Search

We describe the basic ideas of MPI parallelization of the N-body Adaptive Refinement Tree (ART) code. The code uses self-adaptive domain decomposition where boundaries of the domains (parallelepipeds) constantly move -- with many degrees of freedom -- in the search of the minimum of CPU time. The actual CPU time spent by each MPI task on previous time-step is used to adjust boundaries for the next time-step. For a typical decomposition of 5^3 domains, the number of possible changes in boundaries is 3^{84}. We describe two algorithms of finding minimum of CPU time for configurations with a large number of domains. Each MPI task in our code solves the N-body problem where the large-scale distribution of matter outside of the boundaries of a domain is represented by relatively few temporary large particles created by other domains. At the beginning of a zero-level time-step, domains create and exchange large particles. Then each domain advances all its particles for many small time-steps. At the end of the large step, the domains decide where to place new boundaries and re-distribute particles. The scheme requires little communications between processors and is very efficient for large cosmological simulations., Comment: 15 pages, 5 figures, conference "High Performance Computing in Science and Engineering Garching/Munich 2007", Eds. S. Wagner et al, Springer, Berlin 2008

Despite a history that dates back at least a quarter of a century studies of voids in the large--scale structure of the Universe are bedevilled by a major problem: there exist a large number of quite different void--finding algorithms, a fact that has so far got in the way of groups comparing their results without worrying about whether such a comparison in fact makes sense. Because of the recent increased interest in voids, both in very large galaxy surveys and in detailed simulations of cosmic structure formation, this situation is very unfortunate. We here present the first systematic comparison study of thirteen different void finders constructed using particles, haloes, and semi--analytical model galaxies extracted from a subvolume of the Millennium simulation. The study includes many groups that have studied voids over the past decade. We show their results and discuss their differences and agreements. As it turns out, the basic results of the various methods agree very well with each other in that they all locate a major void near the centre of our volume. Voids have very underdense centres, reaching below 10 percent of the mean cosmic density. In addition, those void finders that allow for void galaxies show that those galaxies follow similar trends. For example, the overdensity of void galaxies brighter than $m_B = -20 $ is found to be smaller than about -0.8 by all our void finding algorithms., Comment: 14 pages, version accepted by MNRAS

Thanks to the ever increasing computational power and the development of more sophisticated algorithms, numerical N-body simulations are now uncovering several phenomenological relations between the physical properties of dark matter haloes in position and velocity space. It is the aim of the present work to investigate in detail the dynamical structure of dark matter haloes, as well as its possible dependence on mass and its evolution with redshift up to z=5. We use high-resolution cosmological simulations of individual objects to compute the radially-averaged profiles of several quantities, scaled by the radius Rmax at which the circular velocity attains its maximum value, Vmax. No systematic dependence on mass or cosmic epoch are found within Rmax, and all the different radial profiles are well fit by simple analytical models. However, our results suggest that several properties are not `universal' outside this radius. [Abridged], Comment: Accepted for publication in MNRAS (10 pages, 8 figures)

We explore the dependence of the radial alignment of subhalos on the mass of the host halo they orbit in. As the effect is seen on a broad range of scales including massive clusters as well as galactic systems it only appears natural to explore this phenomenon by means of cosmological simulations covering the same range in masses. We have 25 well resolved host dark matter halos at our disposal ranging from 10^15 Msun/h down to 10^12 Msun/h each consisting of order of a couple of million particles within the virial radius. We observe that subhalos tend to be more spherical than isolated objects. Both the distributions of sphericity and triaxiality of subhalos are Gaussian distributed with peak values of s approx. 0.80 and T approx. 0.56, irrespective of host mass. Interestingly we note that the radial alignment is independent of host halo mass and the distribution of \cos\theta (i.e. the angle between the major axis E_a of each subhalo and the radius vector of the subhalo in the reference frame of the host) is well fitted by a simple power law P(\cos\theta) proportional to \cos^4\theta with the same fitting parameters for all host halos., Comment: 5 pages, 2 figures. MNRAS Letter, in press

