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

TL;DR

DESI's Bright Galaxy Survey final target selection and survey design are presented and validated using Survey Validation data and realistic simulations. The BGS comprises Bright, Faint, and AGN samples drawn from Legacy Surveys DR9 and supplementary catalogs to deliver a dense, high-completeness low-redshift galaxy census over 14,000 deg^2, enabling precise BAO and RSD measurements at $z<0.4$ and enabling multi-tracer and small-scale clustering analyses. The study demonstrates stellar contamination below 1%, fiber assignment efficiency above 80%, and redshift success above 95% across a wide range of observing conditions through carefully designed target cuts and exposure strategies. The Early Data Release demonstrates the approach's success and shows BGS will yield >10 million spectra in 5 years, providing a rich resource for galaxy evolution, dark matter–baryon connections, and tests of gravity at low redshift.

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.

Figures (22)

• Figure 1: The 14,000 ${\rm deg}^2$ footprint for the DESI Bright Galaxy Survey (color map). Imaging from the Legacy Surveys DR9 allows the selection of BGS targets over a larger area, approximately 20,000 deg$^2$ (gray). The color map represents the density of BGS Bright targets (Section \ref{['sec:select']}). During the Survey Validation phases of operations, DESI observed SV1 and the One-Percent Survey to optimize and validate the BGS target selection and survey performance. We mark the tiles observed during SV1 and the One-Percent Survey in red and orange.
• Figure 2: Star-galaxy separation in BGS is performed using a $G_{Gaia} - r_{\rm raw}$ cut. This criterion exploits the fact that the Gaia magnitude is measured from space with a diffraction-limited PSF while the LS $r$ magnitude captures the light from the entire source. LS objects (grey) with $G_{Gaia} - r_{\rm raw} > 0.6$ (black dashed) or objects not in Gaia are classified as galaxies. $G_{Gaia}$ is the $G$-band magnitude from Gaia DR2; $r_{\rm raw}$ is the LS $r$-band magnitude without Galactic extinction correction. In the center and right panels, we present in the color maps the stellar contamination fraction for SV1 and the One-Percent Survey respectively. The contamination fraction is estimated based on Redrock spectral type classification and a minimum redshift cut, $z > 300$ km/s. We only include hexbins with at least 10 BGS targets. Stellar contamination is negligible ($<1\%$) above our star-galaxy separation threshold.
• Figure 3: Targets for the BGS Bright sample are chosen based on the selection cuts described in Section \ref{['sec:select']} and a $r < 19.5$ magnitude cut. In the left panel, we show these cuts (based on fiber magnitude) and the $r < 19.5$ cut (black dashed) on the distribution of $r$ versus $r_{\rm fiber}$ magnitude for LS objects that pass our star-galaxy selection (grey). The contours mark the 11.7, 39.3, 67.5, and 86.4 percentiles of the distribution (dotted). We also include the $r$ and $r_{\rm fiber}$ cuts for the BGS Faint sample (dot dashed). We impose selection cuts on BGS targets in order to minimize the number of spurious objects and mitigate any systematic effects that can affect galaxy clustering analyses. In the right panel, we present the target density of the BGS Bright targets (color map). In total, we have 864 targets/${\rm deg}^{2}$ for the BGS Bright sample.
• Figure 4: BGS Faint targets include objects fainter than BGS Bright, $19.5 < r < 20.175$, that are within custom $r_{\rm fiber}$ - color selection cuts (Eq. \ref{['eq:rfib_color_cut']}). In the left panel, we show the $r_{\rm fiber}$ - color cut (black dashed) on the $r_{\rm fiber}$ versus $(z - W1) - 1.2 (g - r) + 1.2$ distribution of LS objects that pass our star-galaxy separation (grey). The contours mark the 11.7, 39.3, 67.5, and 86.4 percentiles of the distribution (dotted). $(z - W1) - 1.2 (g - r) + 1.2$ is a proxy for the strength of H$\alpha$ and H$\beta$, so BGS Faint targets either have bright fiber magnitudes or strong emission lines. We impose the $r_{\rm fiber}$ - color cut in order to maintain high redshift efficiency for BGS Faint. In the right panel, we present the target density of the BGS Faint targets (color map). In total, we have 533 targets/${\rm deg}^{2}$ for the BGS Faint sample.
• Figure 5: Redshift success rate of spectral simulations run using a nominal exposure time of $t_{\rm nom} = 180$s (blue) as a function of $r$-band magnitude. We include $z$ success rates for spectral simulations run using $t_{\rm nom} = 160$s (orange dashed) and $240$s (green dashed) for comparison. The overall $z$ success rates of all $r < 19.5$ galaxies are presented in the legend. These simulations assume spectra based on realistic continuum templates derived from AGES, matched to $g$, $r$, $z$ LS photometry, and GAMA emission line fluxes. They incorporate realistic noise and throughput for BGS observations. With $t_{\rm nom} = 180{\rm s}$, we predict that the BGS Bright sample can achieve an overall redshift success rate of $95\%$.