Living with Neighbors. IV. Dissecting the Spin$-$Orbit Alignment of Dark Matter Halos: Interacting Neighbors and the Local Large-scale Structure.

Spin$-$orbit alignment (SOA; i.e., the vector alignment between the halo spin and the orbital angular momentum of neighboring halos) provides an important clue to how galactic angular momenta develop. For this study, we extract virial-radius-wise contact halo pairs with mass ratios between 1/10 and 10 from a set of cosmological $N$-body simulations. In the spin--orbit angle distribution, we find a significant SOA in that 52.7%$\pm$0.2% of neighbors are on the prograde orbit. The SOA of our sample is mainly driven by low-mass target halos ($<10^{11.5}h^{-1}M_{\odot}$) with close merging neighbors, corroborating the notion that the tidal interaction is one of the physical origins of SOA. We also examine the correlation of SOA with the adjacent filament and find that halos closer to the filament show stronger SOA. Most interestingly, we discover for the first time that halos with the spin parallel to the filament experience most frequently the prograde-polar interaction (i.e., fairly perpendicular but still prograde interaction; spin--orbit angle $\sim$ 70$^{\circ}$). This instantly invokes the spin-flip event and the prograde-polar interaction will soon flip the spin of the halo to align it with the neighbor's orbital angular momentum. We propose that the SOA originates from the local cosmic flow along the anisotropic large-scale structure, especially that along the filament, and grows further by interactions with neighbors.