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We study evolution of dark matter substructures, especially how they lose the mass and change density profile after they fall in gravitational potential of larger host halos. We develop an analytical prescription that models the subhalo mass evolution and calibrate it to results of N -body numerical simulations of various scales from very small (Earth size) to large (galaxies to clusters) halos. We then combine the results with halo accretion histories, and calculate the subhalo mass function that is physically motivated down to Earth-mass scales. Our results -valid for arbitrary host masses and redshifts -show reasonable agreement with those of numerical simulations at resolved scales. Our analytical model also enables self-consistent calculations of the boost factor of dark matter annhilation, which we find to increase from tens of percent at the smallest (Earth) and intermediate (dwarfs) masses to a factor of several at galaxy size, and to become as large as a factor of ∼10 for the largest halos (clusters) at small redshifts. Our analytical approach can accommodate substructures in the subhalos (sub-subhalos) in a consistent framework, which we find to give up to a factor of a few enhancement to the annihilation boost. Presence of the subhalos enhances the intensity of the isotropic gamma-ray background by a factor of a few, and as the result, the measurement by Fermi Large Area Telescope excludes the annihilation cross section greater than ∼4 × 10 −26 cm 3 s −1 for dark matter masses up to ∼200 GeV.

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We present the results of the "Cosmogrid" cosmological N-body simulation suites based on the concordance LCDM model. The Cosmogrid simulation was performed in a 30 Mpc box with 2048 3 particles. The mass of each particle is 1.28 × 10 5 M , which is sufficient to resolve ultra-faint dwarfs. We found that the halo mass function shows good agreement with the Sheth & Tormen fitting function down to ∼10 7 M . We have analyzed the spherically averaged density profiles of the three most massive halos which are of galaxy group size and contain at least 170 million particles. The slopes of these density profiles become shallower than −1 at the innermost radius. We also find a clear correlation of halo concentration with mass. The mass dependence of the concentration parameter cannot be expressed by a single power law, however a simple model based on the Press-Schechter theory proposed by Navarro et al. gives reasonable agreement with this dependence. The spin parameter does not show a correlation with the halo mass. The probability distribution functions for both concentration and spin are well fitted by the log-normal distribution for halos with the masses larger than ∼10 8 M . The subhalo abundance depends on the halo mass. Galaxy-sized halos have 50% more subhalos than ∼10 11 M halos have.

We present the evolution of dark matter halos in six large cosmological N-body simulations, called the ν 2 GC (New Numerical Galaxy Catalog) simulations on the basis of the ΛCDM cosmology consistent with observational results obtained by the Planck satellite. The largest simulation consists of 8192 3 (550 billion) dark matter particles in a box of 1.12 h −1 Gpc (a mass resolution of 2.20 × 10 8 h −1 M ⊙ ). Among simulations utilizing boxes larger than 1 h −1 Gpc, our simulation yields the highest resolution simulation that has ever been achieved. A ν 2 GC simulation with the smallest box consists of eight billions particles in a box of 70h −1 Mpc (a mass resolution of 3.44 × 10 6 h −1 M ⊙ ). These simulations can follow the evolution of halos over masses of eight orders of magnitude, from small dwarf galaxies to massive clusters. Using the unprecedentedly high resolution and powerful statistics of the ν 2 GC simulations, we provide statistical results of the halo mass function, mass accretion rate, formation redshift, and merger statistics, and present accurate fitting functions for the Planck cosmology. By combining the ν 2 GC simulations with our new semi-analytic galaxy formation model, we are able to prepare mock catalogs of galaxies and active galactic nuclei, which will be made publicly available in the near future.

We introduce the Uchuu suite of large high-resolution cosmological N-body simulations. The largest simulation, named Uchuu, consists of 2.1 trillion (128003) dark matter particles in a box of side-length 2.0$\, h^{-1} \rm Gpc$, with particle mass 3.27 × 108$\, h^{-1} \rm M_{\odot }$. The highest resolution simulation, Shin-Uchuu, consists of 262 billion (64003) particles in a box of side-length 140$\, h^{-1} \rm Mpc$, with particle mass 8.97 × 105$\, h^{-1} \rm M_{\odot }$. Combining these simulations we can follow the evolution of dark matter halos and subhalos spanning those hosting dwarf galaxies to massive galaxy clusters across an unprecedented volume. In this first paper, we present basic statistics, dark matter power spectra, and the halo and subhalo mass functions, which demonstrate the wide dynamic range and superb statistics of the Uchuu suite. From an analysis of the evolution of the power spectra we conclude that our simulations remain accurate from the Baryon Acoustic Oscillation scale down to the very small. We also provide parameters of a mass-concentration model, which describes the evolution of halo concentration and reproduces our simulation data to within 5 per cent for halos with masses spanning nearly eight orders of magnitude at redshift 0 ≤ z ≤ 14. There is an upturn in the mass-concentration relation for the population of all halos and of relaxed halos at z ≳ 0.5, whereas no upturn is detected at z < 0.5. We make publicly available various N-body products as part of Uchuu Data Release 1 on the Skies & Universes site†. Future releases will include gravitational lensing maps and mock galaxy, X-ray cluster, and active galactic nuclei catalogues.

We study the number and the distribution of low mass Pop III stars in the Milky Way. In our numerical model, hierarchical formation of dark matter minihalos and Milky Way sized halos are followed by a high resolution cosmological simulation. We model the Pop III formation in H 2 cooling minihalos without metal under UV radiation of the Lyman-Werner bands. Assuming a Kroupa IMF from 0.15 to 1.0 M ⊙ for low mass Pop III stars, as a working hypothesis, we try to constrain the theoretical models in reverse by current and future observations. We find that the survivors tend to concentrate on the center of halo and subhalos. We also evaluate the observability of Pop III survivors in the Milky Way and dwarf galaxies, and constraints on the number of Pop III survivors per minihalo. The higher latitude fields require lower sample sizes because of the high number density of stars in the galactic disk, the required sample sizes are comparable in the high and middle latitude fields by photometrically selecting low metallicity stars with optimized narrow band filters, and the required number of dwarf galaxies to find one Pop III survivor is less than ten at < 100 kpc for the tip of red giant stars. Provided that available observations have not detected any survivors, the formation models of low mass Pop III stars with more than ten stars per minihalo are already excluded. Furthermore, we discuss the way to constrain the IMF of Pop III star at a high mass range of 10M ⊙ .

In this paper, we describe the implementation and performance of GreeM, a massively parallel TreePM code for large-scale cosmological N-body simulations. GreeM uses a recursive multi-section algorithm for domain decomposition. The size of the domains are adjusted so that the total calculation time of the force becomes the same for all processes. The loss of performance due to non-optimal load balancing is around 4%, even for more than 10 3 CPU cores. GreeM runs efficiently on PC clusters and massively-parallel computers, such as a Cray XT4. The measured calculation speed on Cray XT4 is 5 × 10 4 particles per second per CPU core, for the case of an opening angle of θ = 0.5, if the number of particles per CPU core is larger than 10 6 .