Sources of contamination to weak lensing three-point statistics: constraints from N-body simulations

TL;DR

This paper examines how intrinsic galaxy alignments contaminate third-order weak lensing shear statistics using N-body simulations, highlighting the importance of redshift information and proposing new tools for studying intrinsic alignments.

Contribution

It provides redshift-dependent models of intrinsic alignment contamination and introduces a new observational tool for studying galaxy alignments in weak lensing.

Findings

Third-order shear statistics are more contaminated than second-order.

Contamination can reach ~15% for median redshift 0.7 surveys.

Shallow surveys show intrinsic alignments dominate the signal.

Abstract

We investigate the impact of the observed correlation between a galaxies shape and its surrounding density field on the measurement of third order weak lensing shear statistics. Using numerical simulations, we estimate the systematic error contribution to a measurement of the third order moment of the aperture mass statistic (GGG) from three-point intrinsic ellipticity correlations (III), and the three-point coupling between the weak lensing shear experienced by distant galaxies and the shape of foreground galaxies (GGI and GII). We find that third-order weak lensing statistics are typically more strongly contaminated by these physical systematics compared to second-order shear measurements, contaminating the measured three-point signal for moderately deep surveys with a median redshift z_m ~ 0.7 by ~ 15%. It has been shown that accurate photometric redshifts will be crucial to correct for this effect, once a model and the redshift dependence of the effect can be accurately constrained. To this end we provide redshift-dependent fitting functions to our results and propose a new tool for the observational study of intrinsic galaxy alignments. For a shallow survey with z_m ~ 0.4 we find III to be an order of magnitude larger than the expected cosmological GGG shear signal. Compared to the two-point intrinsic ellipticity correlation which is similar in amplitude to the two-point shear signal at these survey depths, third order statistics therefore offer a promising new way to constrain models of intrinsic galaxy alignments. Early shallow data from the next generation of very wide weak lensing surveys will be optimal for this type of study.