Internal properties and environments of dark matter halos
  We use seven high-resolution $N$-body simulations to study the correlations
among different halo properties (assembly time, spin, shape and substructure),
and how these halo properties are correlated with the large-scale environment
in which halos reside. The large-scale tidal field estimated from halos above a
mass threshold is used as our primary quantity to characterize the large-scale
environment, while other parameters, such as the local overdensity and the
morphology of large-scale structure, are used for comparison. For halos at a
fixed mass, all the halo properties depend significantly on environment,
particularly the tidal field. The environmental dependence of halo assembly
time is primarily driven by local tidal field. The mass of the unbound fraction
in substructure is boosted in strong tidal force region, while the bound
fraction is suppressed. Halos have a tendency to spin faster in stronger tidal
field and the trend is stronger for more massive halos. The spin vectors show
significant alignment with the intermediate axis of the tidal field, as
expected from the tidal torque theory. Both the major and minor axes of halos
are strongly aligned with the corresponding principal axes of the tidal field.
In general, a halo that can accrete more material after the formation of its
main halo on average is younger, is more elongated, spins faster, and contains
a larger amount of substructure. Higher density environments not only provide
more material for halo to accrete, but also are places of stronger tidal field
that tends to suppress halo accretion. The environmental dependencies are the
results of these two competing effects. The tidal field based on halos can be
estimated from observation, and we discuss the implications of our results for
the environmental dependence of galaxy properties.
