Measuring the tidal response of structure formation: anisotropic separate universe simulations using TREEPM

We present anisotropic 'separate universe' simulations that modify the N-body code gadget4 in order to represent a large-scale tidal field through an anisotropic expansion factor. These simulations are used to measure the linear, quasi-linear, and non-linear response of the matter power spectrum to a spatially uniform trace-free tidal field up to wavenumber k = 7 h Mpc(-1). Together with the response to a large-scale overdensity measured in previous work, this completely describes the non-linear matter bispectrum in the squeezed limit. We find that the response amplitude does not approach zero on small scales in physical coordinates, but rather a constant value at z = 0, R-K approximate to 0.5 for k >= 3h Mpc(-1) up to the scale where we consider our simulations reliable, . This shows that even the inner regions of haloes are affected by the large-scale tidal field. We also measure directly the alignment of halo shapes with the tidal field, finding a clear signal that increases with halo mass.

Automated summary

The study uses anisotropic 'separate universe' simulations to examine the response of the matter power spectrum to a large-scale tidal field. The results show that even the inner regions of haloes are affected by the large-scale tidal field, and there is a clear signal of halo shape alignment with the tidal field that increases with halo mass.