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Aims.We investigate properties of superclusters of galaxies found in of the 2dF Galaxy Redshift Survey, and compare them with properties of superclusters from the Millennium Simulation.

Aims.We use the 2dF Galaxy Redshift Survey data to compile catalogues of superclusters for the Northern and Southern regions of the 2dFGRS, altogether 543 superclusters at redshifts $0.009 \leq z \leq 0.2$.

Aims.We compile a supercluster sample using the Sloan Digital Sky Survey Data Release 4, and reanalyse supercluster samples found for the 2dF Galaxy Redshift Survey and for simulated galaxies of the Millennium Run.

The recent years have seen combined measurements of X-ray and (weak) lensing contours for colliding galaxy clusters such as, for instance, the famous "Bullet" cluster. These observations have revealed offsets in the peaks of the baryonic and (dominant) gravitational matter component of order [?]100-200 kpc. Such discrepancies are difficult to explain using modified theories for gravity other than dark matter. Or are they not? Here we use the concept of "phantom dark matter" that is based upon a Newtonian interpretation of the modified Newtonian dynamics (MONDian) gravitational potential. We show that this idea is in fact capable of producing substantial offsets in idealistic density configurations, involving a uniform external field. However, when analyzed in a MONDian cosmological framework we deduce that the size (and probability) of the effect is too small to explain the observed offsets found in the most recent observations, at least in the simplest incarnation of phantom dark matter as discussed here. The lensing centers in merging galaxy clusters are likely very close to the centers of true mass even in a MONDian cosmology. This gives the support to the idea that neutrino-like non-collisional matter might be responsible for the observed offsets of lensing and X-ray peaks.

We systematically study the effects of varying the starting redshift zi for cosmological simulations in the highly nonlinear regime. Our primary focus lies with the (individual) properties of dark matter halos--namely the mass, spin, triaxiality, and concentration--where we find that even substantial variations in zi leave only a small imprint, at least for the probed mass range M [?] [1010, 1013] h -1 M and when investigated at redshift z = 0. We further compare simulations started by using the standard Zel'dovich approximation to runs based upon initial conditions produced with second-order Lagrangian perturbation theory. Here, we observe the same phenomenon, i.e., that differences in the studied (internal) properties of dark matter halos are practically undetectable. These findings are (for the probed mass range) in agreement with other work in the literature. We therefore conclude that the commonly used technique for setting up cosmological simulations leads to stable results at redshift z = 0 for the mass, the spin parameter, the triaxiality, and the concentration of dark matter halos.

We report on a correlation between virial mass M and spin parameter l for dark matter halos forming at redshifts z[?] 10. We find that the spin parameter decreases with increasing halo mass. Interestingly, our analysis indicates that halos forming at later times do not exhibit such a strong correlation, in agreement with the findings of previous studies. We briefly discuss the implications of this correlation for galaxy formation at high redshifts and the galaxy population we observe today.

When analyzing dark matter halos forming in cosmological n-body simulations, it is common practice to obtain the density profile utilizing spherical shells. However, it is also known that the systems under investigation are far from spherically symmetric but, rather, follow a triaxial mass distribution. In this study we present an estimator for the error introduced by spherically averaging an elliptical mass distribution. We systematically investigate the differences arising when using a triaxial density profile under the assumption of spherical symmetry. We show that the variance in the density can be as large as 50? in the outer parts of dark matter halos for extreme (but still credible) axis ratios of 0.55:0.67:1. The inner parts are less affected but still show a scatter at the 16? level for these prolate systems. For more moderate ellipticities, i.e. axis ratios of 0.73:0.87:1, the error is smaller but still as large as 10?20? depending on distance. We further provide a simple formula that allows estimation of this variance as a function of radius for arbitrary axis ratios. We conclude that highly prolate and?or oblate systems are better fit by analytical profiles that take into account the triaxial nature of cosmological objects.

We present results derived from a high-resolution cosmological N-body simulation in which the equations of motion have been changed to account for MOdified Newtonian Dynamics (MOND). It is shown that a low-Ω0MONDian model with an appropriate choice for the normalisation σ8of the primordial density fluctuations can lead to similar clustering properties at redshift z= 0 as the commonly accepted (standard) ΛCDM model. However, such a model shows no significant structures at high redshift with only very few objects present beyond z> 3. For the current implementation of MOND density profiles of gravitationally bound objects at z= 0 can though still be fitted by the universal NFW profile.

AbstractWhen analyzing dark matter halos forming in cosmological n-body simulations, it is common practice to obtain the density profile utilizing spherical shells. However, it is also known that the systems under investigation are far from spherically symmetric but, rather, follow a triaxial mass distribution. In this study we present an estimator for the error introduced by spherically averaging an elliptical mass distribution. We systematically investigate the differences arising when using a triaxial density profile under the assumption of spherical symmetry. We show that the variance in the density can be as large as 50% in the outer parts of dark matter halos for extreme (but still credible) axis ratios of 0.55: 0.67: 1. The inner parts are less affected but still show a scatter at the 16% level for these prolate systems. For more moderate ellipticities, i.e. axis ratios of 0.73: 0.87: 1, the error is smaller but still as large as 10–20% depending on distance. We further provide a simple formula that allows estimation of this variance as a function of radius for arbitrary axis ratios. We conclude that highly prolate and/or oblate systems are better fit by analytical profiles that take into account the triaxial nature of cosmological objects.

