A rising tide: Intrinsic alignments since the turn of the millennium

Abstract

The alignments of galaxies across the large-scale structure of the Universe are known to be a source of contamination for gravitational lensing, but they can also probe cosmology and the physics of galaxy evolution in many ways. In this review, I cover developments in our understanding of intrinsic alignments over the past 25 years on: (1) different approaches to model intrinsic alignments across a range of scales, (2) existing observational constraints, (3) predictions from cosmological numerical $N$-body and hydrodynamical simulations, (4) mitigation strategies to account for their contamination to lensing observables and (5) cosmological and astrophysical applications. While the review focuses mostly on two-point statistics of intrinsic alignments, I also give a summary of other statistics beyond two-point. Finally, I point out some of the open problems hindering the understanding or application of intrinsic alignments and how they might be overcome in the future.

Figures (17)

• Figure 1: An artistic rendering of the distorted path of photons through the large-scale structure. This phenomenon, which distorts observed galaxy shapes in the tangential direction around matter overdensities, is known as gravitational lensing. Credit: Fortuna & Chisari (2025), CC-BY-NC 4.0, adapted from Fortuna & Chisari (2022), CC-BY-NC 4.0.
• Figure 2: An artistic rendering of the intrinsic alignments of galaxies embedded in the large-scale structure of the Universe. Different signals contribute to the overlap alignment of shapes relative to the density field: the alignment of central galaxies with their haloes, the alignment of central galaxies relative to one another, the alignment of satellites within the halo and with the central shape, and the potential alignment of galaxies with filaments. This also leads one to hypothesize about the presence of an alignment around voids. Credit: Fortuna & Chisari (2025), CC-BY-NC 4.0.
• Figure 3: $E$ (left) and $B$-modes (right) of galaxy shapes. Credit: Fortuna and Chisari (2022), CC-BY-NC 4.0.
• Figure 4: A cartoon representation of the assumptions behind the halo model Schneider10. According to the halo model, matter in the Universe is distributed in a collection of spherical haloes. Each halo has a central galaxy whose shape and orientation are aligned pointing towards other haloes, while satellites are sticks pointing radially towards the centre of the halo. Credit: Chisari (2025), CC-BY-NC 4.0. Background image: https://www.needpix.com/photo/1186728/.
• Figure 5: Different contributions to the matter-intrinsic shape power spectrum (left) and intrinsic shape auto-spectrum (right) at $z=0.11$ as predicted by the halo model of intrinsic alignments in the version of Schneider10. The yellow line corresponds to NLA for comparison. Deviations from NLA are evidenced at large $k$ (small scales), where the one-halo satellite-satellite term (dark blue dashed) is seen to dominate the signal. Credit: Figure 4 of Schneider10. Image reproduced with permission from Schneider10, copyright by the author(s).