Galaxy alignments: An overview

TL;DR

This review surveys the physics of galaxy alignments, connecting tidally induced mechanisms to observational signatures across the cosmic web and within haloes. It integrates theory (tidal torque and linear/quadratic alignment models), simulations (N-body and hydrodynamic), and a wealth of observations (from POSS to SDSS and beyond) to explain how alignments arise and impact weak lensing, quantified through two-point statistics like $C_{ m \gamma\gamma}(\ell)$, $w_{g+}$, and related II/GI terms. A central message is that intrinsic alignments are a crucial systematic in cosmic shear analyses but also a rich probe of galaxy formation and the large-scale structure; robust mitigation (nulling, self-calibration, and joint analyses) and improved physical models are essential for Stage IV surveys. The authors anticipate that hydro simulations and halo-model frameworks will mature to describe alignments from Milky Way-sized haloes to massive clusters, with future data from surveys such as Euclid and SKA enabling tighter constraints on the physics of alignments and the underlying cosmology.

Abstract

The alignments between galaxies, their underlying matter structures, and the cosmic web constitute vital ingredients for a comprehensive understanding of gravity, the nature of matter, and structure formation in the Universe. We provide an overview on the state of the art in the study of these alignment processes and their observational signatures, aimed at a non-specialist audience. The development of the field over the past one hundred years is briefly reviewed. We also discuss the impact of galaxy alignments on measurements of weak gravitational lensing, and discuss avenues for making theoretical and observational progress over the coming decade.

Figures (16)

• Figure 1: Left: Sample of simulation particles subsumed into a common halo in an $N$-body simulation. The halo was identified by a variant of the friends-of-friends algorithm, identifying arbitrarily shaped regions with a density above a certain threshold. Increasing this threshold, the halo is decomposed into a number of sub-haloes indicated by the different symbols. Right: Representation of the halo and its substructure by ellipsoids which are determined by the eigenvalues and directions of the inertia tensor. The directions of the angular momenta of the larger sub-haloes as well as of the parent halo are given by the red arrows. Halo shapes and spins are key ingredients for the study of halo and galaxy alignments. © AAS. Reproduced with permission fromBE87.
• Figure 2: Gas density tracing the cosmic web in a subvolume ($12.5\,{\rm Mpc}/h$ comoving horizontally, stacked over $25\,{\rm Mpc}/h$ comoving along the line of sight) of the HORIZON-AGN simulation. The arrows indicate the direction of the smallest eigenvector of the gravitational tidal tensor (given by the Hessian of the gravitational potential at that point), which is expected to align with filamentary structures on average (see e.g. the top left corner). See \ref{['sec:lss']} for more details about the classification of filaments. Reproduced with permission fromCGD+14.
• Figure 3: Sketch of the gravitational lensing signal and its intrinsic alignment contamination. Light travels from the top of the sketch downwards, from the source plane via the lens plane to the plane at the bottom containing the images as seen by an observer. The matter structure (green ellipsoid) deflects the light from the background source galaxies (blue discs) and distorts their images tangentially with respect to the apparent centre of the lens (as seen in the bottom plane). As a consequence, the galaxy images become aligned (GG signal). Galaxies which are physically close to the lens structure (red ellipsoids) may be subjected to forces that cause them to point towards the structure, which results in the alignment of their images (II signal). Images of galaxies close to the lens are then preferentially anti-aligned with the gravitationally sheared images of background galaxies (GI signal).
• Figure 4: Left: Area of the Palomar Sky Survey analysed by Brown64. Dots (circles) mark the positions of galaxies with diameters in excess of 40$\arcsec$ (60$\arcsec$), axis ratios of less than 0.25, and position angles in the range 121 to 135 degrees (East of North, indicated by the lines at the bottom right). Rectangular lines correspond to plate boundaries. Right: Normalised histograms of position angle distributions compiled from table VII of Brown64. The top (bottom) panel shows galaxies with axis ratios $b/a < 0.25$ ($0.25 < b/a < 0.75$). Light grey bars correspond to galaxies inside the ' Pisces Concentration' (PC, roughly comprises the overdensity seen in the left panel), dark grey bars to those outside the concentration. The black horizontal line indicates the expected fraction for a random distribution of galaxy orientations. Reproduced with permission fromBrown64.
• Figure 5: Three strongly elongated Abell clusters analysed by Binggeli82 . The 50 brightest galaxies in a radius of 2 Mpc are plotted as black dots in each case. The brightest galaxy (BCG) is indicated by the circle. Position angles of the BCG and cluster are given by the thin black lines. BM class stands for the morphological Bautz-Morgan classification BM70 of galaxy clusters, where classes I and II are dominated by elliptical BCGs. Reproduced with permission fromBinggeli82© ESO.