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

TL;DR

This study investigates galaxy and cosmic web alignments at redshifts 1-2 using IllustrisTNG simulations, revealing weaker spin alignments than previous models and assessing observational prospects with upcoming surveys.

Contribution

It provides new insights into galaxy-cosmic web alignments at high redshift and compares simulation results, highlighting the dependence on subgrid physics and observational challenges.

Findings

Alignment signals are consistent with previous studies at similar redshifts.

Spin alignment magnitude is approximately 2.4 times weaker than in Horizon-AGN simulation.

Upcoming Subaru PFS GE survey can detect shape alignment at z~1 with high significance.

Abstract

Galaxy formation theories predict that galaxy shapes and angular momenta have non-random alignments with the cosmic web. This leads to so-called intrinsic alignment between pairs of galaxies, which is important to quantify as a nuisance parameter for weak lensing. We study galaxy-cosmic web alignment 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 this signal may have significant dependence on subgrid physics. Based on IllustrisTNG, we then construct mock observational spectroscopic surveys that can probe shape-cosmic web alignment 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 alignment signal that depend on the projected direction -- this induces significant errors in the observed alignment. We predict a $5.3\sigma$ detection of IllustrisTNG's shape alignment 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 alignment signal strengths between the TNG and HorizonAGN simulations suggests that PFS GE should be able to more easily constrain the latter's stronger signal.