We present a new method – called HINORA (HIgh-NOise RANdom SAmple Consensus) – for the identification of regular structures in 3D point distributions. Motivated by the possible existence of the so-called Council of Giants, that is, a ring of 12 massive galaxies surrounding the Local Group in the Local Sheet with a radius of 3.75 Mpc, we apply HINORA to the Local Volume Galaxy catalogue confirming its existence. When varying the lower limit of K-band luminosity of the galaxy entering the catalogue, we further report on the existence of another ring-like structure in the Local Volume that now contains the Milky Way and M31. However, this newly found structure is dominated by low-mass (satellite) galaxies. While we here simply present the novel method as well as its first application to observational data, follow-up work using numerical simulations of cosmic structure formation shall shed light into the origin of such regular patterns in the galaxy distribution. Further, the method is equally suited to identify similar (or even different) structures in various kinds of astrophysical data (e.g. locating the actual ‘baryonic-acoustic oscillation spheres’ in galaxy redshift surveys).

The differences between cold dark matter (CDM) and warm dark matter (WDM) in the formation of a group of galaxies are examined by running two identical simulations, where in the WDM case the initial power spectrum has been altered to mimic a 1-keV dark matter particle. The CDM initial conditions were constrained to reproduce at z = 0 the correct local environment within which a ‘Local Group’ (LG) of galaxies may form. Two significant differences between the two simulations are found. While in the CDM case a group of galaxies that resembles the real LG forms, the WDM run fails to reproduce a viable LG, instead forming a diffuse group which is still expanding at z = 0. This is surprising since, due to the suppression of small-scale power in its power spectrum, WDM is naively expected to only affect the collapse of small haloes and not necessarily the dynamics on a scale of a group of galaxies. Furthermore, the concentration of baryons in halo centre is greater in CDM than in WDM and the properties of the discs differ.

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 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.

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.

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.

To understand cluster formation and evolution it is important to understand the evolution of cluster substructure. The work described here focuses upon a set of fully self-consistent numerical simulations within a $\Lambda$CDM cosmology. Within the simulations we follow in detail the temporal and spatial properties of individually tracked satellite galaxies. The host galaxy clusters were chosen to sample a variety of formation histories, ages, and triaxialities. Despite their obvious differences, we find striking similarities within the associated substructure populations. Namely, the satellite galaxy orbital distributions are indistinguishable between the galaxy clusters.To search for other articles by the author(s) go to: http://adsabs.harvard.edu/abstract_service.html