Cosmic Web-halo Connection between Twin Universes

Both simulation and observational data have shown that the spin and shape of dark matter halos are correlated with their nearby large-scale environment. As structure formation on different scales is strongly coupled, it is tricky to disentangle the formation of a halo from that of the large-scale environment, making it difficult to infer which is the driving force behind the correlation between halo spin/shape and the large-scale structure. In this paper, we use N-body simulations to produce twin universes that share the same initial conditions on small scales but that are different on large scales. This is achieved by changing the random seeds for the phase of those k modes smaller than a given scale in the initial conditions. In this way, we are able to disentangle the formation of halo and large-scale structure, making it possible to investigate how halo spin and shape correspond to the change of environment on large scales. We identify matching halo pairs in the twin simulations as those sharing the maximum number of identical particles within them. Using these matched halo pairs, we study the cross match of halo spin and the correlation with the large-scale structure. It is found that when the large-scale environment changes (eigenvector) between the twin simulations, the halo spin has to rotate accordingly, although not significantly, to maintain the universal correlation seen in each simulation. Our results suggest that the large-scale structure is the main factor to drive the correlation between halo properties and their environment.