We study the distribution function (DF) of dark matter particles in haloes of mass range 10^{14}--10^{15}\Msun. In the numerical part of this work we measure the DF for a sample of relaxed haloes formed in the simulation of a standard \LambdaCDM model. The DF is expressed as a function of energy E and the absolute value of the angular momentum L, a form suitable for comparison with theoretical models. By proper scaling we obtain the results that do not depend on the virial mass of the haloes. We demonstrate that the DF can be separated into energy and angular momentum components and propose a phenomenological model of the DF in the form f_{E}(E)[1+L^{2}/(2L_{0}^{2})]^{-\beta_{\infty}+\beta_{0}}L^{-2\beta_{0}}. This formulation involves three parameters describing the anisotropy profile in terms of its asymptotic values (\beta_{0} and \beta_{\infty}) and the scale of transition between them (L_{0}). The energy part f_{E}(E) is obtained via inversion of the integral for spatial density. We provide a straightforward numerical scheme for this procedure as well as a simple analytical approximation for a typical halo formed in the simulation. The DF model is extensively compared with the simulations: using the model parameters obtained from fitting the anisotropy profile, we recover the DF from the simulation as well as the profiles of the dispersion and kurtosis of radial and tangential velocities. Finally, we show that our DF model reproduces the power-law behaviour of phase space density Q=\rho(r)/\sigma^{3}(r)., Comment: 16 pages, 12 figures, final version accepted for publication in MNRAS

We summarize the method of mass modelling of galaxy clusters based on reproducing the dispersion and kurtosis of the projected velocity distribution of galaxies. The models are parametrized within the framework of the NFW density profile, characterized by the virial mass and concentration, together with the constant anisotropy of galaxy orbits. The use of velocity dispersion alone does not allow to constrain all the three parameters from kinematic data due to the mass-anisotropy degeneracy. The degeneracy is broken by introducing the fourth velocity moment, the kurtosis. We tested the method based on fitting both moments on mock data sets drawn from simulated dark matter haloes and showed it to reproduce reliably the properties of the haloes. The method has been applied to estimate the mass, concentration and anisotropy of more than 20 clusters which allowed us to confirm, for the first time using kinematic data, the mass-concentration relation found in N-body simulations., Comment: 6 pages, 5 figures, contribution to the proceedings of XIX Rencontres de Blois

To study the local Hubble flow, we have run constrained dark matter (DM) simulations of the Local Group (LG) in the concordance LCDM and OCDM cosmologies, with identical cosmological parameters apart from the Lambda term. The simulations were performed within a computational box of 64 h^{-1}Mpc centred on the LG. The initial conditions were constrained by the observed peculiar velocities of galaxies and positions of X-ray nearby clusters of galaxies. The simulations faithfully reproduce the nearby large scale structure, and in particular the Local Supercluster and the Virgo cluster. LG-like objects have been selected from the DM halos so as to closely resemble the dynamical properties of the LG. Both the LCDM and OCDM simulations show very similar local Hubble flow around the LG-like objects. It follows that, contrary to recent statements, the dark energy (DE) does not manifest itself in the local dynamics., Comment: 10 pages, 7 figures, submitted to MNRAS

We investigate the origin and evolution of fossil groups in a concordance LCDM cosmological simulation. We consider haloes with masses between $(1-5)\times10^{13} \hMsun$ and study the physical mechanisms that lead to the formation of the large gap in magnitude between the brightest and the second most bright group member, which is typical for these fossil systems. Fossil groups are found to have high dark matter concentrations, which we can relate to their early formation time. The large magnitude-gaps arise after the groups have build up half of their final mass, due to merging of massive group members. We show that the existence of fossil systems is primarily driven by the relatively early infall of massive satellites, and that we do not find a strong environmental dependence for these systems. In addition, we find tentative evidence for fossil group satellites falling in on orbits with typically lower angular momentum, which might lead to a more efficient merger onto the host. We find a population of groups at higher redshifts that go through a ``fossil phase'': a stage where they show a large magnitude-gap, which is terminated by renewed infall from their environment., Comment: 9 pages and 8 figures, submitted to MNRAS