We present results derived from a high-resolution cosmological N-body simulation in which the equations of motion have been changed to account for MOdified Newtonian Dynamics (MOND). It is shown that a low-Ω0 MONDian model with an appropriate choice for the normalisation σ8 of the primordial density fluctuations can lead to similar clustering properties at redshift z=0 as the commonly accepted (standard) ΛCDM model. However, such a model shows no significant structures at high redshift with only very few objects present beyond z>3. For the current implementation of MOND density profiles of gravitationally bound objects at z=0 can though still be fitted by the universal NFW profile.

In this contribution a broad overview of the methodologies of cosmological N-body simulations and a short introduction explaining the general idea behind such simulations is presented. After explaining how to set up the initial conditions using a set of N particles two (diverse) techniques are presented for evolving these particles forward in time under the influence of their self-gravity. One technique (tree codes) is solely based upon a sophistication of the direct particle–particle summation whereas the other method relies on the continuous (de-)construction of arbitrarily shaped grids and is realized in adaptive mesh refinement codes.

AbstractIn this contribution a broad overview of the methodologies of cosmological N-body simulations and a short introduction explaining the general idea behind such simulations is presented. After explaining how to set up the initial conditions using a set of Nparticles two (diverse) techniques are presented for evolving these particles forward in time under the influence of their self-gravity. One technique (tree codes) is solely based upon a sophistication of the direct particle–particle summation whereas the other method relies on the continuous (de-)construction of arbitrarily shaped grids and is realized in adaptive mesh refinement codes.

Nearby clusters such as Virgo and Coma possess galaxy distributions that tend to be aligned with the principal axis of the cluster itself. This has also been confirmed by a recent statistical analysis of some 300 Abell clusters, where the effect has been linked to the dynamical state of the cluster. Moreover, the orbits of satellite galaxies in galactic systems like our own Milky Way also demonstrate a high degree of anisotropy--the so-called Holmberg effect, the origin of which has been the subject of debate for more than 30 years. This study presents the analysis of cosmological simulations focusing on the orbits of satellite galaxies within dark matter halos. The apocenters of the orbits of these satellites are preferentially found within a cone of opening angle ~40deg around the major axis of the host halo, in accordance with the observed anisotropy found in galaxy clusters. We do, however, note that a link to the dynamical age of the cluster is not well established, as both of our oldest dark matter halos do show a clear anisotropy signal. Further analysis connects this distribution to the infall pattern of satellites along the filaments: the orbits are determined rather by the environment of the host halo than some "dynamical selection" during their life within the host's virial radius.

AbstractWe present a statistical analysis of the interactions between satellite galaxies in cosmological dark matter halos taken from fully self-consistent high-resolution simulations of galaxy clusters. We show that the number distribution of satellite encounters has a tail that extends to as many as three to four encounters per orbit. On average 30% of the substructure population had at least one encounter (per orbit) with another satellite galaxy. However, this result depends on the age of the dark matter host halo with a clear trend for more interactions in younger systems. We also report a correlation between the number of encounters and the distance of the satellites to the centre of the cluster — satellite galaxies closer to the centre experience more interactions. However, this can be simply explained by the radial distribution of the substructure population and merely reflects the fact that the density of satellites is higher in those regions.In order to find substructure galaxies we applied (and present) a new technique based upon the N-body code MLAPM. This new halo finder MHF (MLAPM’s halo finder) acts with exactly the same accuracy as the N-body code itself and is therefore free of any bias and spurious mismatch between simulation data and halo finding precision related to numerical effects.

We present a statistical analysis of the interactions between satellite galaxies in cosmological dark matter halos taken from fully self-consistent high-resolution simulations of galaxy clusters. We show that the number distribution of satellite encounters has a tail that extends to as many as three to four encounters per orbit. On average 30% of the substructure population had at least one encounter (per orbit) with another satellite galaxy. However, this result depends on the age of the dark matter host halo with a clear trend for more interactions in younger systems. We also report a correlation between the number of encounters and the distance of the satellites to the centre of the cluster — satellite galaxies closer to the centre experience more interactions. However, this can be simply explained by the radial distribution of the substructure population and merely reflects the fact that the density of satellites is higher in those regions.In order to find substructure galaxies we applied (and present) a new technique based upon the N-body code MLAPM. This new halo finder MHF (MLAPM’s halo finder) acts with exactly the same accuracy as the N-body code itself and is therefore free of any bias and spurious mismatch between simulation data and halo finding precision related to numerical effects.

