Resolving Cosmic Structure Formation with the Millennium-II Simulation
  We present the Millennium-II Simulation (MS-II), a very large N-body
simulation of dark matter evolution in the concordance LCDM cosmology. The
MS-II assumes the same cosmological parameters and uses the same particle
number and output data structure as the original Millennium Simulation (MS),
but was carried out in a periodic cube one-fifth the size (100 Mpc/h) with 5
times better spatial resolution (a Plummer equivalent softening of 1.0 kpc/h)
and with 125 times better mass resolution (a particle mass of 6.9 \times 10^6
Msun/h). By comparing results at MS and MS-II resolution, we demonstrate
excellent convergence in dark matter statistics such as the halo mass function,
the subhalo abundance distribution, the mass dependence of halo formation
times, the linear and nonlinear autocorrelations and power spectra, and halo
assembly bias. Together, the two simulations provide precise results for such
statistics over an unprecedented range of scales, from halos similar to those
hosting Local Group dwarf spheroidal galaxies to halos corresponding to the
richest galaxy clusters. The "Milky Way" halos of the Aquarius Project were
selected from a lower resolution version of the MS-II and were then resimulated
at much higher resolution. As a result, they are present in the MS-II along
with thousands of other similar mass halos. A comparison of their assembly
histories in the MS-II and in resimulations of 1000 times better resolution
shows detailed agreement over a factor of 100 in mass growth. We publicly
release halo catalogs and assembly trees for the MS-II in the same format
within the same archive as those already released for the MS.