The cosmic ultraviolet background (UVB) heats the intergalactic medium (IGM), as a result the gas in dark matter halos below a certain mass is too hot to cool within a Hubble time. The UVB effectively suppresses the formation of dwarf galaxies. Using high resolution cosmological hydrodynamical simulations we show that photo heating leads to small baryon fractions in halos below ~ 6x10^9 h^{-1}M_sun, independent of the cosmic environment. The simulations are carried out assuming a homogeneous UVB with flux densities as given by Haardt & Madau (1996). A halo may stop to condense gas significantly after the universe is reionised, namely when its mass falls below the characteristic mass scale set by the photo heating. Assuming a spherical halo model we derive this characteristic mass analytically and identify the main mechanisms that prevent the gas from cooling in small halos. The theoretically derived characteristic mass is smaller than the one obtained from observations. Increasing the energy per ionising photon by a factor between four and eight would be sufficient to reconcile both. This is equivalent to an average temperature of the IGM of ~ 10^4 K. In this sense the faint end of the luminosity function may serve as a calorimeter for the IGM., Comment: To appear in Proceedings of IAU Symp #244, "Dark Galaxies and Lost Baryons", June 2007, 5 pages including 3 figures

We present the mass and X-ray temperature functions derived from a sample of more than 15,000 galaxy clusters of the MareNostrum Universe cosmological SPH simulations. In these simulations, we follow structure formation in a cubic volume of 500/h Mpc on a side assuming cosmological parameters consistent with either the first or third year WMAP data and gaussian initial conditions. We compare our numerical predictions with the most recent observational estimates of the cluster X-ray temperature functions and find that the low normalization cosmological model inferred from the 3 year WMAP data results is barely compatible with the present epoch X-ray cluster abundances. We can only reconcile the simulations with the observational data if we assume a normalization of the Mass-Temperature relation which is a factor of 2.5--3 smaller than our non-radiative simulations predict. This deviation seems to be too large to be accounted by the effects of star formation or cooling in the ICM, not taken into account in these simulations., Comment: 4 pages, 3 figures. Accepted for publication in The Astrophysical Journal Letters

The MareNostrum Universe is one of the largest cosmological SPH simulation done so far. It consists of $1024^3$ dark and $1024^3$ gas particles in a box of 500 $h^{-1}$ Mpc on a side. Here we study the shapes and spins of the dark matter and gas components of the 10,000 most massive objects extracted from the simulation as well as the gas fraction in those objects. We find that the shapes of objects tend to be prolate both in the dark matter and gas. There is a clear dependence of shape on halo mass, the more massive ones being less spherical than the less massive objects. The gas distribution is nevertheless much more spherical than the dark matter, although the triaxiality parameters of gas and dark matter differ only by a few percent and it increases with cluster mass. The spin parameters of gas and dark matter can be well fitted by a lognormal distribution function. On average, the spin of gas is 1.4 larger than the spin of dark matter. We find a similar behavior for the spins at higher redshifts, with a slightly decrease of the spin ratios to 1.16 at $z=1.$ The cosmic normalized baryon fraction in the entire cluster sample ranges from $Y_b = 0.94$, at $z=1$ to $Y_b = 0.92$ at $z=0$. At both redshifts we find a slightly, but statistically significant decrease of $Y_b$ with cluster mass., Comment: 7 pages, 6 figures. Accepted for publication in The Astrophysical Journal

The MareNostrum Universe is one of the biggest SPH cosmological simulations done so far. It contains more than 2 billion particles (2 times 1024^3) in a 500 Mpc/h cubic volume. This simulation has been performed on the MareNostrum supercomputer at the Barcelona Supercomputer Center. We have obtained more than 0.5 million halos with masses greater than a typical Milky Way galaxy halo. We report results about the halo mass function, the shapes of dark matter and gas distributions in halos, the baryonic fraction in galaxy clusters and groups, baryon oscillations in the dark matter and the halo power spectra as well as the distribution and evolution of the gas fraction at large scales., Comment: 6 pages, 6 figures. To appear in the proceedings of the XLIst Rencontres de Moriond, XXVIth Astrophysics Moriond Meeting: "From Dark Halos to Light", La Thuile, 12-18 March 2006. ed. L. Tresse, S. Maurogordato, J. Tran Than Van. see also the the web page of the The Marenostrum Numerical Cosmology Project: http://astro.ft.uam.es/~marenostrum/

