Abstract

Contributed Talk - Splinter LargeScale

Thursday, 10 September 2026, 14:00   (MW-2235)

Galaxy halo shapes from lens-shear-shear statistics

Silvestre-Rosello,E. and Linke, L. et al
University of Innsbruck

The morphology of galaxy-scale dark matter halos encodes key information about the nature of dark matter, galaxy assembly histories and intrinsic alignment mechanisms. Moreover, cosmological analysis based on the halo model would benefit from realistic halo shapes on galaxy scale. Since dark matter extends much farther from the center of galaxy than luminous matter, weak lensing is a powerful tool to study halo shapes. Second-order weak lensing estimators, like stacking, require orienting the halos in the direction of the observed galaxy light distribution. Therefore, these measurements suffer from a fundamental degeneracy between the intrinsic halo ellipticity and the misalignment angle between baryonic and dark matter components, making it impossible to disentangle halo shapes from intrinsic alignments. To break this degeneracy, we introduce a novel estimator of the halo shape, ⟨ε²⟩, based on the three-point lens-shear-shear correlation functions (GGN). We find that, in any realistic setting, GGN is heavily contaminated by cosmic shear, and that a random subtraction is not enough, so higher-order effects have to be included in the correction, like reduced shear and magnification bias. We model GGN for a distribution of elliptical NFW halos with the decomposition in ellipticity moments, measure GGN with the package orpheus and find the best-fit ⟨ε²⟩ with a Bayesian framework. We validate the method through controlled tests on the Millennium Simulation and forecast the detectability of ⟨ε²⟩ in the first large data release of Euclid, Euclid-DR1, and in the final data release of Euclid. Despite its intrinsically lower signal-to-noise ratio relative to two-point statistics, Stage IV surveys such as Euclid provide an optimal observational framework for a first measurement of ⟨ε²⟩.