How the cosmic web induces intrinsic alignments of galaxies

TL;DR

This paper investigates how the cosmic web influences galaxy intrinsic alignments, using hydrodynamical simulations to understand their impact on weak lensing measurements and potential systematic errors.

Contribution

It provides the first estimates of galaxy intrinsic alignments from the Horizon-AGN simulation, linking galaxy spins to the tidal field and their correlations.

Findings

Galaxy spins are spatially correlated within the simulation.

Intrinsic alignments depend on galaxy populations and are a significant systematic.

Theoretical models qualitatively explain the observed alignments.

Abstract

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.