The aim of this paper is to study the efficiency of different approaches to interloper treatment in dynamical modelling of galaxy clusters. Using cosmological N-body simulation of standard LCDM model, we select 10 massive dark matter haloes and use their particles to emulate mock kinematic data in terms of projected galaxy positions and velocities as they would be measured by a distant observer. Taking advantage of the full 3D information available from the simulation, we select samples of interlopers defined with different criteria. The interlopers thus selected provide means to assess the efficiency of different interloper removal schemes found in the literature. We study direct methods of interloper removal based on dynamical or statistical restrictions imposed on ranges of positions and velocities available to cluster members. In determining these ranges, we use either the velocity dispersion criterion or a maximum velocity profile. We also generalize the common approaches taking into account both the position and velocity information. Another criterion is based on the dependence of the commonly used virial mass and projected mass estimators on the presence of interlopers. We find that the direct methods exclude on average 60-70 percent of unbound particles producing a sample with contamination as low as 2-4 percent. Next, we consider indirect methods of interloper treatment which are applied to the data stacked from many objects. In these approaches, interlopers are treated in a statistical way as a uniform background which modifies the distribution of cluster members. Using a Bayesian approach, we reproduce the properties of composite clusters and estimate the probability of finding an interloper as a function of distance from the object centre., Comment: 15 pages, 14 figures, final version accepted for publication in A&A

We study the mass distribution in six nearby (z<0.06) relaxed Abell clusters of galaxies A0262, A0496, A1060, A2199, A3158 and A3558. Given the dominance of dark matter in galaxy clusters we approximate their total density distribution by the NFW formula characterized by virial mass and concentration. We also assume that the anisotropy of galactic orbits is reasonably well described by a constant and that galaxy distribution traces that of the total density. Using the velocity and position data for 120-420 galaxies per cluster we calculate, after removal of interlopers, the profiles of the lowest-order even velocity moments, dispersion and kurtosis. We then reproduce the velocity moments by jointly fitting the moments to the solutions of the Jeans equations. Including the kurtosis in the analysis allows us to break the degeneracy between the mass distribution and anisotropy and constrain the anisotropy as well as the virial mass and concentration. The method is tested in detail on mock data extracted from N-body simulations of dark matter haloes. We find that the best-fitting galactic orbits are remarkably close to isotropic in most clusters. Using the fitted pairs of mass and concentration parameters for the six clusters we conclude that the trend of decreasing concentration for higher masses found in cosmological N-body simulations is consistent with the data. By scaling the individual cluster data by mass we combine them to create a composite cluster with 1465 galaxies and perform a similar analysis on such sample. The estimated concentration parameter then lies in the range 1.5 < c < 14 and the anisotropy parameter in the range -1.1 < \beta < 0.5 at the 95 percent confidence level., Comment: 11 pages, 9 figures, final version accepted for publication in MNRAS

Using high resolution DM simulations we study the shape of dark matter halos. Halos become more spherical with decreasing mass. This trend is even more pronounced for the inner part of the halo. Angular momentum and shape are correlated. The angular momenta of neighboring halos are correlated., Comment: Comments: 4 pages, 3 figures. To appear in the proceedings of the XXIst IAP Colloquium "Mass Profiles and Shapes of Cosmological Structures", Paris July 4-9, 2005, France, Editors G. Mamon, F. Combes, C. Deffayet, B. Fort, EAS Publications Series

