Galaxies are the most abundant and readily observed objects in the Universe beyond our own Milky Way. Deep exposures in otherwise empty patches of sky contain large numbers of faint and small galaxy images at increasingly larger distances from Earth and thus at an earlier time in the history of the Universe (Ferguson et al., 2000). This makes galaxies prime candidates for studying the properties and the evolution of the large-scale structure of the Universe. Analysing their spatial and spectral light distributions in combination with their dynamical properties, one can infer the physical processes that govern galaxy formation and attempt to explain the plethora of galaxy types and structures one observes. In addition, galaxies physically trace the density peaks of the overall matter distribution, and their light samples the properties of spacetime along the line of sight, which allows for direct constraints on cosmological models and the non-luminous, more exotic ingredients of the Universe (e.g. Zwicky 1937, Refsdal 1966).
The fundamental properties of galaxy images that can still be extracted from the faintest, most distant objects include the position on the sky, the total flux, the apparent size, as well as the lowest-order deviation from a circular morphology given by a measure of ellipticity (e.g. the ratio of minor to major axis) and orientation (e.g. the angle of the major axis with respect to a reference direction). These quantities have been exploited to varying degrees in studies of cosmology and galaxy evolution, for instance in galaxy clustering (using the distribution of galaxy positions; e.g. Zehavi et al., 2011, Sánchez et al., 2014) and gravitational magnification as well as measurements of the inherent size distribution (using size and flux; e.g. Schmidt et al., 2012, Mosleh et al., 2012).
Of particular interest are the orientations of galaxy images, because one expects them to be random for a sufficiently large sample of galaxies in a homogeneous and isotropic universe. Physical processes that locally violate isotropy may be indicated by either (a) any net preferred orientation, or alignment, with respect to some reference direction in an ensemble of galaxy images; or, (b) any non-vanishing correlation between galaxy orientations. Such processes have been linked e.g. to tidal gravitational forces acting on galaxies during formation and at later evolutionary stages. Moreover, the gravitational lensing effect by the large-scale structure induces a coherent apparent distortion in galaxy images. The resulting galaxy alignments can thus be used to constrain models of lensing effects, but only in the absence of (or with a well-defined model for) other sources of coherent alignment, which constitute a nuisance signal in this case.
Interest in galaxy alignments dates back to the early twentieth century (see Section 3.1), when the extragalactic nature of nebulae was not even established. Contradictory results obtained from the slowly increasing galaxy samples indicated that galaxy alignments are challenging to measure reliably. Substantial stochasticity in most signals suppresses their signal-to-noise ratio, as does the fact that three-dimensional alignments are diluted due to projection on the sky. The low signal-to-noise ratio in samples of a few thousand galaxies, which were typical for most of the last century, was paired with large spurious ellipticities and alignments induced e.g. by telescope movement, optics, or photographic plate artefacts (e.g. Hu et al., 2006). Furthermore, it is a relatively recent insight that the non-linear propagation of noise from the pixels to the shape of the image alone causes biases in the measurement of galaxy orientation and shape Viola et al., 2014.
However, the past decade has seen a dramatic acceleration of progress in this field, which can largely be attributed to the following developments:
With the cosmological concordance model firmly established (Planck Collaboration et al., 2015a), there is now a robust framework upon which the more intricate models of galaxy alignments can build. This includes the cold dark matter paradigm, in which dark matter with negligible kinetic energy governs structure formation, as well as the bottom-up scenario of structure formation starting with small dark matter haloes that coalesce into ever-larger objects which eventually host galaxies.
Astrophysics has entered a golden era of large imaging and spectroscopic surveys, the first and foremost of which was the Sloan Digital Sky Survey (SDSS; York et al., 2000). These surveys are finally able to provide the galaxy sample sizes and the quality of shape measurements for significant and robust detections of galaxy alignments.
Computational power is constantly increasing such that one can nowadays run N-body simulations in cosmological volumes (Efstathiou et al., 1985), with sufficient mass and spatial resolutions to obtain precise measurements of galaxy or halo shape and orientation, which implies that alignment signals can be predicted robustly and at high statistical significance. This numerical effort is critical to better understand and model the highly non-linear physics expected in alignment processes. Some recent hydrodynamic simulations have also incorporated the physical processes of gas and stars, thus enabling a more direct link between theory and observations (e.g. Vogelsberger et al., 2014).
A number of on-going and planned cosmological galaxy surveys (for details see Section 8) will use weak gravitational lensing by large-scale structure as a key probe of our cosmological model. The small but coherent galaxy shape distortions due to gravitational lensing are partly degenerate with local, physically-induced (and hence dubbed intrinsic) galaxy alignments, which could thus constitute a limiting systematic effect. This has further boosted the interest in a better understanding of galaxy alignments.
The current research into galaxy alignments can roughly be split into two branches according to the main drivers of this field: the study of galaxy alignments with the elements of the cosmic web, such as clusters of galaxies, filaments, and voids, with the purpose of directly testing models of galaxy formation and evolution, and the measurement of pairwise alignments in large, broadly-defined galaxy samples with the goal of quantifying and mitigating bias in cosmological surveys, using similar datasets, statistics, and analysis methodology as in the corresponding measurements of gravitational lensing. One goal of this review is to take a synoptic viewpoint on these branches and treat them as two approaches to the same science goal – a deeper understanding of the physics of galaxy alignments and its implications for galaxy evolution and cosmology.
This work provides an overview on the subject and attempts to limit the previous knowledge required to follow its contents to the basics of extragalactic astrophysics and cosmology. There is a plethora of ways in which galaxies can align with the multitude of structures that populate the Universe, and we attempt to categorise and structure the vast amount of research done over the past century. We will focus on correlations which involve galaxy ellipticities or position angles, as well as the ellipticities of the underlying dark matter distribution, including the latters observational proxies such as the distribution of satellite galaxies.
After brief summaries of the basics of galaxy formation and evolution, the relevant gravitational lensing effects, and tidal alignment theory in Section 2, this overview highlights the developments of the twentieth century (Section 3). We then review recent work, proceeding from small-scale alignments inside an overdense region such as a galaxy cluster (Section 4), to alignments between clusters and with the cosmic web (Section 5), to alignments of broadly defined galaxy populations (Section 6). Section 7 summarises the impact of alignments on cosmological signals and corresponding mitigation strategies, followed by an outlook on future developments of the field in Section 8. This work is part of a topical volume on galaxy alignments consisting of three papers in total. The two companion papers take a more detailed and technical approach, covering theory, modelling, and simulations (Kiessling et al., 2015) as well as observational results and the impact on cosmology (Kirk et al., 2015). Troxel & Ishak (2014) also provided a recent review on galaxy alignments, with a focus on aspects related to weak gravitational lensing.