KiDS+GAMA: Intrinsic alignment model constraints for current and future
  weak lensing cosmology
  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 ($\sim9\sigma$) 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.