Using cosmological N-body simulations we study the line-of-sight velocity distribution of dark matter haloes focusing on the lowest-order even moments, dispersion and kurtosis, and their application to estimate the mass profiles of cosmological structures. For each of the ten massive haloes selected from the simulation box we determine the virial mass, concentration and the anisotropy parameter. In order to emulate observations from each halo we choose randomly 300 particles and project their velocities and positions along the line of sight and on the surface of the sky, respectively. After removing interlopers we calculate the profiles of the line-of-sight velocity moments and fit them with the solutions of the Jeans equations. The estimates of virial mass, concentration parameter and velocity anisotropy obtained in this way are in good agreement with the values found from the full 3D analysis., Comment: 2 pages, 1 figure, poster contribution to the proceedings of the XXIst IAP Colloquium "Mass Profiles and Shapes of Cosmological Structures", Paris 4-9 July 2005, Editors: G. Mamon, F. Combes, C. Deffayet, B. Fort, EDP Sciences, in press

The Abell 1689 galaxy cluster has recently become a subject of intensive study. Thanks to its intermediate redshift (z=0.183) its mass distribution can be reconstructed using numerous methods including gravitational lensing, galaxy kinematics and X-ray imaging. The methods used to yield conflicting mass estimates in the past and recently the cluster mass distribution has been claimed to be in conflict with standard CDM scenarios due to rather large concentration and steep mass profile obtained from detailed studies of Broadhurst et al. using lensing. By studying in detail the kinematics of about 200 galaxies with measured redshifts in the vicinity of the cluster we show that the cluster is probably surrounded by a few structures, quite distant from each other, but aligned along the line of sight. We support our arguments by referring to cosmological N-body simulations and showing explicitly that distant, non-interacting haloes can produce entangled multi-peak line-of-sight velocity distributions similar to that in A1689. We conclude that it is difficult to estimate the cluster mass reliably from galaxy kinematics, but the value we obtain after applying a simple cut-off in velocity agrees roughly with the mass estimated from lensing. The complicated mass distribution around the cluster may however increase the uncertainty in the determination of the density profile shape obtained with weak lensing., Comment: 5 pages, 4 figures, revised version accepted for publication in MNRAS Letters

We present a statistical analysis of voids in the 2dF galaxy redshift survey (2dFGRS). In order to detect the voids, we have developed two robust algorithms. We define voids as non-overlapping maximal spheres empty of halos or galaxies with mass or luminosity above a given one. We search for voids in cosmological $N$-Body simulations to test the performance of our void finders. We obtain and analyze the void statistics for several volume-limited samples for the North Galactic Strip (NGP) and the South Galactic Strip (SGP) constructed from the 2dFGRS full data release. We find that the results obtained from the NGP and the SGP are statistically compatible. From the results of several statistical tests we conclude that voids are essentially uncorrelated, with at most a mild anticorrelation and that there is a dependence of the void number density on redshift at least at the 99.5% confidence level. We develop a technique to correct the distortion caused by the fact that we use the redshift as the radial coordinate. We calibrate this technique with mock catalogues and find that the correction might be of some relevance to carry out accurate inferences from void statistics. We study the statistics of the galaxies inside nine nearby voids. We find that galaxies in voids are not randomly distributed: they form structures like filaments. We also obtain the galaxy number density profile in voids. This profile follow a similar but steeper trend to that follow by halos in voids., Comment: 16 pages, 10 figures, replaced to match the published version in MNRAS

Using cosmological N-body simulations we study the radial velocity distribution in dark matter haloes focusing on the lowest-order even moments, dispersion and kurtosis. We determine the properties of ten massive haloes in the simulation box approximating their density distribution by the NFW formula characterized by the virial mass and concentration. We also calculate the velocity anisotropy parameter of the haloes and find it mildly radial and increasing with distance from the halo centre. The radial velocity dispersion of the haloes shows a characteristic profile with a maximum, while the radial kurtosis profile decreases with distance starting from a value close to Gaussian near the centre. We therefore confirm that dark matter haloes possess intrinsically non-Gaussian, flat-topped velocity distributions. We find that the radial velocity moments of the simulated haloes are quite well reproduced by the solutions of the Jeans equations obtained for the halo parameters with the anisotropy measured in the simulations. We also study the radial velocity moments for a composite cluster made of ten haloes out to ten virial radii. In this region the velocity dispersion decreases systematically to reach the value of the background, while kurtosis increases from below to above the Gaussian value of 3 signifying a transition from a flat-topped to a strongly peaked velocity distribution with respect to the Gaussian, which can be interpreted as the dominance of ordered flow with a small dispersion. We illustrate the transition by showing explicitly the velocity distribution of the composite cluster in a few radial bins., Comment: 5 pages, 4 figures, minor changes, accepted for publication in MNRAS Letters

