We analyze the halo occupation distribution (HOD) and two-point correlation function of galaxy-size dark matter halos using high-resolution dissipationless simulations of the concordance flat ÃCDM model. The halo samples include both the host halos and the ''subhalos,'' distinct gravitationally bound halos within the virialized regions of larger host systems. We find that the HOD, the probability distribution for a halo of mass M to host a number of subhalos N, is similar to that found in semianalytic and N-body+gasdynamics studies. Its first moment, hN i M , has a complicated shape consisting of a step, a shoulder, and a power-law high-mass tail. The HOD can be described by Poisson statistics at high halo masses but becomes sub-Poisson for hN i M P 4. We show that the HOD can be understood as a combination of the probability for a halo of mass M to host a central galaxy and the probability to host a given number N s of satellite galaxies. The former can be approximated by a steplike function, while the latter can be well approximated by a Poisson distribution, fully specified by its first moment. The first moment of the satellite HOD can be well described by a simple power law hN s i / M with % 1 for a wide range of number densities, redshifts, and different power spectrum normalizations. This formulation provides a simple but accurate model for the halo occupation distribution found in simulations. At z ¼ 0, the twopoint correlation function (CF) of galactic halos can be well fitted by a power law down to $100 h À1 kpc with an amplitude and slope similar to those of observed galaxies. The dependence of correlation amplitude on the number density of objects is in general agreement with results from the Sloan Digital Sky Survey. At redshifts z k 1, we find significant departures from the power-law shape of the CF at small scales, where the CF steepens because of a more pronounced one-halo component. The departures from the power law may thus be easier to detect in high-redshift galaxy surveys than at the present-day epoch. They can be used to put useful constraints on the environments and formation of galaxies. If the deviations are as strong as indicated by our results, the assumption of the single power law often used in observational analyses of high-redshift clustering is dangerous and is likely to bias the estimates of the correlation length and slope of the correlation function.

Using six high‐resolution dissipationless simulations with a varying box size in a flat Lambda cold dark matter (ΛCDM) universe, we study the mass and redshift dependence of dark matter halo shapes for Mvir= 9.0 × 1011− 2.0 × 1014 h−1 M⊙, over the redshift range z= 0–3, and for two values of σ8= 0.75 and 0.9. Remarkably, we find that the redshift, mass and σ8 dependence of the mean smallest‐to‐largest axis ratio of haloes is well described by the simple power‐law relation 〈s〉= (0.54 ± 0.02)(Mvir/M*)−0.050±0.003, where s is measured at 0.3Rvir, and the z and σ8 dependences are governed by the characteristic non‐linear mass, M*=M*(z, σ8). We find that the scatter about the mean s is well described by a Gaussian with σ∼ 0.1, for all masses and redshifts. We compare our results to a variety of previous works on halo shapes and find that reported differences between studies are primarily explained by differences in their methodologies. We address the evolutionary aspects of individual halo shapes by following the shapes of the haloes through ∼100 snapshots in time. We determine the formation scalefactor ac as defined by Wechsler et al. and find that it can be related to the halo shape at z= 0 and its evolution over time.

We investigate the dependence of dark matter halo clustering on halo formation time, density profile concentration, and subhalo occupation number, using high-resolution numerical simulations of a LCDM cosmology. We confirm results that halo clustering is a function of halo formation time at fixed mass, and that this trend depends on halo mass. For the first time, we show unequivocally that halo clustering is a function of halo concentration and show that the dependence of halo bias on concentration, mass, and redshift can be accurately parameterized in a simple way:. Interestingly, the scaling between bias and concentration changes sign with the value of M/M * : high concentration (early forming) objects cluster more strongly for M < ∼ M * , while low concentration (late forming) objects cluster more strongly for rare high-mass halos, M > ∼ M * . We show the first explicit demonstration that host dark halo clustering depends on the halo occupation number (of dark matter subhalos) at fixed mass, and discuss implications for halo model calculations of dark matter power spectra and galaxy clustering statistics. The effect of these halo properties on clustering is strongest for early-forming dwarf-mass halos, which are significantly more clustered than typical halos of their mass. Our results suggest that isolated low-mass galaxies (e.g. low surface-brightness dwarfs) should have more slowly-rising rotation curves than their more clustered counterparts, and may have consequences for the dearth of dwarf galaxies in voids. They also imply that self calibrating richness-selected cluster samples with their clustering properties might overestimate cluster masses and bias cosmological parameter estimation.

We analyze the effect of dissipation on the shapes of dark matter (DM) halos using high-resolution cosmological gasdynamics simulations of clusters and galaxies in the LCDM cosmology. We find that halos formed in simulations with gas cooling are significantly more spherical than corresponding halos formed in adiabatic simulations. Gas cooling results in an average increase of the principle axis ratios of halos by ~ 0.2-0.4 in the inner regions. The systematic difference decreases slowly with radius but persists almost to the virial radius. We argue that the differences in simulations with and without cooling arise both during periods of quiescent evolution, when gas cools and condenses toward the center, and during major mergers. We perform a series of high-resolution N-body simulations to study the shapes of remnants in major mergers of DM halos and halos with embedded stellar disks. In the DM halo-only mergers, the shape of the remnants depends only on the orbital angular momentum of the encounter and not on the internal structure of the halos. However, significant shape changes in the DM distribution may result if stellar disks are included. In this case the shape of the DM halos is correlated with the morphology of the stellar remnants.Comment: Accepted for publication in ApJL, 5 pages, 3 figures, LaTeX (uses emulateapj5.sty

A large scale SPH+N-body simulation (GADGET) of the concordance LCDM universe is used to investigate orientation and angular momentum of galaxy clusters at z=0 in connection with their recent accretion histories. The basic cluster sample comprises the 3000 most massive friends-of-friends halos found in the 500 Mpc/h simulation box. Two disjoint sub-samples are constructed, using the mass ratio of the two most massive progenitors at z=0.5 m_2 / m_1 (m_1 < m_2), namely a recent major merger sample and a steady accretion mode sample. The mass of clusters in the merger sample is on average ~43% larger than the mass of the two progenitors (m_1 + m_2), whereas in the steady accretion mode sample a smaller increase of ~25% is found. The separation vector connecting the two most massive progenitor halos at z=0.5 is strongly correlated with the orientation of the cluster at z=0. The angular momentum of the clusters in the recent major merger sample tends to be parallel to orbital angular momentum of the two progenitors, whereas the angular momentum of the steady accretion mode sample is mainly determined by the angular momentum of the most massive progenitor. The long range correlations for the major and the minor principal axes of cluster pairs extend to distances of ~100 Mpc/h. Weak angular momentum correlations are found for distances < 20 Mpc/h. Within these ranges the major axes tend to be aligned with the connecting line of the cluster pairs whereas minor axes and angular momenta tend to be perpendicular to this line. A separate analysis of the two sub-samples reveals that the long range correlations are independent of the mass accretion mode. Thus orientation and angular momentum of galaxy clusters is mainly determined by the accretion along the filaments independently of the particular accretion mode.Comment: 11 pages, 8 figures, replaced to match version accepted for publication in MNRA