Intrinsic alignment from multiple shear estimates: A first application to data and forecasts for Stage IV

TL;DR

This paper introduces a novel method to measure galaxy intrinsic alignment using multiple shear estimates, applies it to DES data, and forecasts its effectiveness for future LSST data to improve weak lensing cosmology.

Contribution

It is the first application of a shear estimator difference method to data for intrinsic alignment measurement and provides forecasts for LSST's capabilities.

Findings

Null detection of intrinsic alignment in DES data.

Forecasts indicate specific shear estimator differences needed for LSST to detect IA.

Method addresses systematics like selection functions and biases.

Abstract

Without mitigation, the intrinsic alignment (IA) of galaxies poses a significant threat to achieving unbiased cosmological parameter constraints from precision weak lensing surveys. Here, we apply for the first time to data a method to extract the scale dependence of the IA contribution to galaxy-galaxy lensing, which takes advantage of the difference in alignment signal as measured by shear estimators with different sensitivities to galactic radii. Using data from Year 1 of the Dark Energy Survey, with shear estimators METACALIBRATION and IM3SHAPE, we investigate and address method systematics including non-trivial selection functions, differences in weighting between estimators, and multiplicative bias. We obtain a null detection of IA, which appears qualitatively consistent with existing work. We then forecast the application of this method to Rubin Observatory Legacy Survey of Space and Time (LSST) data and place requirements on a pair of shear estimators for detecting IA and constraining its 1-halo scale dependence. We find that for LSST Year 1, shear estimators should have at least a $40\%$ difference in IA amplitude, and the Pearson correlation coefficient of their shape noise should be at least $\rho=0.50$, to ensure a $1\sigma$ detection of IA and a constraint on its 1-halo scale dependence with a signal-to-noise ratio greater than $1$. For Year 10, a $1\sigma$ detection and constraint become possible for $20\%$ differences in alignment amplitude and $\rho=0.50$.