Tidal alignment of galaxies

TL;DR

This paper presents an advanced analytic model for galaxy intrinsic alignments based on tidal alignment theory, incorporating nonlinear corrections and comparing with observations, improving understanding of IA in cosmic shear analyses.

Contribution

The paper introduces a comprehensive nonlinear tidal alignment model that accounts for density weighting, bias dependence, and halo-scale effects, enhancing the accuracy of IA predictions.

Findings

Nonlinear corrections significantly improve IA modeling on small scales.

Density weighting causes bias-dependent IA amplitude, affecting luminosity dependence.

The model accurately describes observed alignments of luminous red galaxies.

Abstract

We develop an analytic model for galaxy intrinsic alignments (IA) based on the theory of tidal alignment. We calculate all relevant nonlinear corrections at one-loop order, including effects from nonlinear density evolution, galaxy biasing, and source density weighting. Contributions from density weighting are found to be particularly important and lead to bias dependence of the IA amplitude, even on large scales. This effect may be responsible for much of the luminosity dependence in IA observations. The increase in IA amplitude for more highly biased galaxies reflects their locations in regions with large tidal fields. We also consider the impact of smoothing the tidal field on halo scales. We compare the performance of this consistent nonlinear model in describing the observed alignment of luminous red galaxies with the linear model as well as the frequently used "nonlinear alignment model," finding a significant improvement on small and intermediate scales. We also show that the cross-correlation between density and IA (the "GI" term) can be effectively separated into source alignment and source clustering, and we accurately model the observed alignment down to the one-halo regime using the tidal field from the fully nonlinear halo-matter cross correlation. Inside the one-halo regime, the average alignment of galaxies with density tracers no longer follows the tidal alignment prediction, likely reflecting nonlinear processes that must be considered when modeling IA on these scales. Finally, we discuss tidal alignment in the context of cosmic shear measurements.