The SDSS-IV extended Baryonic Oscillation Spectroscopic Survey: Luminous Red Galaxy Target Selection

TL;DR

This study presents a robust, multi‑wavelength approach to selecting Luminous Red Galaxies for the SDSS‑IV/eBOSS survey, combining SDSS optical imaging with WISE infrared data to efficiently target galaxies at $0.6<z<1.0$ for precise BAO and RSD measurements. The authors detail an algorithm that uses updated photometric calibrations, forced WISE photometry, and carefully designed color cuts (notably $r-i>0.98$, $r-W1>2(r-i)$, and $i-z>0.625$) to minimize stellar contamination while meeting a target density of $\sim50\ \mathrm{deg}^{-2}$. They assess redshift reliability using SDSS/BOSS pipelines plus visual checks, finding $\sim89\%$ secure redshifts among inspected targets, with $68$–$72\%$ of targets yielding robust $0.6<z<1.0$ redshifts in practice and $9\%$ stellar contamination; homogeneity tests show strong uniformity in much of the footprint but some degradation in certain regions due to zero‑point and extinction systematics. The work demonstrates the feasibility and limitations of achieving a uniform, high‑redshift LRG sample and lays groundwork for precise BAO/RSD analyses with eBOSS, complemented by higher‑redshift tracers and galaxy evolution studies.

Abstract

We describe the algorithm used to select the Luminous Red Galaxy (LRG) sample for the extended Baryon Oscillation Spectroscopic Survey (eBOSS) of the Sloan Digital Sky Survey IV (SDSS-IV) using photometric data from both the SDSS and the Wide-Field Infrared Survey Explorer (WISE). LRG targets are required to meet a set of color selection criteria and have z-band and i-band MODEL magnitudes z < 19.95 and 19.9 < i < 21.8, respectively. Our algorithm selects roughly 50 LRG targets per square degree, the great majority of which lie in the redshift range 0.6 < z < 1.0 (median redshift 0.71). We demonstrate that our methods are highly effective at eliminating stellar contamination and lower-redshift galaxies. We perform a number of tests using spectroscopic data from SDSS-III/BOSS to determine the redshift reliability of our target selection and its ability to meet the science requirements of eBOSS. The SDSS spectra are of high enough signal-to-noise ratio that at least 89% of the target sample yields secure redshift measurements. We also present tests of the uniformity and homogeneity of the sample, demonstrating that it should be clean enough for studies of the large-scale structure of the universe at higher redshifts than SDSS-III/BOSS LRGs reached.

Figures (10)

• Figure 1: Optical/infrared color-color plot for galaxies observed by WISE and CFHT LS with photometric redshifts from the COSMOS survey. Blue symbols represent galaxies with photometric redshifts of z $<$ 0.6, red diamonds represent galaxies at 0.6 $<$ z $<$ 1.0, and cyan triangles represent galaxies at z $>$ 1.0. Stars are represented by green diamonds. The triangular area depicts the broad selection presented in Equations \ref{['eqn:nom_cuts1']} and \ref{['eqn:nom_cuts2']}. Photometric redshifts are taken from the COSMOS photo-z catalog of ilbert_2008 and optical photometry is from the catalog of gwyn_2011, transformed to SDSS passbands. The conversion relation can be found at CFHT LS webpage.
• Figure 2: Plots of $r-i$, $i-z$ and $r-W1$ color as a function of redshift for 15,000 LRGs targeted via a BOSS ancillary program DR12, which utilized broader selection criteria than those used for eBOSS. The blue lines represent the cuts applied as part of the eBOSS target selection algorithm. Selecting objects with r-i $>$ 0.98, i-z $>$ 0.625 and $r-w1 >2*(r-i)$ rejects a significant number of $z < 0.6$ galaxies while missing relatively few $z > 0.6$ LRGs.
• Figure 3: Schematic flow chart for the eBOSS LRG target selection algorithm. All quantities are corrected for Galactic extinction. Refer to the text for a full description of all of the quantities shown in this figure.
• Figure 4: Redshift histogram of 2,119 visually inspected LRGs (blue bar) observed with eBOSS. The median redshift of confirmed galaxies is 0.712 (black line), with 9$\%$ stellar contamination (red bar). We use only objects with secure redshifts ($\texttt{z\_conf} > 1$) here.
• Figure 5: Representative spectra of galaxies from the eBOSS LRG sample, smoothed with a 21 pixel boxcar kernel. Shown are four LRGs covering the entire redshift regime of $0.6~\hbox{$\sim$}\hbox{$<$}$$\,z~\hbox{$\sim$}\hbox{$<$}$$\,1$. Flux errors are plotted in red while the template model fits are in blue. Black curves depict the observed spectra.