Galaxy alignments: Observations and impact on cosmology

TL;DR

Galaxy alignments, which reflect non-random orientations of galaxies, provide insights into galaxy formation, evolution, and large-scale structure, but also pose a systematic challenge for weak lensing cosmology.

Contribution

This review synthesizes key measurements, formalism, and mitigation techniques related to galaxy alignments and their impact on cosmological studies.

Findings

Galaxy alignments depend on galaxy type, scale, and environment.

Intrinsic alignments can significantly bias weak lensing measurements.

Mitigation techniques can reduce contamination in cosmic shear surveys.

Abstract

Galaxy shapes are not randomly oriented, rather they are statistically aligned in a way that can depend on formation environment, history and galaxy type. Studying the alignment of galaxies can therefore deliver important information about the physics of galaxy formation and evolution as well as the growth of structure in the Universe. In this review paper we summarise key measurements of galaxy alignments, divided by galaxy type, scale and environment. We also cover the statistics and formalism necessary to understand the observations in the literature. With the emergence of weak gravitational lensing as a precision probe of cosmology, galaxy alignments have taken on an added importance because they can mimic cosmic shear, the effect of gravitational lensing by large-scale structure on observed galaxy shapes. This makes galaxy alignments, commonly referred to as intrinsic alignments, an important systematic effect in weak lensing studies. We quantify the impact of intrinsic alignments on cosmic shear surveys and finish by reviewing practical mitigation techniques which attempt to remove contamination by intrinsic alignments.