[论文解读] Three-point intrinsic alignments of galaxies and haloes in the FLAMINGO simulations

Casper Vedder, Thomas Bakx+3 more|arXiv (Cornell University)|Jan 25, 2026

Galaxies: Formation, Evolution, Phenomena被引用 0

一句话总结

The paper measures and models the three-point intrinsic alignment statistics for galaxies and haloes in the FLAMINGO hydrodynamical simulations, and tests a tree-level EFT of IA against alternative models across configurations and redshifts.

ABSTRACT

Third-order statistics provide information beyond two-point measures, but extracting this information requires accurate and consistent modelling. We measure and detect the three-point correlation function and third-order aperture-mass statistics of intrinsic alignments (IA) for galaxies and for haloes with $M_{ m halo} > 10^{13}\,{ m M}_\odot$ in the $(2.8\,\mathrm{Gpc})^3$ simulation volume of the FLAMINGO hydrodynamical simulation suite. We model the third-order aperture-mass statistics and show that on large scales both the galaxy and halo samples are well described by the tree-level effective field theory (EFT) of IA across the three dark matter density-shape combinations and a wide range of triangle configurations, with the alignment amplitude consistent with that inferred from two-point statistics. We compare the full EFT to several other models: a version neglecting the velocity-shear term, the non-linear alignment model (NLA), and to a reduced EFT assuming co-evolution relations that follow from the assumption that alignment is linear in Lagrangian space. The first two models yield biased constraints on the alignment amplitude, but the reduced EFT performs remarkably well, achieving a low reduced chi-squared and minimal bias. We examine the redshift and mass dependence of the higher-order bias parameters, finding that the linear Lagrangian bias assumption is approximately satisfied across the explored halo mass and redshift ranges for both galaxies and haloes, suggesting that the galaxies broadly follow the alignment properties of their host haloes. These co-evolution relations can be valuable for photometric shear surveys, where limited constraining power on IA parameters favours models with fewer free parameters.

研究动机与目标

• Motivate the use of third-order statistics to extract non-Gaussian information in intrinsic alignments for cosmology.
• Measure the 3PCF and third-order aperture-mass statistics of IA for galaxies and haloes in a large FLAMINGO volume.
• Develop and compare EFT-based IA models to phenomenological alternatives across configurations and redshifts.
• Assess co-evolution relations and the applicability of simplified IA models for photometric shear surveys.

提出的方法

• Compute 2PCF and 3PCF of IA using projection-specified estimators for shapes and positions.
• Use the aperture-mass formalism to compress the 3PCF into E- and B-modes via the M_ap statistic.
• Adopt a tree-level effective field theory (EFT) of IA and compare to a version without velocity-shear, the nonlinear alignment model (NLA), and a reduced EFT with linear Lagrangian bias assumptions.
• Employ a multipole decomposition and the TreeCorr/Orpheus toolchain to estimate 3PCFs and convert to aperture-mass statistics.
• Project 3D shapes to 2D with a projection operator and decompose into E- and B-modes; relate to three-point statistics through specified kernels.
• Analyze redshift and mass dependence of higher-order IA bias parameters to test co-evolution relations.

Figure 1: The upper panel shows the ellipticity distribution of either galaxies or haloes given a certain mass cut at $z=0$ . The ellipticities are obtained via the simple, non-iterative, inertia tensor using only particles within the half mass radius of the object. The lower panel shows the corresp

实验结果

研究问题

• RQ1 How well does the tree-level EFT of intrinsic alignments describe the galaxy and halo 3-point statistics in FLAMINGO across triangle configurations?
• RQ2 Do simplified or reduced IA models (e.g., reduced EFT with linear Lagrangian bias) reproduce the full EFT results with low bias and good fit quality?
• RQ3 What are the redshift and halo-mass dependences of higher-order IA bias parameters, and do co-evolution relations hold for galaxies and their host haloes?
• RQ4 How do the different IA models compare in constraining the IA amplitude when faced with higher-order statistics?
• RQ5 Can third-order IA statistics provide robust information for photometric shear surveys with limited parameter freedom?

主要发现

• 在大尺度上，银河样本和晕样本在三种暗物质密度-形状组合以及广泛的三角配置范围内，均可被树级IA有效理论描述。
• 从三点统计推断的对齐振幅与二点统计的一致。
• 无速度-剪切的EFT和NLA模型对IA振幅给出有偏差的约束，而具有共进化关系的简化EFT在减小的卡方拟合值和最小化偏差方面表现出色。
• 在所探索的晕质量和红shift范围内，线性拉格朗日偏差假设对银河与晕均大致成立，表明银河在总体上遵循其宿主晕的对齐特性。
• 共进化关系对于参数受限较弱的光度剪切 surveys 有价值，倾向于自由参数较少的模型。

Figure 3: Alignment around galaxy pairs (red dots) separated by a distance $d_{3}$ . The colour map represents the amplitude of the total alignment signal on a logarithmic scale. The black lines indicate the orientation of the galaxies. The grey circle has a radius of $d_{3}/2$ and is centered at $(