The halo model as a versatile tool to predict intrinsic alignments
  Intrinsic alignments (IAs) of galaxies are an important contaminant for
cosmic shear studies, but the modelling is complicated by the dependence of the
signal on the source galaxy sample. In this paper, we use the halo model
formalism to capture this diversity and examine its implications for Stage-III
and Stage-IV cosmic shear surveys. We account for the different IA signatures
at large and small scales, as well for the different contributions from
central/satellite and red/blue galaxies, and we use realistic mocks to account
for the characteristics of the galaxy populations as a function of redshift. We
inform our model using the most recent observational findings: we include a
luminosity dependence at both large and small scales and a radial dependence of
the signal within the halo. We predict the impact of the total IA signal on the
lensing angular power spectra, including the current uncertainties from the IA
best-fits to illustrate the range of possible impact on the lensing signal: the
lack of constraints for fainter galaxies is the main source of uncertainty for
our predictions of the IA signal. We investigate how well effective models with
limited degrees of freedom can account for the complexity of the IA signal.
Although these lead to negligible biases for Stage-III surveys, we find that,
for Stage-IV surveys, it is essential to at least include an additional
parameter to capture the redshift dependence.
