We present the Millennium-II Simulation ( MS-II ) , a very large N -body simulation of dark matter evolution in the concordance \Lambda CDM 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 h ^ { -1 } { Mpc } ) with 5 times better spatial resolution ( a Plummer equivalent softening of 1.0 h ^ { -1 } { kpc } ) and with 125 times better mass resolution ( a particle mass of 6.9 \times 10 ^ { 6 } h ^ { -1 } M _ { \odot } ) .
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 .