Statistical separation of weak gravitational lensing and intrinsic ellipticities based on galaxy colour information

TL;DR

This paper proposes a method to separate weak gravitational lensing signals from intrinsic galaxy alignments using galaxy colour information, enabling more accurate cosmological measurements.

Contribution

It introduces a technique leveraging galaxy morphology to isolate lensing signals from intrinsic alignments, improving measurement precision and robustness.

Findings

Intrinsic alignment spectra can be measured with high significance.

Model parameters can be estimated at percent-level accuracy.

Misclassification has minimal impact on parameter estimation.

Abstract

Intrinsic alignments of galaxies are recognised as one of the most important systematic in weak lensing surveys on small angular scales. In this paper we investigate ellipticity correlation functions that are measured separately on elliptical and spiral galaxies, for which we assume the generic alignment mechanisms based on tidal shearing and tidal torquing, respectively. Including morphological information allows to find linear combinations of measured ellipticity correlation functions which suppress the gravitational lensing signal completely or which show a strongly boosted gravitational lensing signal relative to intrinsic alignments. Specifically, we find that $(i)$ intrinsic alignment spectra can be measured in a model-independent way at a significance of $\Sigma\simeq 60$ with a wide-angle tomographic survey such as Euclid's, $(ii)$ intrinsic alignment model parameters can be determined at percent-level precision, $(iii)$ this measurement is not impeded by misclassifying galaxies and assuming a wrong alignment model, $(iv)$ parameter estimation from a cleaned weak lensing spectrum is possible with almost no bias and $(v)$ the misclassification would not strongly impact parameter estimation from the boosted weak lensing spectrum.