Dark Energy Survey Year 6 Results: MagLim++ Lens Sample Selection and Measurements of Galaxy Clustering

TL;DR

This work presents MagLim++, a refined DES Year 6 galaxy sample optimized for cosmological clustering analyses. It combines a bin-adaptive SOM-based color-space cut and a bin-specific star-galaxy separation with two complementary regression-based corrections (ENet and ISD) to robustly model residual selection-function systematics. The authors measure the two-point angular clustering $w(\theta)$ for 9,186,205 galaxies over 4,031 deg$^2$ across six tomographic bins with high signal-to-noise, and place cosmological constraints on $\Omega_m$ and $b_i\sigma_8$ using $w(\theta)$ alone and in combination with galaxy-galaxy lensing and cosmic shear data (3×2pt). They demonstrate that MagLim++ achieves strong suppression of additive contamination while maintaining sensitivity to large-scale structure, and they quantify systematic uncertainties through extensive null tests, mock-based debiasing, and external-LSS tracer cross-checks. The resulting constraints, including $\Omega_m = 0.311^{+0.023}_{-0.035}$ and per-bin $b_i\sigma_8$ values, illustrate robust, scalable performance in a photometric clustering regime relevant for future LSST/Euclid-like surveys.

Abstract

Galaxy clustering is a sensitive probe of the expansion history and growth of structure of the universe, and key degeneracies can be broken by combining these data with measurements of cosmic shear and galaxy-galaxy lensing (a so-called 3$\times$2pt analysis). The largest and least biased statistical samples of galaxies for use in clustering analyses can be collected photometrically through large imaging surveys. However, selecting clean photometric subsamples for cosmology are crucial for avoiding contamination that can bias cosmological constraints. Here we present the MagLim++ galaxy sample, selected to optimize for cosmological constraining power and incorporating an array of novel quality cuts to identify and remove residual contamination. This sample comes from the full six years of observations from the Dark Energy Survey. We present measurements of the two-point angular clustering ($w(θ)$) of 9,186,205 galaxies distributed over 4031 sq. degrees and in six tomographic redshift bins centered at $\bar{z}\approx$ [0.31, 0.44, 0.62, 0.78, 0.90, 1.01]. These measurements are used as part of the 3$\times$2pt and other DES Y6 legacy cosmological analyses in companion works. We describe the battery of null tests and mitigation schemes implemented to address observational, astrophysical, and methodological systematics in the analysis. The resulting $w(θ)$ measurements have a S/N = 149 (90.2 for linear scales only), which we use to place galaxy-clustering-only constraints on the matter density of the Universe, $Ω_m=0.311^{+0.023}_{-0.035}$, and amplitude of galaxy clustering in each redshift bin, $b_iσ_8=[1.16^{+0.04}_{-0.06},\ 1.40^{+0.04}_{-0.06},\ 1.57^{+0.04}_{-0.06},\ 1.59^{+0.04}_{-0.05},\ 1.50^{+0.04}_{-0.05},\ 1.74^{+0.06}_{-0.08}]$.

Figures (24)

• Figure 1: Distribution of MagLim++ galaxies, selected via a photo-$z$-dependent $i$-band limiting magnitude given by Eq. \ref{['eq:ilim_maglim']}
• Figure 2: Redshift distribution of MagLim++ galaxies, as estimated in y6-lenspz. Bin 2 is dashed since $\gamma_t$ and $w(\theta)$ measurements using this bin are discarded in the Y6 3$\times$2pt cosmology analyses in order to meet unblinding criteria (c.f. y6-3x2pt).
• Figure 3: Color distributions of MagLim galaxies in each redshift bin, along with the star-galaxy cut (red) optimized to isolate and remove residual stellar contamination. Iso-density contours are $[25, 50, 75, 90, 95, 97, 98] \%$-iles, and points subsampled by a factor of 200.
• Figure 4: Example of how the density of objects in as a function of stellar density changes as one moves from the base Y6 MagLim sample (blue), to applying bin-optimized star-galaxy separation (orange), to additionally applying photo-$z$ quality cuts to get the final MagLim++ sample (green). Once stellar contamination is removed, the impacts of stellar obscuration becomes apparent. Density is normalized to the average density for MagLim++, and no galaxy weights have been applied. Gray bars indicate the area distribution of stellar density in the footprint.
• Figure 5: Three different measures of photo-$z$ uncertainty for galaxies in self-organizing map cells (see Sec. \ref{['sec:somcuts']}), except for the lower right subplot, which shows the average predicted photo-$z$ of galaxies in each cell. The different measures largely agree, but have different sensitivity to outliers. Objects in cells with photo-$z$ error above a certain threshold are cut from the sample (black).