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 { \scriptsize { IA } } } ^ { \textrm { B } } = 0.21 ^ { +0.37 } _ { -0.36 } to be consistent with zero .
We make robust detections ( \sim 9 \sigma ) for red galaxies , with A _ { \textrm { \scriptsize { 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 .