Validation of the Scientific Program for the Dark Energy Spectroscopic Instrument

TL;DR

DESI's Survey Validation demonstrates that the instrument, target-selection algorithms, and data-processing pipeline can meet the stringent requirements for a five-year, 14{,}000 deg$^2$ BAO survey. By validating deep spectroscopy across MWS, BGS, LRG, ELG, and QSO tracers and optimizing exposure times, SV provides robust redshift distributions and near-final cosmology forecasts. The One-Percent Survey confirms high fiber-assignment and redshift-completeness across tracers, enabling early clustering measurements and mock-catalog calibration. Cosmological forecasts indicate DESI will deliver sub-percent BAO precision and competitive growth-rate measurements, yielding a DETF FoM well above SDSS and establishing DESI as a leading Stage-IV dark-energy experiment. These results underpin the five-year plan and data-release strategy for the DESI collaboration, with broad implications for cosmology and Milky Way science.

Abstract

The Dark Energy Spectroscopic Instrument (DESI) was designed to conduct a survey covering 14,000 deg$^2$ over five years to constrain the cosmic expansion history through precise measurements of Baryon Acoustic Oscillations (BAO). The scientific program for DESI was evaluated during a five month Survey Validation (SV) campaign before beginning full operations. This program produced deep spectra of tens of thousands of objects from each of the stellar (MWS), bright galaxy (BGS), luminous red galaxy (LRG), emission line galaxy (ELG), and quasar target classes. These SV spectra were used to optimize redshift distributions, characterize exposure times, determine calibration procedures, and assess observational overheads for the five-year program. In this paper, we present the final target selection algorithms, redshift distributions, and projected cosmology constraints resulting from those studies. We also present a `One-Percent survey' conducted at the conclusion of Survey Validation covering 140 deg$^2$ using the final target selection algorithms with exposures of a depth typical of the main survey. The Survey Validation indicates that DESI will be able to complete the full 14,000 deg$^2$ program with spectroscopically-confirmed targets from the MWS, BGS, LRG, ELG, and quasar programs with total sample sizes of 7.2, 13.8, 7.46, 15.7, and 2.87 million, respectively. These samples will allow exploration of the Milky Way halo, clustering on all scales, and BAO measurements with a statistical precision of 0.28% over the redshift interval $z<1.1$, 0.39% over the redshift interval $1.1<z<1.9$, and 0.46% over the redshift interval $1.9<z<3.5$.

Figures (18)

• Figure 1: The field centers for the fields designed to test MWS, BGS, LRG, ELG, and quasar selections and spectroscopic performance in the DESI Target Selection Validation program. The light gray regions show the full imaging footprint available from Bok and Mayall imaging while the dark gray regions show the full imaging footprint available from the DECam imaging. The black outline shows the footprint of the Dark Energy Survey (DES). Details on the imaging can be found in Section \ref{['sec:dr9']}.
• Figure 2: Example of the sky coverage of one DESI tile centered at ($\alpha$, $\delta$) = (0, 0). The white circles display the individual fiber patrol regions. Left: an image that spans four degrees on a side, illustrating the entire DESI focal plane. Right: the smaller region identified by the red square in the left panel. DESI Main Survey Dark Targets are circled (LRGs in red, ELGs in cyan, and quasars in yellow). The background imaging is a $grz$-band composite image from the DESI Legacy Surveys. The region was chosen to demonstrate that some fibers have limited number of targets accessible while others can be heavily oversubscribed. For example, even though this region will be covered by five or more tiles, the six ELG targets accessible to the central fiber close to the bottom of the image are unlikely to be observed because there is a quasar and seven LRG targets also within reach. There are also regions that are not accessible to fibers at all. For example, the hole near the middle of the right panel is the location of a fiducial that is used to calibrate the focal plane coordinate system.
• Figure 3: The distribution of stellar targets for the Milky Way Survey program as a function of color and proper motion. The two density peaks correspond to the thin disk (redder colors, higher proper motions) and the metal-poor halo and thick disk (bluer colors, lower proper motions). The blue-, red- and green-shaded regions indicate the three primary MWS target classes. All stars in the magnitude range $16 < r < 19$ are selected in one of these three categories. We do not apply any proper motion selection to stars bluer than $g-r < 0.7$ (MWS Main Blue, blue region). We divide redder stars ($g-r>0.7$) into MWS Main Broad (higher proper motion, green region) and MWS Main Red (lower proper motion, red region) using a magnitude-dependent threshold, shown by the hatched region. We give MWS Main Red stars the same fiber assignment priority as those in the MWS Main Blue sample, because they are more likely to be distant giant stars in the stellar halo. Conversely, we give MS Main Broad stars a lower fiber assignment priority. We use a more stringent proper motion threshold for fainter stars because true giants at larger distances have lower proper motions: the fiber assignment priority of a larger fraction of nearby disk stars can then be reduced without introducing a bias against high velocity giants.
• Figure 4: Representation of the target selection algorithm for the BGS program. Left panel: Star-galaxy separation is performed using a $G_{Gaia} - r> 0.6$ cut (black dashed line) using $Gaia$ and Legacy Survey photometry. Middle panel: The BGS Bright sample (blue) is identified using the boundaries shown by the dashed lines in the $r$ and $r_{\rm fiber}$ magnitudes. No object fainter than $r_{\rm fiber} = 22.9$ is included in the BGS Bright sample. Right panel: The BGS Faint sample (orange) includes objects fainter than BGS Bright, $19.5 < r < 20.175$, passing the ($r_{\rm fiber}$ and $(z - W1) - 1.2 (g - r) + 1.2$) cuts, illustrated by the black dashed lines.
• Figure 5: Selection boundaries for the LRG targets in the DECaLS footprint. Redshifts are color-coded using the DESI spectroscopic redshifts. The upper left panel shows the stellar rejection cut, with point sources (almost all of which are stars) in gray. The upper right panel shows the cuts that remove lower-redshift galaxies and bluer galaxies. The lower left panel shows the color-magnitude cut that shapes the redshift distribution. The lower right panel shows the magnitude limit in $z$-band fiber magnitude that ensures sufficient signal-to-noise for DESI spectroscopic observations. The objects along the diagonal line are classified as point sources in the imaging and have fixed fiber-flux to total flux ratio.