Target Selection and Validation of DESI Luminous Red Galaxies

Abstract

The Dark Energy Spectroscopic Instrument (DESI) is carrying out a 5-year survey that aims to measure the redshifts of tens of millions of galaxies and quasars, including 8 million luminous red galaxies (LRGs) in the redshift range of 0.4 < z < ∼ 1.0. Here we present the selection of the DESI LRG sample and assess its spectroscopic performance using data from Survey Validation (SV) and the first 2 months of the Main Survey. The DESI LRG sample, selected using g, r, z and W 1 photometry from the DESI Legacy Imaging Surveys, is highly robust against imaging systematics. The sample has a target density of 605 deg −2 and a comoving number density of 5 × 10 −4 h 3 Mpc −3 in 0.4 < z < 0.8; this is significantly higher density than previous LRG surveys (such as SDSS, BOSS and eBOSS) while also extends to much higher redshifts. After applying a bright star veto mask developed for the sample, 98.9% of the observed LRG targets yield confident redshifts (with a catastrophic failure rate of 0.2% in the confident redshifts), and only 0.5% of the LRG targets are stellar contamination. The LRG redshift efficiency varies with source brightness and effective exposure time, and we present a simple model that accurately characterizes this dependence. In the appendices, we describe the extended LRG samples observed during SV.

The DESI Survey Validation: Results from Visual Inspection of Bright Galaxies, Luminous Red Galaxies, and Emission Line Galaxies

“…We use the VI galaxy catalogs obtained from DESI spectra with long exposure times to validate the performance of the algorithm, Redrock (Bailey et al 2022), that DESI adopts for recovering the redshifts of galaxies at the designed nominal exposure times, i.e. effective exposure time ∼ 1000s (800 < T eff < 1200s) for ELGs (Raichoor et al 2022a) and LRGs (Zhou et al 2022) and ∼ 180s (160 < T eff < 200s) for bright galaxies (Hahn et al 2022). For those galaxies with deep co-added spectra, we use all the data from individual exposures to produce several spectra with similar exposure times to the designed exposure time of the Main Survey.…”

“…Results for LRGs -Similarly to BGS, for LRGs, we include the redshift measurements from Redrock in the selection criteria (see also Zhou et al 2022): z Redrock < 1.5. The middle panel of Figure 7 shows the good redshift inclusion fraction and good redshift purity of LRGs as a function of log 10 ∆χ 2 and the vertical dashed line shows the current adopted value ∆χ 2 = 15.…”

“…Together with the target density and the fiber assignment rate, these redshift efficiency values are used to estimate the final galaxy densities with redshifts. The final estimations of galaxy densities are described in Dawson et al (2022) and the target selection papers of each tracer, Hahn et al (2022), Raichoor et al (2022a), andZhou et al (2022).…”

“…• Luminous red galaxies (LRGs) (Zhou et al 2022), a population of massive passive galaxies selected based on their colors and brightness. The DESI LRG sample has a significantly higher target density than that of any previous LRG surveys, including the SDSS LRG survey (Eisenstein et al 2001), Baryon Oscillation Spectroscopic Survey (BOSS) (Reid et al 2016) and extended Baryon Oscillation Spectroscopic Survey (eBOSS) (Prakash et al 2016), and also probes higher redshifts with a designed redshift coverage from z ∼ 0.4 to z ∼ 1.1.…”

The Target-selection Pipeline for the Dark Energy Spectroscopic Instrument

“…Target selection approaches for DESI are, themselves, varied and extensive. Other publications that accompany this work include a paper describing the DESI Survey Validation (SV) phase (DESI collaboration et al 2023a), two papers describing how visual inspection of spectra of targets acquired during SV produced truth tables to inform target selection for the DESI Main Survey (Alexander et al 2022;Lan et al 2022), and a series of five papers detailing the selection of DESI bright-time and dark-time science targets (Chaussidon et al 2022a;Zhou et al 2022;Raichoor et al 2022;Cooper et al 2022;Hahn et al 2022, see also §4.1.1).…”