The Dark Energy Spectroscopic Instrument (DESI) embarked on an ambitious 5 yr survey in 2021 May to explore the nature of dark energy with spectroscopic measurements of 40 million galaxies and quasars. DESI will determine precise redshifts and employ the baryon acoustic oscillation method to measure distances from the nearby universe to beyond redshift z > 3.5, and employ redshift space distortions to measure the growth of structure and probe potential modifications to general relativity. We describe the significant instrumentation we developed to conduct the DESI survey. This includes: a wide-field, 3.°2 diameter prime-focus corrector; a focal plane system with 5020 fiber positioners on the 0.812 m diameter, aspheric focal surface; 10 continuous, high-efficiency fiber cable bundles that connect the focal plane to the spectrographs; and 10 identical spectrographs. Each spectrograph employs a pair of dichroics to split the light into three channels that together record the light from 360–980 nm with a spectral resolution that ranges from 2000–5000. We describe the science requirements, their connection to the technical requirements, the management of the project, and interfaces between subsystems. DESI was installed at the 4 m Mayall Telescope at Kitt Peak National Observatory and has achieved all of its performance goals. Some performance highlights include an rms positioner accuracy of better than 0.″1 and a median signal-to-noise ratio of 7 of the [O ii] doublet at 8 × 10−17 erg s−1 cm−2 in 1000 s for galaxies at z = 1.4–1.6. We conclude with additional highlights from the on-sky validation and commissioning, key successes, and lessons learned.

The Dark Energy Spectroscopic Instrument (DESI) Survey has obtained a set of spectroscopic measurements of galaxies to validate the final survey design and target selections. To assist in these tasks, we visually inspect DESI spectra of approximately 2500 bright galaxies, 3500 luminous red galaxies (LRGs), and 10,000 emission-line galaxies (ELGs) to obtain robust redshift identifications. We then utilize the visually inspected redshift information to characterize the performance of the DESI operation. Based on the visual inspection (VI) catalogs, our results show that the final survey design yields samples of bright galaxies, LRGs, and ELGs with purity greater than 99%. Moreover, we demonstrate that the precision of the redshift measurements is approximately 10 km s−1 for bright galaxies and ELGs and approximately 40 km s−1 for LRGs. The average redshift accuracy is within 10 km s−1 for the three types of galaxies. The VI process also helps improve the quality of the DESI data by identifying spurious spectral features introduced by the pipeline. Finally, we show examples of unexpected real astronomical objects, such as Lyα emitters and strong lensing candidates, identified by VI. These results demonstrate the importance and utility of visually inspecting data from incoming and upcoming surveys, especially during their early operation phases.

We study how the cosmic environment affects galaxy evolution in the Universe by comparing the metallicities of dwarf galaxies in voids with dwarf galaxies in more dense regions. Ratios of the fluxes of emission lines, particularly those of the forbidden [O III] and [S II] transitions, provide estimates of a region's electron temperature and number density. From these two quantities and the emission line fluxes [O II] λ3727, [O III] λ4363, and [O III] λλ4959, 5007, we estimate the abundance of oxygen with the Direct T e method. We estimate the metallicity of 42 blue, star-forming void dwarf galaxies and 89 blue, star-forming dwarf galaxies in more dense regions using spectroscopic observations from the Sloan Digital Sky Survey Data Release 7, as re-processed in the MPA-JHU value-added catalog. We find very little difference between the two sets of galaxies, indicating little influence from the large-scale environment on their chemical evolution. Of particular interest are a number of extremely metal-poor dwarf galaxies that are less prevalent in voids than in the denser regions.

We study how the void environment affects the formation and evolution of galaxies in the universe by comparing the ratio of dark matter halo mass to stellar mass of galaxies in voids with galaxies in denser regions. Using spectroscopic observations from the SDSS MaNGA DR15, we estimate the dark matter halo mass of 641 void galaxies and 937 galaxies in denser regions. We use the velocity from the Hα emission line to measure the rotation curve of the galaxies since the kinematics of the interstellar medium is smoother than the stellar kinematics. We find that neither the stellar-to-halo-mass relation nor the relationship between the gas-phase metallicity and the ratio of dark matter halo mass to stellar mass is affected by the void environment. We also observe no difference in the distribution of the ratio of dark matter halo mass to stellar mass between void galaxies and galaxies in denser regions, implying that the shape of the dark matter halo profile is independent of a galaxy's environment.

We study how the void environment affects the chemical evolution of galaxies in the universe by comparing the oxygen and nitrogen abundances of dwarf galaxies in voids with dwarf galaxies in denser regions. Using spectroscopic observations from the Sloan Digital Sky Survey Data Release 7, we estimate the oxygen, nitrogen, and neon abundances of 889 void dwarf galaxies and 672 dwarf galaxies in denser regions. We use the Direct T e method for calculating the gas-phase chemical abundances in the dwarf galaxies because it is best suited for low-metallicity, low-mass (dwarf) galaxies. A substitute for the [O II] λ3727 doublet is developed, permitting oxygen abundance estimates of SDSS dwarf galaxies at all redshifts with the Direct T e method. We find that void dwarf galaxies have about the same oxygen abundance and Ne/O ratio as dwarf galaxies in denser environments. However, we find that void dwarf galaxies have slightly higher neon (∼10%) abundances than dwarf galaxies in denser environments. The opposite trend is seen in both the nitrogen abundance and N/O ratio: void dwarf galaxies have slightly lower nitrogen abundances (∼5%) and lower N/O ratios (∼7%) than dwarf galaxies in denser regions. Therefore, we conclude that the void environment has a slight influence on dwarf galaxy chemical evolution. Our mass-N/O relationship shows that the secondary production of nitrogen commences at a lower stellar mass in void dwarf star-forming galaxies than in dwarf star-forming galaxies in denser environments. We also find that star-forming void dwarf galaxies have higher H I masses than the star-forming dwarf galaxies in denser regions. Our starforming dwarf galaxy sample demonstrates a strong anti-correlation between the sSFR and N/O ratio, providing evidence that oxygen is produced in higher-mass stars than those which synthesize nitrogen. The lower N/O ratios and smaller stellar mass for secondary nitrogen production seen in void dwarf galaxies may indicate both delayed star formation as predicted by ΛCDM cosmology and a dependence of cosmic downsizing on the large-scale environment. A shift toward slightly higher oxygen abundances and higher H I masses in void dwarf galaxies could be evidence of larger ratios of dark matter halo mass to stellar mass in voids compared with denser regions.