Intrinsic Alignments Between Galaxies and the Cosmic Web at $z\sim 1-2$ in the IllustrisTNG Simulations

Galaxy formation theories predict that galaxy shapes and angular momenta have intrinsic alignments (IA) with the cosmic web, which provides an observational test of various theories, and is important to quantify as a nuisance parameter for weak lensing. We study galaxy IA in the IllustrisTNG suite of hydrodynamical simulations at redshifts 1 and 2, finding that alignment trends are consistent with previous studies. However, we find that the magnitude of the spin alignment signal is $\sim 2.4 \times$ weaker than seen in previous studies of the Horizon-AGN simulation, suggesting that IA may have significant dependence on subgrid physics. Based on IllustrisTNG, we then construct mock observational spectroscopic surveys that can probe shape-cosmic web IA at $z \sim 1-2$, modeled on the low-$z$ galaxy redshift and IGM tomography surveys on the upcoming Subaru Prime Focus Spectrograph Galaxy Evolution (PFS GE) survey. However, even over box sizes of $L=205 h^{-1} \mathrm{Mpc}$, we find that global anisotropies induce a sample variance in the 2D projected IA signal that depend on the projected direction -- this induces significant errors in the observed alignment. We predict a $5.3\sigma$ detection of IlustrisTNG's shape IA signal at $z \sim 1$ from Subaru PFS GE, although a detection would be challenging at $z \sim 2$. However, a rough rescaling of the relative IA signal strengths between the TNG and HorizonAGN simulations suggests that PFS GE should be able to constrain the latter's stronger signal.