Intrinsic alignments of galaxies in the EAGLE and cosmo-OWLS simulations

TL;DR

This study uses EAGLE and cosmo-OWLS simulations to analyze galaxy intrinsic alignments across various scales and halo masses, revealing dependencies on galaxy shape, mass, and the subset of stars used for orientation measurements.

Contribution

It provides a comprehensive analysis of galaxy alignments in hydrodynamical simulations across a wide mass and scale range, highlighting the importance of stellar component selection.

Findings

Alignment strength decreases with galaxy separation.

Massive haloes and aspherical galaxies show stronger alignments.

Galaxy alignments are weaker than dark matter halo alignments.

Abstract

We report results for the alignments of galaxies in the EAGLE and cosmo-OWLS simulations as a function of galaxy separation and halo mass. The combination of these hydro-cosmological simulations enables us to span four orders of magnitude in halo mass ($10.7<log_{10}(M_{200}/[h^{-1}M_\odot])<15$) and a large range of separations ($-1<log_{10}(r/[h^{-1}Mpc])< 2$). We focus on two classes of alignments: the orientations of galaxies with respect to either the directions to, or the orientations of, surrounding galaxies. We find that the strength of the alignment is a strongly decreasing function of the distance between galaxies. The orientation-direction alignment can remain significant up to ~100 Mpc, for galaxies hosted by the most massive haloes in our simulations. Galaxies hosted by more massive subhaloes show stronger alignment. At a fixed halo mass, more aspherical or prolate galaxies exhibit stronger alignments. The spatial distribution of satellites is anisotropic and significantly aligned with the major axis of the main host halo. The major axis of satellite galaxies, when all stars are considered, are preferentially aligned towards the centre of the main host halo. The predicted projected direction-orientation alignment, $\epsilon_{g+}(r_{p})$, is in broad agreement with recent observations when only stars within the typical observable extent of a galaxy are used to define galaxy orientations. We find that the orientation-orientation alignment is weaker than the orientation-direction alignment on all scales. Overall, the strength of galaxy alignments depends strongly on the subset of stars that are used to measure the orientations of galaxies and it is always weaker than the alignment of the dark matter haloes. Thus, alignment models that use halo orientation as a direct proxy for galaxy orientation will overestimate the impact of intrinsic alignments on weak lensing analyses.