Abstract
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.
Highlights
We present the Millennium-II Simulation (MS-II), a very large N -body simulation of dark matter evolution in the concordance ΛCDM cosmology
Simulations with volumes large enough to probe the statistics of large-scale structure and resolution high enough to resolve subhalo dynamics within galaxy halos are critical for this quest
Throughout this paper we have shown that Millennium Simulation (MS)-II results agree extremely well with those from the MS, so we can combine the two simulations to cover an even broader range of physical scales
Summary
Perhaps the most widely-used N -body simulation of cosmological structure formation to date has been the Millennium Simulation (Springel et al 2005, hereafter MS), which followed more than ten billion particles within a simulation volume of (500 h−1 Mpc)3 This provided sufficient mass resolution to see the formation of halos hosting 0.1 L galaxies and sufficient volume to obtain good statistical samples of rare objects such as massive cluster halos and luminous quasars. The strong clustering of matter within a few very massive clumps can create serious problems with respect to parallelization: it is much more difficult to split such a particle distribution into optimal computational domains than is the case if the matter distribution is more homogeneous In spite of these challenges, it is essential to have simulations that probe the structure of galaxy-scale dark matter halos with high mass resolution and over a large enough region to include a sizable and representative sample of objects. Throughout this paper, all logarithms without specified bases are natural logarithms
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