In Space Science Reviews
http://arxiv.org/abs/1504.05465

For a PDF version of the article, click here.

GALAXY ALIGNMENTS: OBSERVATIONS AND IMPACT ON COSMOLOGY

Donnacha Kirk 1, Michael L. Brown 2, Henk Hoekstra 3, Benjamin Joachimi 1, Thomas D. Kitching 4, Rachel Mandelbaum 5, Cristóbal Sifón 3, Marcello Cacciato 3, Ami Choi 6, Alina Kiessling 7, Adrienne Leonard 1, Anais Rassat 8, Björn Malte Schäfer 9

1 Department of Physics & Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
2 Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK
3 Leiden Observatory, Leiden University, PO Box 9513, NL-2300 RA Leiden, Netherlands
4 Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
5 McWilliams Center for Cosmology, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213, USA
6 Scottish Universities Physics Alliance, Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK
7 Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA
8 Laboratoire d'astrophysique (LASTRO), Ecole Polytechnique Fédérale de Lausanne (EPFL), Observatoire de Sauverny, CH-1290 Versoix, Switzerland
9 Astronomisches Recheninstitut, Zentrum für Astronomie der Universität Heidelberg, Philosophenweg 12, 69120 Heidelberg, Germany


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 astrophysics of galaxy formation and evolution as well as the growth of structure in the Universe. In this review paper we summarise key measurements of intrinsic 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 took 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 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.

Keywords: galaxies: evolution; galaxies: haloes; galaxies: interactions; large-scale structure of Universe; gravitational lensing: weak


Table of Contents

INTRODUCTION

QUANTIFYING ORIENTATIONS AND SHAPES
Using orientations
Spin alignments
Measuring shapes
Shape measurement systematics
Intrinsic alignment measurements and cosmic shear

SHAPE CORRELATIONS
Two-point correlation functions
Estimators of the two-point correlation functions
Projected correlation functions
Using correlation functions to test intrinsic alignment models
Tests for systematics

OBSERVATIONS OF ALIGNMENT IN LARGE GALAXY SAMPLES
Late-type galaxies
Early-type Galaxies
Other Large-Scale Measurements

ENVIRONMENTALLY DEPENDENT ALIGNMENTS
Galaxy position alignments in the field and the Local Group
Galaxy alignments within galaxy groups and clusters
Galaxy alignments with voids
Galaxy alignments with filaments and sheets
Alignments between galaxy groups and clusters

IMPACT ON COSMOLOGY & MITIGATION
Quantifying Impact
Exploiting redshift dependence
Parameterisation and marginalisation
Self-calibration
Higher-order cosmic shear statistics
Probes of the unlensed galaxy shape
Radio polarisation as a tracer of intrinsic orientation
Rotational velocities as a tracer of intrinsic orientation

SUMMARY & OUTLOOK

REFERENCES

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