Intrinsic alignments of galaxies in multiple projections

¹ Institute for Theoretical Physics, Utrecht University Princetonplein 5 3584 CC Utrecht, The Netherlands
² Leiden Observatory, Leiden University P.O. Box 9513 2300 RA Leiden, The Netherlands

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 9 September 2025
Accepted: 22 November 2025

Abstract

The intrinsic alignments of galaxies can be measured and modelled to gain cosmological information and further improve our understanding of the interactions between galaxies, as well as to mitigate their effects on gravitational weak lensing studies. Hydrodynamical simulations are often used to constrain priors or calibrate models. Therefore, obtaining the maximum amount of information possible from these simulations is imperative. In this work, we combined the information of shapes projected over two or three axes (x, y, z), for intrinsic alignment signals (w_g+,ξ∼_g+,2), showing a consistent gain in signal-to-noise ratio (S/N) for all cases studied using TNG300-1. The gain in S/N is found to be higher for the addition of the second projection than for the third, and it is also higher for shapes calculated using the reduced inertia tensor, rather than the simple one. The two shape samples studied, n_★ > 300 and log(M_★ h/M_⊙) > 10.5, where the latter has a much higher signal amplitude, show similar gains in S/N when more projections are added. We also modelled the correlation functions with the non-linear alignment model for scales greater than 6 Mpc/h. The S/N gains on the non-linear alignment amplitude, A_IA, and galaxy bias, b_g, are higher than those seen for the full measurements, indicating potential advantages for future works, particularly on larger scales with an increased uncertainty on the alignment signals. Using multiple projection axes increases the overall S/N, enabling a more efficient use of numerically expensive hydrodynamical simulations.