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 trick to
disentangle the formation of halo with the large-scale environment, making it
difficult to infer which is the driving force for the correlation between halo
spin/shape with the large-scale structure. In this paper, we use N-body
simulation to produce twin Universes that share the same initial conditions on
small scales but 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 each other. Using these matched halo pairs, we
study the cross match of halo spin and their 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.
