Measuring the Tidal Response of Structure Formation: Anisotropic
  Separate Universe Simulations using TreePM
  We present anisotropic "separate universe" simulations which modify the
N-body code Gadget-4 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 nonlinear response of the matter power spectrum to a spatially
uniform trace-free tidal field up to wavenumber $k = 7 h \text{Mpc}^{-1}$.
Together with the response to a large-scale overdensity measured in previous
work, this completely describes the nonlinear 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 $z=0$,
$R_K\approx 0.5$ for $k \geq 3 h\text{Mpc}^{-1}$ up to the scale where we
consider our simulations reliable, $k \leq 7 h\text{Mpc}^{-1}$ at $z=0$. 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 which increases with halo mass.
