Origin of the anomalous intrinsic-alignment and goodness-of-fit signals in the DECADE NGC weak-lensing data

The Dark Energy Camera All Data Everywhere (DECADE) 'DECam 13k' cosmic shear analysis - the largest to date, combining the DECADE northern and southern Galactic cap (NGC and SGC) datasets with the Dark Energy Survey (DES) Year 3 sample (270 million galaxies over 13,000 deg^2) - reports two unexplained anomalies localized in its DECADE NGC component: a statistically significant nonzero amplitude of the quadratic tidal-torquing term of the tidally aligned tidally torqued (TATT) intrinsic-alignment model ($a_2=-2.47^{+0.56}_{-0.65}$, a 2.9-sigma preference that is robust to scale cuts, baryon modeling, and data-vector subsplits, with no identified physical origin), and localized $\chi^2$ excesses in three tomographic bin pairs of the $\xi_+$ shear-correlation signal that depress the full-analysis goodness of fit to $p\approx0.007$. Neither the sibling DECADE SGC and DES Y3 vectors nor the successor DES Year 6 analysis shows a comparable intrinsic-alignment preference (DES Y6 prefers the simpler non-linear alignment (NLA) model). The open problem is to determine whether these signals are genuine astrophysics or artifacts - for the intrinsic-alignment signal, distinguishing genuine quadratic alignment of the source population from galaxy-selection, environmental, or instrumental causes, for example via cross-correlation of DECADE galaxy shapes with spectroscopic samples. The outcome bears directly on whether nonlinear intrinsic-alignment terms must be calibrated into Stage-IV (next-generation; e.g. LSST and Euclid) weak-lensing analyses.

Background

Weak gravitational lensing ('cosmic shear') measures the coherent distortion of distant galaxy images by the intervening matter distribution. Two-point correlations of galaxy shapes - conventionally summarized by the shear correlation functions $\xi_{\pm}(\theta)$ - measured in tomographic redshift bins constrain $S_8\equiv\sigma_8(\Omega_{\rm m}/0.3)^{0.5}$ - the amplitude of matter clustering - and are a central probe of the late-time Universe. The Dark Energy Camera All Data Everywhere (DECADE) project measured galaxy shapes over 5,400 deg$^2$ of the northern Galactic cap (NGC; 107 million galaxies) and 3,356 deg$^2$ of the southern cap (SGC; 63 million galaxies); combined with the Dark Energy Survey Year 3 (DES Y3) sample, the 'DECam 13k' analysis of Anbajagane et al. (arXiv:2509.03582) is the largest cosmic shear analysis to date, yielding $S_8=0.805\pm0.019$ under $\Lambda$CDM.

Two astrophysical contaminants must be modeled in such analyses. First, baryonic feedback - gas heated by active galactic nuclei and supernovae - redistributes matter and suppresses the non-linear matter power spectrum by about 25% around $k\simeq4\,h/{\rm Mpc}$ at $z=0$; the DECam 13k analysis found that small-scale shear information is consumed by self-calibrating the baryon model rather than improving cosmological constraints. Second, intrinsic alignment (IA): galaxy shapes are correlated with the local gravitational tidal field, contaminating the lensing measurement. The standard linear prescription is the non-linear alignment (NLA) model; the more flexible Tidally Aligned Tidally Torqued (TATT) model of Blazek et al. adds a quadratic 'tidal-torquing' term with amplitude $a_2$, a density-weighted alignment term with amplitude $b_{\rm TA}$, and redshift-evolution exponents $\eta_{1,2}$, alongside the linear alignment amplitude $a_1$.

The DECADE NGC data present two unexplained anomalies, both localized to that sub-dataset. (i) The NGC TATT analysis prefers a statistically significant nonzero quadratic alignment amplitude, $a_2=-2.47^{+0.56}_{-0.65}$, a 2.9-sigma preference (Anbajagane et al., arXiv:2502.17677, DECADE IV). The DECADE V summary text quotes this preference with the opposite sign; the sign convention used here follows the primary DECADE IV measurement. The preference is robust - it persists when all angular scales are used, when baryon suppression is modeled, and when the highest-$\chi^{2}$ bin pairs are dropped - and DECADE IV's empirical tests excluded image-quality variations and specific galaxy subsets as causes, yet the authors state they 'do not have a clean explanation for the exact origin of the IA constraint in our data'. Notably, neither the SGC data nor DES Y3 show any IA preference, and the successor DES Year 6 analysis (DES Collaboration, arXiv:2602.10065) prefers NLA over TATT with a Bayesian evidence ratio of about 17. Direct IA measurements with spectroscopic samples exist for luminous red galaxies (Samuroff et al., arXiv:2212.11319) but not for the faint, blue-dominated DECADE source population. (ii) The goodness of fit of the full DECam 13k $\Lambda$CDM analysis is low ($p\approx0.007$), traced to excess $\chi^{2}$ in the $\xi_{+}$ measurements of three NGC tomographic bin pairs, (1,1), (2,4), and (4,4); the authors' forty-six catalog-split and survey-property tests found no correlated cause, and they attributed the excess scatter to statistical fluctuations - an absence-of-correlation conclusion rather than a positive identification of its origin. Dropping these bin pairs raises $p$ to 0.14 without changing any parameter posterior, and the IA preference is independent of them. Because the choice of IA model shifts $S_8$ by up to about $1\sigma$ in current analyses, whether the DECADE NGC IA preference is genuine physics or an artifact bears directly on IA modeling for Stage-IV surveys (LSST, Euclid). The DECADE shear catalogs, data vectors, and likelihoods are publicly available, enabling independent reanalysis.

Problem Statement

Determine the origin of the two unexplained anomalous signals in the DECADE NGC weak-lensing data vector, i.e. establish whether each is a genuine astrophysical signal or an artifact.

Primary component: identify, with empirical evidence, whether the statistically significant nonzero TATT quadratic intrinsic-alignment amplitude ($a_2\approx-2.5$ at 2.9 sigma under the published DECADE analysis conventions) arises from (a) genuine quadratic intrinsic alignment of the DECADE source population, (b) galaxy-selection or environmental effects, (c) residual observational systematics, or (d) an interplay of these - and reproduce the published amplitude, sign, and significance within the quoted uncertainties, including the preference's stability across the published variant analyses - at least the use of all angular scales, explicit baryon modeling, and removal of the highest-$\chi^2$ bin pairs.

Secondary component (same dataset, established to be independent of the intrinsic-alignment preference): identify the origin of the localized $\xi_+$ $\chi^2$ excesses in tomographic bin pairs (1,1), (2,4), and (4,4), or positively confirm their tentative attribution to statistical fluctuations.

The two components share one diagnostic objective: a validated account of why the largest single dataset in the DECam 13k combination behaves anomalously where its sibling datasets do not. Both components are mandatory and jointly required: a complete account of the task must resolve both anomalies, and resolving either one alone is not sufficient. Plausible identification routes include direct IA measurements from cross-correlation of DECADE galaxy shapes with spectroscopic samples (e.g. the Dark Energy Spectroscopic Instrument, DESI), IA-constrained analyses of source subsamples, targeted systematics tests, and forward modeling; the public DECADE shear catalogs, data vectors, and likelihoods enable independent reanalysis.

References

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