[Paper Review] The DESI Experiment Part I: Science,Targeting, and Survey Design

DESI Collaboration, Amir Aghamousa+291 more|arXiv (Cornell University)|Oct 31, 2016

Astronomy and Astrophysical Research1,202 citations

TL;DR

DESI is a Stage IV ground-based dark energy experiment outlining its science goals, target selection across four classes, and survey design to measure BAO, RSD, and Ly-α forest with over 30 million galaxy and quasar redshifts.

ABSTRACT

DESI (Dark Energy Spectroscopic Instrument) is a Stage IV ground-based dark energy experiment that will study baryon acoustic oscillations (BAO) and the growth of structure through redshift-space distortions with a wide-area galaxy and quasar redshift survey. To trace the underlying dark matter distribution, spectroscopic targets will be selected in four classes from imaging data. We will measure luminous red galaxies up to $z=1.0$. To probe the Universe out to even higher redshift, DESI will target bright [O II] emission line galaxies up to $z=1.7$. Quasars will be targeted both as direct tracers of the underlying dark matter distribution and, at higher redshifts ($ 2.1 < z < 3.5$), for the Ly-$α$ forest absorption features in their spectra, which will be used to trace the distribution of neutral hydrogen. When moonlight prevents efficient observations of the faint targets of the baseline survey, DESI will conduct a magnitude-limited Bright Galaxy Survey comprising approximately 10 million galaxies with a median $z\approx 0.2$. In total, more than 30 million galaxy and quasar redshifts will be obtained to measure the BAO feature and determine the matter power spectrum, including redshift space distortions.

Motivation & Objective

• Motivate the DESI project as a next-generation dark energy probe (Stage IV) leveraging BAO and redshift-space distortions.
• Define four spectroscopic target classes to trace dark matter via imaging data and reach z up to 1.7 for galaxies and z up to 3.5 for Ly-α forest.
• Design a wide-area survey strategy to obtain >30 million galaxy and quasar redshifts for BAO and growth measurements.

Proposed method

• Target selection from imaging data to create four spectroscopic samples (LRG up to z=1, [O II] emitters up to z=1.7, quasars for direct tracing and Ly-α forest at 2.1<z<3.5, Bright Galaxy Survey at z~0.2).
• Survey design aimed at wide-area coverage to maximize BAO measurements and the matter power spectrum including redshift-space distortions (RSD).
• Use of Ly-α forest in quasar spectra as a probe of neutral hydrogen distribution at high redshift.
• Plan to achieve >30 million redshifts by integrating galaxy and quasar targets across four classes.

Experimental results

Research questions

• RQ1 How can imaging data be optimized to select four spectroscopic target classes for DESI to trace dark matter across the desired redshift range?
• RQ2 What survey design (area, depth, observing strategy) is required to measure the BAO feature and growth of structure with redshift-space distortions and Ly-α forest signals?
• RQ3 What is the expected yield of galaxy and quasar redshifts needed to constrain dark energy and the matter power spectrum?
• RQ4 How can Ly-α forest observations from quasars be utilized to extend DESI's reach to high redshift and complement galaxy clustering measurements.

Key findings

• DESI aims to measure BAO and the growth of structure through redshift-space distortions with a wide-area spectroscopic survey.
• Targets include four classes: luminous red galaxies up to z=1, bright [O II] emission-line galaxies up to z=1.7, quasars for direct tracing and Ly-α forest up to z~3.5, and a Bright Galaxy Survey at z~0.2 when observing conditions are poor.
• The survey plans to obtain over 30 million galaxy and quasar redshifts to map the BAO feature and determine the matter power spectrum.
• Quasars are used both as tracers of the underlying dark matter and as backlights for Ly-α forest analyses, enabling high-redshift measurements.
• The design integrates target selection, survey strategy, and data products to optimize measurements of cosmic expansion and structure growth.