# Testing the Impact of Satellite Anisotropy on Large and Small Scale Intrinsic Alignments using Hydrodynamical Simulations

## TL;DR

This study uses hydrodynamical simulations to show that galaxy satellite anisotropy significantly impacts intrinsic alignment models, potentially biasing cosmological parameters in future weak lensing surveys.

## Contribution

It quantifies the effect of satellite anisotropy on intrinsic alignments and highlights the limitations of spherical halo models in cosmological analyses.

## Key findings

- Satellite anisotropy contributes 30-40% to small-scale alignment correlations.
- Impact on IA power spectra amplitude is about 20% at scales >1 h^{-1} Mpc.
- Potential biases in cosmological parameters like S_8 and w are around 1.5 sigma.

## Abstract

Galaxy intrinsic alignments (IAs) have long been recognised as a significant contaminant to weak lensing-based cosmological inference. In this paper we seek to quantify the impact of a common modelling assumption in analytic descriptions of intrinsic alignments: that of spherically symmetric dark matter halos. Understanding such effects is important as the current generation of intrinsic alignment models are known to be limited, particularly on small scales, and building an accurate theoretical description will be essential for fully exploiting the information in future lensing data. Our analysis is based on a catalogue of 113,560 galaxies between $z=0.06-1.00$ from MassiveBlack-II, a hydrodynamical simulation of box length $100 h^{-1}$ Mpc. We find satellite anisotropy contributes at the level of $\geq 30-40\%$ to the small scale alignment correlation functions. At separations larger than $1 h^{-1}$ Mpc the impact is roughly scale-independent, inducing a shift in the amplitude of the IA power spectra of $\sim20\%$. These conclusions are consistent across the redshift range and between the MassiveBlack-II and Illustris simulations. The cosmological implications of these results are tested using a simulated likelihood analysis. Synthetic cosmic shear data is constructed with the expected characteristics (depth, area and number density) of a future LSST-like survey. Our results suggest that modelling alignments using a halo model based upon spherical symmetry could potentially induce cosmological parameter biases at the $\sim 1.5\sigma$ level for $S_8$ and $w$.