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 MNRAS

We study structure formation in cosmological void regions using high-resolution hydrodynamical simulations. Despite being significantly underdense, voids are populated abundantly with small dark matter halos which should appear as dwarf galaxies if their star formation is not suppressed significantly. We here investigate to which extent the cosmological UV-background photo-evaporates baryons out of halos of dwarf galaxies, and thereby limits their cooling and star formation rates. Assuming a Haardt & Madau UV-background with reionisation at redshift z=6, our samples of simulated galaxies show that halos with masses below a characteristic mass of M_c(z=0) = 6.5 x 10^9 h^{-1} M_sun are baryon-poor, but in general not completely empty, because baryons that are in the condensed cold phase or are already locked up in stars resist evaporation. In halos with mass M < M_c, we find that photo-heating suppresses further cooling of gas. The redshift and UV-background dependent characteristic mass M_c(z) can be understood from the equilibrium temperature between heating and cooling at a characteristic overdensity of \delta ~ 1000. If a halo is massive enough to compress gas to this density despite the presence of the UV background, gas is free to `enter' the condensed phase and cooling continues in the halo, otherwise it stalls. By analysing the mass accretion histories of dwarf galaxies in voids, we show that they can build up a significant amount of condensed mass at early times before the epoch of reionisation. Later on, the amount of mass in this phase remains roughly constant, but the masses of the dark matter halos continue to increase. (abridged), Comment: revised version as accepted by MNRAS, 15 pages, 15 figures, new simulation results and a significantly extended discussion have been included

We study motions of galaxies in galaxy clusters formed in the concordance LCDM cosmology. We use high-resolution cosmological simulations that follow dynamics of dark matter and gas and include various physical processes critical for galaxy formation: gas cooling, heating and star formation. Analysing motions of galaxies and the properties of intracluster gas in the sample of eight simulated clusters at z=0, we study velocity dispersion profiles of the dark matter, gas, and galaxies. We measure the mean velocity of galaxy motions and gas sound speed as a function of radius and calculate the average Mach number of galaxy motions. The simulations show that galaxies, on average, move supersonically with the average Mach number of ~1.4, approximately independent of the cluster-centric radius. The supersonic motions of galaxies may potentially provide an important source of heating for the intracluster gas by driving weak shocks and via dynamical friction, although these heating processes appear to be inefficient in our simulations. We also find that galaxies move faster than the dark matter particles in clusters. The magnitude of the velocity bias, b_v~1.1, is, however, smaller than the bias estimated for subhalos in dissipationless simulations. Interestingly, we find velocity bias in the tangential component of the velocity dispersion, but not in the radial component. Finally, we find significant random bulk motions of gas. The typical gas velocities are of order ~20-30% of the gas sound speed. These random motions provide about 10% of the total pressure support in our simulated clusters. The non-thermal pressure support, if neglected, will bias measurements of the total mass in the hydrostatic analyses of the X-ray cluster observations., Comment: accepted for publication in MNRAS, 11 pages, 6 figures, added explanation of the measured average Mach number and discussion of the velocity bias

