Beyond the Halo: Redefining environment with unbound matter in N-body
  simulations
  Approximately half of the matter in the Universe is "unbound" at z = 0,
according to N-body simulations such as the Millennium Run. Here, we use the
milli-Millennium simulation to examine the distribution of unbound matter in
relation to the dark matter halos which host galaxies. We measure the unbound
matter within two types of windows, using a halo dependent radius and a fixed
radius at several different scales. We also consider the timescales over which
a halo can accrete the local unbound matter at z = 2 and z = 0. Finally, we
compare the unbound matter to observable properties of galaxies, such as local
galaxy count environment and stellar mass. We find that halos at z = 2 can
accrete far more of the nearby unbound matter over a Hubble time than halos at
z = 0 and that 78% of particles within 5 $R_{vir}$ of a halo at z = 2 will be
accreted by z = 0, compared to 36% of particles within 5 $h^{-1}$ Mpc of the
halo. We also find that galaxy count environment is closely related to the
amount of nearby unbound matter when measured on the same scale.
