KiDS+GAMA: Intrinsic alignment model constraints for current and future weak lensing cosmology

¹ Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
e-mail: hj@star.ucl.ac.uk
² Leiden Observatory, Leiden University, PO Box 9513, Leiden 2300 RA, The Netherlands
³ Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁴ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312 Giessenbachstrasse, 85741 Garching, Germany
⁵ Scottish Universities Physics Alliance, Institute for Astronomy, University of Edinburgh, Blackford Hill, Edinburgh EH9 3HJ, UK
⁶ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received: 23 November 2018
Accepted: 14 February 2019

Abstract

We directly constrain the non-linear alignment (NLA) model of intrinsic galaxy alignments, analysing the most representative and complete flux-limited sample of spectroscopic galaxies available for cosmic shear surveys. We measure the projected galaxy position-intrinsic shear correlations and the projected galaxy clustering signal using high-resolution imaging from the Kilo Degree Survey (KiDS) overlapping with the GAMA spectroscopic survey, and data from the Sloan Digital Sky Survey. Separating samples by colour, we make no significant detection of blue galaxy alignments, constraining the blue galaxy NLA amplitude A_IA^B = 0.21_−0.36^+0.37 to be consistent with zero. We make robust detections (∼9σ) for red galaxies, with A_IA^R = 3.18_−0.46^+0.47, corresponding to a net radial alignment with the galaxy density field, and we find no evidence for any scaling of alignments with galaxy luminosity. We provide informative priors for current and future weak lensing surveys, an improvement over de facto wide priors that allow for unrealistic levels of intrinsic alignment contamination. For a colour-split cosmic shear analysis of the final KiDS survey area, we forecast that our priors will improve the constraining power on S₈ and the dark energy equation of state w₀, by up to 62% and 51%, respectively. Our results indicate, however, that the modelling of red/blue-split galaxy alignments may be insufficient to describe samples with variable central/satellite galaxy fractions.