The Environmental Dependence of Halo Intrinsic Alignments: Stronger Signals in Underdense Regions

The intrinsic alignment (IA) of galaxies and the dark matter haloes that host them is one of the leading astrophysical systematics for weak-lensing cosmology, yet how the IA signal depends on the large-scale environment in which haloes reside is not yet fully characterised. We use the high-resolution N-body simulations of the Dark Quest suite to measure the environmental dependence of the IA of dark matter haloes over the redshift range z=0.1 -- 1.5. We quantify each halo's environment through the overdensity delta 8, defined from the number of neighbouring haloes within 8, h-1Mpc, and we isolate the environmental effect from its degeneracy with the halo-mass dependence by comparing the most overdense and most underdense haloes constructed to share the same halo-mass distribution. We find that haloes in underdense environments exhibit systematically larger IA amplitudes A IA than haloes of the same mass in overdense environments, by a factor of about1.5 -- 1.8, and that this trend persists across the mass and redshift ranges probed, strengthening towards low redshift. Using an orientation-only (unit-ellipticity) estimator, we further show that this environmental contrast is driven by a combination of two effects: haloes in underdense regions are both intrinsically less spherical and more strongly aligned with the large-scale tidal field than their overdense counterparts of the same mass. These results indicate that the large-scale environment is a non-negligible variable in modelling halo and galaxy alignments, and may be a particularly important factor for beyond-two-point weak-lensing analyses.