Cosmological simulation in tides: power spectra and halo shape responses, and shape assembly bias

TL;DR

This paper develops a tidal simulation method to measure the response of matter power spectra and halo shapes to large-scale tidal fields, revealing new dependencies and providing insights for intrinsic alignment studies in cosmology.

Contribution

It introduces an improved tidal simulation technique with corrected initial conditions and quantifies the tidal responses of power spectra and halo shapes, including their secondary dependencies.

Findings

Quantitative measurement of linear tidal response of matter power spectrum.

Universal relation between shape bias and halo bias with a new fitting formula.

First discovery of secondary dependence of shape bias on halo concentration and axis ratio.

Abstract

The well-developed separate universe technique enables accurate calibration of the response of any observable to an isotropic long-wavelength density fluctuation. The large-scale environment also hosts tidal modes that perturb all observables anisotropically. As in the separate universe, both the long tidal and density modes can be absorbed by an effective anisotropic background, on which the interaction and evolution of the short modes change accordingly. We further develop the tidal simulation method, including proper corrections to the second order Lagrangian perturbation theory (2LPT) to generate initial conditions of the simulations. We measure the linear tidal responses of the matter power spectrum, at high redshift from our modified 2LPT, and at low redshift from the tidal simulations. Our results agree qualitatively with previous works, but exhibit quantitative differences in both cases. We also measure the linear tidal response of the halo shapes, or the shape bias, and find its universal relation with the linear halo bias, for which we provide a fitting formula. Furthermore, analogous to the assembly bias, we study the secondary dependence of the shape bias, and discover for the first time dependence on halo concentration and axis ratio. Our results provide useful insights for studies of the intrinsic alignment as source of either contamination or information. These effects need to be correctly taken into account when one uses intrinsic alignments of galaxy shapes as a precision cosmological tool.