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

Abstract

The Dark Energy Spectroscopic Instrument (DESI) Survey has obtained a set of spectroscopic measurements of galaxies for validating the final survey design and target selections. To assist these tasks, we visually inspect (VI) DESI spectra of approximately 2,500 bright galaxies, 3,500 luminous red galaxies, and 10,000 emission line galaxies, to obtain robust redshift identifications. We then utilize the VI redshift information to characterize the performance of the DESI operation. Based on the VI catalogs, our results show that the final survey design yields samples of bright galaxies, luminous red galaxies, and emission line galaxies with purity greater than 99%. Moreover, we demonstrate that the precision of the redshift measurements is approximately 10 km/s for bright galaxies and emission line galaxies and approximately 40 km/s for luminous red galaxies. The average redshift accuracy is within 10 km/s for the three types of galaxies. The VI process also helps to 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 Lyman α 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.

Target Selection and Validation of DESI Emission Line Galaxies

“…The dv distribution is reported in the bottom panel of Figure 16, and can be reasonably modeled by the weighted sum of two Gaussian distributions centered on zero with widths of 5 and 20 km.s −1 . Eventually, if we use the same statistical measurement as Lan et al (2022) we measure for the ELG LOP (ELG VLO) sample MAD(dv) × 1.48/ √ 2 ∼ 7 km.s −1 (9 km.s −1 ), where MAD is the median absolute deviation, in agreement with Lan et al (2022). This z spec precision is the best among DESI tracers (Lan et al 2022;Alexander et al 2022), as the z spec is based on sharp emission lines.…”

“…Eventually, if we use the same statistical measurement as Lan et al (2022) we measure for the ELG LOP (ELG VLO) sample MAD(dv) × 1.48/ √ 2 ∼ 7 km.s −1 (9 km.s −1 ), where MAD is the median absolute deviation, in agreement with Lan et al (2022). This z spec precision is the best among DESI tracers (Lan et al 2022;Alexander et al 2022), as the z spec is based on sharp emission lines. We present the fraction of ELG spectra which is selected by our reliability criterion from Equation 1.…”

“…four times the nominal Main survey EFFTIME SPEC, so that the observations provide secure data to study the faint end of the explored samples. Three of those tiles have much higher EFFTIME SPEC (7,000 -15,000s) and were used to build a truth table of about ten thousand ELG spectra with Visual Inspection (VI) (Lan et al 2022).…”

“…DESI Collaboration et al (2022) present an overview of the DESI spectroscopic observations and the tracers used by those papers, and Myers et al (2022) present how those target selections are implemented in DESI. Lan et al (2022) and Alexander et al (2022) present the construction of spectroscopic truth tables based from visual inspection (VI) for the galaxies (BGS, LRG, ELG) and the QSO targets, respectively. The MWS sample is presented in Cooper et al (2022), the BGS sample is presented in Hahn et al (2022), the LRG sample is presented in Zhou et al (2022), the ELG sample is presented in this paper, and the QSO sample is presented in Chaussidon et al (2022).…”

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).…”