A theoretical estimate of intrinsic ellipticity bispectra induced by angular momenta alignments

TL;DR

This paper provides a theoretical analysis of the intrinsic ellipticity bispectra caused by galaxy alignments, revealing their significant impact on small-scale cosmic shear measurements and potential for cosmological insights.

Contribution

It introduces an analytical model for intrinsic alignments using tidal torque theory and compares their bispectra to gravitational lensing signals across different configurations.

Findings

Intrinsic alignments dominate on small angular scales (<20 arcmin) in Euclid-like surveys.

III-alignments exceed lensing signals by over an order of magnitude at high multipoles (~3000).

GGI-alignments are consistently weaker than shear bispectra across all scales.

Abstract

Intrinsically aligned galaxy shapes are one of the most important systematics in cosmic shear measurements. So far theoretical studies of intrinsic alignments almost exclusively focus on their statistics at the two-point level. Results from numerical simulations, however, suggest that third-order measures might be even stronger affected. We therefore investigate the (angular) bispectrum of intrinsic alignments. In our fully analytical study we describe intrinsic galaxy ellipticities by a physical alignment model, which makes use of tidal torque theory. We derive expressions for the various combinations of intrinsic and gravitationally induced ellipticities, i.e. III-, GII- and GGI-alignments, and compare our results to the shear bispectrum, the GGG-term. The latter is computed using hyper-extended perturbation theory. Considering equilateral and squeezed configurations we find that for a Euclid-like survey intrinsic alignments (III-alignments) start to dominate on angular scales smaller than 20 arcmin and 13 arcmin, respectively. This sensitivity to the configuration-space geometry may allow to exploit the cosmological information contained in both the intrinsic and gravitationally induced ellipticity field. On smallest scales (l ~ 3000) III-alignments exceed the lensing signal by at least one order of magnitude. The amplitude of the GGI-alignments is the weakest. It stays below that of the shear field on all angular scales irrespective of the wave-vector configuration.