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

TL;DR

This work measures position–intrinsic shear and galaxy clustering signals in flux-limited GAMA and SDSS samples using KiDS imaging, to constrain the non-linear alignment (NLA) model and its luminosity variant. Red galaxies show a robust radial IA signal with $A_{ extrm{IA}} \\approx 3.18^{+0.47}_{-0.46}$, while blue galaxies are consistent with zero, and no strong luminosity scaling is detected; blue and red populations show clear environmental dependencies, especially among centrals and satellites. The authors provide informative IA priors from these measurements and demonstrate that applying them in a colour-split KiDS-like forecast can improve constraints on $S_{8}$ and $w_{0}$ by up to ~50%, highlighting the importance of realistic IA modelling for upcoming surveys. They also discuss limitations of current NLA/LA prescriptions and advocate more advanced halo-based, environment-aware IA models to fully capture the complexity of galaxy alignments across populations and environments. Overall, the work delivers representative IA constraints with direct relevance to current and planned weak-lensing cosmology, and it lays groundwork for increasingly refined priors and models in the era of LSST, Euclid, and WFIRST.

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 $A_{\textrm{IA}}^{\textrm{B}}=0.21^{+0.37}_{-0.36}$ to be consistent with zero. We make robust detections ($\sim9σ$) for red galaxies, with $A_{\textrm{IA}}^{\textrm{R}}=3.18^{+0.47}_{-0.46}$, 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 $S_{8}$ and the dark energy equation of state $w_{0}$, 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.

Figures (16)

• Figure 1: Left: Galaxy rest-frame colour-magnitude diagram, where we choose a cut in $g-r$ to isolate the red sequence in GAMA and SDSS. Right: Sample absolute $r$-band magnitude-redshift diagram. The total distribution of GAMA and SDSS galaxies is shown, binned in hexagonal cells with a colour scale corresponding to the counts in cells. Coloured contours indicate $75\%$ and $95\%$ of galaxies in a sample. Colour/redshift cuts are shown as dashed green lines, and the apparent leakage of contours is due to the grid-size used in kernel density estimation.
• Figure 2: Measured galaxy clustering for our blue (top) and red (bottom) galaxy samples. Solid curves illustrate the best-fit linear clustering per sample (Eq. \ref{['eq:gg_hankel']}). The vertical dashed line indicates $r_{p}=6\,h^{-1}{\rm{Mpc}}$, below which scales are excluded from fitting (Section \ref{['sec:likelihoods']}).
• Figure 3: Measured galaxy position-intrinsic shear correlations for our blue (top) and red (bottom) galaxy samples. Best-fit NLA models are shown as solid curves, and the vertical dashed line indicates $r_{p}=6\,h^{-1}{\rm{Mpc}}$, below which scales are excluded from fitting (Section \ref{['sec:likelihoods']}). The best-fit LA model to SR is shown as a dot-dashed line.
• Figure 4: Galaxy clustering (top) and position-intrinsic shear correlations (bottom) measured in the full KiDS$+$GAMA and SDSS Main datasets. Solid lines illustrate the best-fit NLA model, and dot-dashed lines the LA. The vertical dashed line indicates $r_{p}=6\,h^{-1}{\rm{Mpc}}$, below which scales are excluded from fitting (Section \ref{['sec:likelihoods']}).
• Figure 5: Posterior probability contours of our fitted galaxy bias $b_{\rm{g}}$, NLA amplitude $A$ and luminosity power-law $\beta$ parameters, for red (left) and blue (right) galaxies. The filled (unfilled) contours are for the NLA (NLA-$\beta$) models. Dashed grey lines mark values of zero for IA parameters.