Target Selection and Validation of DESI Emission Line Galaxies

TL;DR

This paper presents the DESI Emission Line Galaxy target selection and validation framework for both the SV1 and Main Survey. It defines two Main ELG subsamples (ELG_LOP and ELG_VLO) using a g-fiber magnitude cut and a g-r vs r-z color box, supported by Legacy Survey imaging and Tractor photometry, with a third ELG_HIP subsample to aid cross-correlation studies. The SV1 program expands the photometric space to test reliability and refine the Main selection, including extensions in color space and faint targets. Using DESI spectroscopic data from SV1, One-Percent, and Main surveys, the authors establish a reliability criterion based on [O II] flux and DELTACHI2, quantify catastrophic redshift rates, and evaluate redshift-efficiency and distributions across footprints. The results show that the Main ELG selection can achieve >400 reliable z per deg^2 in both 0.6<z<1.1 and 1.1<z<1.6, confirming DESI’s capability to map large-scale structure with high density ELG tracers while highlighting areas for continued calibration against imaging systematics and sky-subtraction challenges.

Abstract

The Dark Energy Spectroscopic Instrument (DESI) will precisely constrain cosmic expansion and the growth of structure by collecting $\sim$40 million extra-galactic redshifts across $\sim$80\% of cosmic history and one third of the sky. The Emission Line Galaxy (ELG) sample, which will comprise about one-third of all DESI tracers, will be used to probe the Universe over the $0.6 < z < 1.6$ range, which includes the $1.1<z<1.6$ range, expected to provide the tightest constraints. We present the target selection of the DESI SV1 Survey Validation and Main Survey ELG samples, which relies on the Legacy Surveys imaging. The Main ELG selection consists of a $g$-band magnitude cut and a $(g-r)$ vs.\ $(r-z)$ color box, while the SV1 selection explores extensions of the Main selection boundaries. The Main ELG sample is composed of two disjoint subsamples, which have target densities of about 1940 deg$^{-2}$ and 460 deg$^{-2}$, respectively. We first characterize their photometric properties and density variations across the footprint. Then we analyze the DESI spectroscopic data obtained since December 2020 during the Survey Validation and the Main Survey up to December 2021. We establish a preliminary criterion to select reliable redshifts, based on the \oii~flux measurement, and assess its performance. Using that criterion, we are able to present the spectroscopic efficiency of the Main ELG selection, along with its redshift distribution. We thus demonstrate that the the main selection with higher target density sample should provide more than 400 deg$^{-2}$ reliable redshifts in both the $0.6<z<1.1$ and the $1.1<z<1.6$ ranges.

Figures (20)

• Figure 1: Sky maps of the Galactic dust extinction (top), the imaging $g$-band depth (middle), and the extinction-corrected $g$-band depth (bottom) for the imaging data used to select the DESI ELG targets. Depths are 5$\sigma$ depths for a 0.45 arcsec radius exponential profile, typical of DESI ELG targets. The thick black line represents a 14 000 deg$^2$ footprint covered by DESI. The Galactic plane is displayed as a solid line and the Sagittarius plane as a dashed line.
• Figure 1: Comparison of the HSC/DR2 $z_{\rm phot}$ to the DESI Main Survey ELG $z_{\rm spec}$ measurements, using $\sim$175 thousand matched targets with a $z_{\rm spec}$ passing our reliability criterion of Equation \ref{['eq:zcrit']}. Lef-hand panels: scatter plots color-coded according to the average HSC risk parameter (see text); the solid lines in the bottom panel are the median values of $\Delta z = (z_{\rm phot} - z_{\rm spec}) / (1 + z_{\rm spec})$ for three subsamples: all the matches (black), the 50 percent lower risk parameter values (cyan), and the 25 percent lower risk parameter values (red); the shaded regions show the $1.48 \times \rm MAD(\Delta z)$ values. Right-hand panels: redshift distributions for those three subsamples; in each panel, the filled (empty, respectively) histogram is for the DESI $z_{\rm spec}$ (HSC $z_{\rm phot}$, respectively). The DESI $z_{\rm spec}$ distribution with no cut on risk is displayed with a black, hatched histogram.
• Figure 2: Cumulative distribution of the logarithm of the photometric SNR (see text) in the selection band for the DESI Main Survey dark tracers over the nominal DESI footprint. For the ELG targets, the two Main Survey selections are shown.
• Figure 3: Main ELG target selection cuts in the $g-r$ vs. $r-z$ diagram. The ELG_LOP and the ELG_VLO selections are displayed as solid black lines and dashed red lines, respectively. The background symbols describe a $g_{\rm fib} < 24.1$ sample; the color-coding indicates the mean HSC/DR2 $z_{\rm phot}$ measurements and the transparency scales with the logarithm of the density. The thin black contour indicates the stellar locus. Tracks show stellar evolutions for a highly star-forming (green solid line) and a less star-forming (red dashed line) galaxy observed at $z_{\rm spec}$ 2.0, 1.6, 1.1, 0.6, 0.1 (square, down triangle, circle, up triangle, star, respectively), see text for more details.
• Figure 4: SV1 ELG target selection cuts in the $g-r$ vs. $r-z$ diagram (top) and $(g-r) + 1.2 \times (r-z)$ vs $g_{\rm fib}$ diagram (bottom). The ELG_LOP and the ELG_VLO selections are displayed as solid black lines and dashed red lines, respectively. The background symbols describe a $g_{\rm fib} < 25.0$ sample; the color-coding indicates the mean HSC/DR2 $z_{\rm phot}$ measurements and the transparency scales with the logarithm of the density. Contour and tracks are as in Figure \ref{['fig:maingrz']}.