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 equipped 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 spanning 14,000 deg 2 . In this work, we present and validate the final BGS target selection and survey design. From DR9 of 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 AGN). BGS will observe these targets using exposure times that are dynamically scaled to achieve homogeneous completeness and visit each point of the footprint three times on average. We use early spectroscopic observations from the Survey Validation programs conducted prior to the main survey along with realistic simulations to show that BGS can successfully complete this strategy and make optimal use of 'bright' time, when the moon is above the horizon. Specifically, we demonstrate that BGS targets have stellar contamination below 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 the BGS Bright and Faint samples will achieve >95% redshift success rates across a broad range of galaxies, with no significant dependence on observing conditions. Overall, BGS meets the requirements necessary for an extensive range of scientific applications. BGS will yield the most precise Baryon Acoustic Oscillations and Redshift-Space Distortions (RSD) measurements at z < 0.4 to date. It also presents unique opportunities to exploit new methods that require highly complete and dense galaxy samples (e.g. N -point statistics, multi-tracer RSD). BGS further provides a powerful tool to study galaxy populations including dwarf galaxies, galaxy groups and clusters, and the relations between galaxies and dark matter.

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

“…In the Main Survey, two samples, BGS bright and BGS faint, will be selected (Hahn et al 2022). The VI quality distributions for the BGS bright and faint targets satisfying the Main Survey selection are shown in black and purple respectively.…”

“…This is expected given that the BGS bright sample is selected based on a minimum set of simple selection criteria for cosmological analysis. On the other hand, the BGS faint sample is designed to use special color-magnitude selection crite-ria to maximize the redshift success rate of the sample (Hahn et al 2022).…”

“…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 BGS -To estimate the redshift purity and inclusion fraction, we include redshift measurements from Redrock in the selection criteria (see also Hahn et al 2022): z Redrock < 0.6. The top panel of Figure 7 shows the good redshift inclusion fraction and good redshift purity for BGS as a function of ∆χ 2 .…”

“…• Bright galaxies (bright galaxy survey, BGS) (Hahn et al 2022), a galaxy sample whose redshifts can be obtained during the bright time of the observations together with the Milky Way stars (Cooper et al 2022). The BGS consists of two selections, (1) BGS bright -a magnitude limit sample with r < 19.5, which is similar to the depth of the Galaxy And Mass Assembly (GAMA) survey (Driver et al 2011) and about 2 magnitude deeper than the main galaxy sample from the Sloan Digital Sky Survey (Strauss et al 2002), and (2) BGS faint -a sample of fainter 19.5 < r < 20.175 galaxies with a color-selection to achieve high redshift efficiency.…”