Cosmological N-Body Simulations with a Large-Scale Tidal Field

TL;DR

This paper introduces anisotropic 'separate universe' N-body simulations incorporating large-scale tidal fields to measure their impact on structure formation, providing new insights into the nonlinear matter bispectrum and developing a related halo model.

Contribution

It presents the first measurement of the nonlinear tidal response function using anisotropic simulations with a large-scale tidal field, extending understanding beyond density responses.

Findings

Tidal response remains below large-scale perturbation theory predictions even nonlinearly.

Developed a halo model incorporating tidal effects that aligns with simulation measurements.

Complete description of the nonlinear matter bispectrum in the squeezed limit.

Abstract

In this paper we carry out anisotropic "separate universe" simulations by including a large-scale tidal field in the N-body code \textsc{gadget}4 using an anisotropic expansion factor $A_{ij}$. We use the code in a pure \textit{particle-mesh} (PM) mode to simulate the evolution of 16 realizations of an initial density field with and without a large-scale tidal field, which are then used to measure the \textit{response function} describing how the tidal field influences structure formation in the linear and non-linear regimes. Together with the previously measured response to a large scale overdensity, this completely describes the nonlinear matter bispectrum in the squeezed limit. We find that, contrary to the density response, the tidal response never significantly exceeds the large-scale perturbation-theory prediction even on nonlinear scales for the redshift range we discuss. We develop a simple halo model that takes into account the effect of the tidal field and compare it with our direct measurement from the anisotropic N-body simulations.