 < z < 3.5?
• RQ3 Can DESI’s high-throughput, high-stability spectrograph enable radial velocity measurements of stars with precision ~1 km/s for Milky Way kinematics?
• RQ4 What are the most promising scientific extensions of DESI beyond 2025, particularly in high-redshift galaxy surveys and Lyman-alpha forest studies?
• RQ5 How can DESI complement upcoming imaging surveys like LSST, Euclid, and SPHEREx through targeted, high-precision spectroscopy?

Key findings

• DESI achieved spectroscopic first-light in October 2019, with full commissioning completed by January 2020, enabling the start of the 5-year survey in summer 2020.
• The instrument achieved over 50% throughput from top of atmosphere to detected photons, with 5000 fibers and a 1.5" fiber core, enabling high-efficiency observations.
• The survey will yield 34 million redshifts: 2.4 million quasars, 4 million luminous red galaxies (0.3 < z < 1), and 18 million emission-line galaxies (0.6 < z < 1.6), with Lyman-alpha forest measurements along 600,000 lines of sight at z > 2.
• The bright time program will survey 10 million galaxies to r ≈ 19.5, achieving a median redshift of ~0.2 and covering a volume over 10× larger than SDSS MAIN and 2dF GRS.
• The velocity precision of ~1 km/s is maintained due to the bench-mounted, thermally stable spectrograph design, enabling high-accuracy radial velocity measurements.
• Annual data releases with selection functions and mock catalogs are planned, supporting robust cosmological clustering and systematics studies.