We present the study of ten random realisations of a density field characterised by a cosmological power spectrum P(k) at redshift z = 50. The reliability of such initial conditions for N-body simulations is tested with respect to their correlation properties. The power spectrum P(k) and the mass variance σM(r) do not show detectable deviations from the desired behaviour in the intermediate range of scales between the mean interparticle distance and the simulation volume. The estimator for ξ(r) is too noisy to detect any reliable signal at the initial redshift z = 50. The particle distributions are then evolved forward until z = 0. This allows us to explore the cosmic variance stemming from the random nature of the initial conditions. With cosmic variance we mean the fact that a simulation represents a single realisation of the stochastic initial conditions whereas the real Universe contains many realisations of regions of the size of the box; this problem affects most importantly the scales at about the fundamental mode. We study morphological descriptors of the matter distribution such as the genus, as well as the internal properties of the largest object(s) forming in the box. We find that the scatter is at least comparable to the scatter in the fundamental mode.

AbstractWe present the study of ten random realisations of a density field characterised by a cosmological power spectrum P(k) at redshift z= 50. The reliability of such initial conditions for N-body simulations is tested with respect to their correlation properties. The power spectrum P(k) and the mass variance sM(r) do not show detectable deviations from the desired behaviour in the intermediate range of scales between the mean interparticle distance and the simulation volume. The estimator for ?(r) is too noisy to detect any reliable signal at the initial redshift z= 50. The particle distributions are then evolved forward until z= 0. This allows us to explore the cosmic variance stemming from the random nature of the initial conditions. With cosmic variance we mean the fact that a simulation represents a single realisation of the stochastic initial conditions whereas the real Universe contains many realisations of regions of the size of the box; this problem affects most importantly the scales at about the fundamental mode. We study morphological descriptors of the matter distribution such as the genus, as well as the internal properties of the largest object(s) forming in the box. We find that the scatter is at least comparable to the scatter in the fundamental mode.

We compare the observed power spectrum of matter found in our earlier papers with analytical power spectra. We extrapolate the observed power spectra on small scales to find the linear power spectrum of matter. We consider spatially flat cold and mixed dark matter models with the cosmological constant as well as open models. We fix the Hubble constant and the baryon density in the middle of the allowed range and vary the density parameter and the cosmological constant. We determine the primordial power spectrum of matter using the power spectrum of matter and the transfer functions of analytical models. We take two different spectra suggested by observations: one with a sharp maximum at 120 h-1 Mpc and a second one with a broader maximum, as found for regions with rich and medium-rich superclusters of galaxies, respectively. For both models, the primordial power spectra have a break in amplitude; in the case of the spectrum with a sharp maximum the break is sharp. We conclude that a scale-free primordial power spectrum is excluded if presently available data on the distribution of clusters and galaxies represent the true mass distribution of the universe.

We study the growth of galaxy masses, via gas accretion and galaxy mergers. We introduce a toy model that describes (in a single equation) how much baryonic mass is accreted and retained into galaxies as a function of halo mass and redshift. In our model, the evolution of the baryons differs from that of the dark matter because 1) gravitational shock heating and AGN jets suppress gas accretion mainly above a critical halo mass of Mshock~ 1012M⊙; 2) the intergalactic medium after reionisation is too hot for accretion onto haloes with circular velocities vcirc≲ 40 km   s-1; 3) stellar feedback drives gas out of haloes, mainly those with vcirc≲ 120    km   s-1. We run our model on the merger trees of the haloes and sub-haloes of a high-resolution dark matter cosmological simulation. The galaxy mass is taken as the maximum between the mass given by the toy model and the sum of the masses of its progenitors (reduced by tidal stripping). Designed to reproduce the present-day stellar mass function of galaxies, our model matches fairly well the evolution of the cosmic stellar density. It leads to the same z= 0 relation between central galaxy stellar and halo mass as the one found by abundance matching and also as that previously measured at high mass on SDSS centrals. Our model also predicts a bimodal distribution (centrals and satellites) of stellar masses for given halo mass, in very good agreement with SDSS observations. The relative importance of mergers depends strongly on stellar mass (more than on halo mass). Massive galaxies with mstars> mcrit~ Ωb/ΩmMshock~ 1011M⊙acquire most of their final mass through mergers (mostly major and gas-poor), as expected from our model’s shutdown of gas accretion at high halo masses. However, although our mass resolution should see the effects of mergers down to mstars≃ 1010.6h-1M⊙, we find that mergers are rare for mstars≲ 1011h-1M⊙. This is a consequence of the curvature of the stellar vs. halo mass relation set by the physical processes of our toy model and found with abundance matching. So gas accretion must be the dominant growth mechanism for intermediate and low mass galaxies, including dwarf ellipticals in clusters. The contribution of galaxy mergers terminating in haloes with mass Mhalo< Mshock(thus presumably gas-rich) to the mass buildup of galaxies is small at all masses, but accounts for the bulk of the growth of ellipticals of intermediate mass (~1010.5h-1M⊙), which we predict must be the typical mass of ULIRGs.