How the cosmic web induces intrinsic alignments of galaxies
  Intrinsic alignments are believed to be a major source of systematics for
future generation of weak gravitational lensing surveys like Euclid or LSST.
Direct measurements of the alignment of the projected light distribution of
galaxies in wide field imaging data seem to agree on a contamination at a level
of a few per cent of the shear correlation functions, although the amplitude of
the effect depends on the population of galaxies considered. Given this
dependency, it is difficult to use dark matter-only simulations as the sole
resource to predict and control intrinsic alignments. We report here estimates
on the level of intrinsic alignment in the cosmological hydrodynamical
simulation Horizon-AGN that could be a major source of systematic errors in
weak gravitational lensing measurements. In particular, assuming that the spin
of galaxies is a good proxy for their ellipticity, we show how those spins are
spatially correlated and how they couple to the tidal field in which they are
embedded. We also present theoretical calculations that illustrate and
qualitatively explain the observed signals.
