DESI Bright Galaxy Survey: Final Target Selection, Design, and Validation

Abstract: Over the next five years, the Dark Energy Spectroscopic Instrument (DESI) will use 10 spectrographs with 5000 fibers on the 4m Mayall Telescope at Kitt Peak National Observatory to conduct the first Stage-IV dark energy galaxy survey. At $z < 0.6$, the DESI Bright Galaxy Survey (BGS) will produce the most detailed map of the Universe during the dark energy dominated epoch with redshifts of >10 million galaxies over 14,000 deg$^2$. In this work, we present and validate the final BGS target selection and survey design. From the Legacy Surveys, BGS will target a $r < 19.5$ magnitude-limited sample (BGS Bright); a fainter $19.5 < r < 20.175$ sample, color-selected to have high redshift efficiency (BGS Faint); and a smaller low-z quasar sample. BGS will observe these targets using exposure times, scaled to achieve uniform completeness, and visit each point on the footprint three times. We use observations from the Survey Validation programs conducted prior to the main survey along with realistic simulations to show that BGS can complete its strategy and make optimal use of `bright' time. We demonstrate that BGS targets have stellar contamination <1% and that their densities do not depend strongly on imaging properties. We also confirm that BGS Bright will achieve >80% fiber assignment efficiency. Finally, we show that BGS Bright and Faint will achieve >95% redshift success rates with no significant dependence on observing conditions. BGS meets the requirements for an extensive range of scientific applications. BGS will yield the most precise Baryon Acoustic Oscillations and Redshift-Space Distortions measurements at $z < 0.4$. It also presents opportunities to exploit new methods that require highly complete and dense galaxy samples (e.g. N-point statistics, multi-tracers). BGS further provides a powerful tool to study galaxy populations and the relations between galaxies and dark matter.

Overview of the DESI Bright Galaxy Survey: Final Target Selection, Design, and Validation

The paper "DESI Bright Galaxy Survey: Final Target Selection, Design, and Validation" presents a comprehensive overview of the methodology and strategy behind the DESI (Dark Energy Spectroscopic Instrument) Bright Galaxy Survey (BGS). This survey component is a critical element of DESI's ambitious plan to map the large-scale structure of the universe by acquiring redshifts of over 10 million galaxies at $z < 0.6$. This survey aims to deliver a detailed map of the universe during the epoch dominated by dark energy, assembling the most significant low-redshift sample observed to date.

Target Selection and Survey Strategy

The target selection utilizes data from the ninth Data Release (DR9) of the Legacy Surveys, specifically targeting galaxies using a combination of magnitude limits and color criteria to ensure high redshift efficiency and low contamination from stars. The BGS is divided into three main samples:

1. BGS Bright: A magnitude-limited sample ($r < 19.5$) ensuring a dense and complete spectroscopic sample of galaxies.
2. BGS Faint: Consists of galaxies in the range $19.5 < r < 20.175$, selected with refined color criteria to maintain high redshift efficiency.
3. BGS AGN: Includes lower-redshift quasar samples to enhance the completeness of AGN and quasar samples within the dataset.

The survey design and target selection were validated through extensive simulations and a Survey Validation phase, which demonstrated the methodologies could achieve greater than 95% redshift success rates with excellent celestial object discrimination capabilities.

Survey Implementation and Goals

The implementation of DESI's BGS is designed to optimize the use of bright time conditions. The survey strategically utilizes 5000 fibers distributed across 10 spectrographs on the 4-meter Mayall Telescope at Kitt Peak National Observatory. Each fiber observes a segment of the sky to collect galaxy spectra that span a wavelength range from 360 nm to 980 nm. Scheduled for completion over five years, the BGS will cover approximately 14,000 square degrees of the sky.

The goals of this survey are to:

• Provide the most precise measurements of low-redshift Baryon Acoustic Oscillations (BAO) and Redshift Space Distortions (RSD), offering critical insights into the nature of dark energy.
• Enable new science through dense galaxy samples that improve small-scale clustering measurements, multi-tracer analyses, and studies of galaxy evolution and dark matter halo occupation.

Validation and Clustering Results

Validation of the survey's target selection and strategies was achieved through early spectroscopic observations during Survey Validation. These early results confirmed stellar contamination below 1% and high uniformity in target density, independent of imaging or observational conditions. The overall structure of the galaxy samples also confirmed the effectiveness of the fiber assignment strategy in optimizing the survey's efficiency and completeness.

Conclusions and Implications

The DESI Bright Galaxy Survey is poised to make a significant contribution to our understanding of the universe's large-scale structure and the dynamics of dark energy. By systematically mapping galaxies over a vast expanse of the sky with unprecedented detail and precision, the survey will offer vital information, enhancing cosmological constraints and providing datasets for further exploration of galaxy evolution, clustering, and systemic effects in cosmology. The strategies and design instantiated in this paper ensure that the BGS will deliver on its promise to refine our cosmic maps, enabling a deeper understanding of fundamental cosmic phenomena. This will also lay the groundwork for future advancements in the study of dark matter, cosmic acceleration, and other astrophysical inquiries.