We analyze the mass accretion histories (MAHs) and density profiles of cluster- size halos with virial masses of 0.6-2.5x10^14/h Msun in a flat LCDM cosmology. In agreement with previous studies,we find that the concentration of the density distribution is tightly correlated with the halo's MAH and its formation redshift.During the period of fast mass growth the concentration remains approximately constant and low c_v ~ 3-4,while during the slow accretion stages it increases with decreasing redshift as c_v ~ (1+z)^-1.We consider fits of three widely discussed analytic density profiles to the simulated clusters focusing on the most relaxed inner regions.We find that there is no unique best fit analytic profile for all the systems.If,however,a cluster is best fit by a particular analytic profile at z=0,the same is usually true at earlier epochs out to z ~ 1-2.The local logarithmic slope of the density profiles at 3% of the virial radius ranges from -1.2 to -2.0,a remarkable diversity for the relatively narrow mass range of our cluster sample.For all the studied clusters the logarithmic slope becomes shallower with decreasing radius without reaching an asymptotic value down to the smallest resolved scale < ~1% of the virial radius.We do not find a clear correlation of the inner slope with the formation redshift or the shape of the halo's MAH.We do find, however,that during the period of rapid mass growth the density profiles can be well described by a single power law rho(r) ~ r^-gamma with gamma ~ 1.5-2.The relatively shallow power law slopes result in low concentrations at these stages of evolution.This indicates that the inner power law like density distribution of halos is built up during the periods of rapid mass accretion and active merging,while outer steeper profile is formed when the mass accretion is slow., Comment: 15 pages,8 figures, LaTeX(uses emulateapj5.sty); matches published version

We analyze the halo occupation distribution (HOD), the probability for a halo of mass M to host a number of subhalos N, and two-point correlation function of galaxy-size dark matter halos using high-resolution dissipationless simulations of the concordance flat LCDM 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 first moment of the HOD, (M), has a complicated shape consisting of a step, a shoulder, and a power law high-mass tail. The HOD can be described by a Poisson statistics at high halo masses but becomes sub-Poisson for <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 Ns of satellite galaxies. The former can be approximated by a step-like function, while the latter can be well approximated by a Poisson distribution, fully specified by its first moment (M). We find that ~M^b with b~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=0, the two-point correlation function (CF) of galactic halos can be well fit by a power law down to ~100/h kpc with an amplitude and slope similar to those of observed galaxies. At redshifts z>~1, we find significant departures from the power-law shape of the CF at small scales. 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 likely to bias the estimates of the correlation length and slope of the correlation function., Comment: To appear in July 1, 2004 issue of ApJ, 18 pages, 12 figures, LaTeX (uses emulateapj5.sty). This version matches the version in press

Using high resolution N-body simulations we address the problem of emptiness of giant 20 Mpc/h diameter voids found in the distribution of bright galaxies. Are the voids filled by dwarf galaxies? Do cosmological models predict too many small dark matter haloes inside the voids? Can the problems of cosmological models on small scales be addressed by studying the abundance of dwarf galaxies inside voids? We find that voids in the distribution of 10^12 Msun/h haloes (expected galactic magnitudes ~ M_*) are almost the same as the voids in 10^11 Msun/h haloes. Yet, much smaller haloes with masses 10^9 Msun/h and circular velocities v_circ about 20 km/s readily fill the voids: there should be almost 1000 of these haloes in a 20 Mpc/h void. A typical void of diameter 20 Mpc/h contains about 50 haloes with v_circ > 50 km/s. The haloes are arranged in a pattern, which looks like a miniature Universe: it has the same structural elements as the large-scale structure of the galactic distribution of the Universe. There are filaments and voids; larger haloes are at the intersections of filaments. The only difference is that all masses are four orders of magnitude smaller. There is severe (anti)bias in the distribution of haloes, which depends on halo mass and on the distance from the centre of the void. Large haloes are more antibiased and have a tendency to form close to void boundaries. The mass function of haloes in voids is different from the ``normal'' mass function. It is much steeper for high masses resulting in very few M33-type galaxies (v_circ about 100 km/s). We present an analytical approximation for the mass function of haloes in voids., Comment: 12 pages, 10 figures (paper with high resolution figure 2 available at http://www.aip.de/People/SGottloeber/papers/void_struc.ps.gz (8.5 Mb)), MNRAS

The relative orientation of clusters' major elongation axes and clusters' angular momentum is studied using a large N-body simulation in a box of 500 Mpc/h base length for a standard LambdaCDM model. Employing the technique of mark correlation functions, we successfully separated the correlations in the orientation from the well known clustering signal traced by the two-point correlation function. The correlations in the orientation are highly significant for our sample of 3000 clusters. We found an alignment of neighboring clusters, i.e. an enhanced probability of the major elongation axes of neighboring cluster pairs to be in parallel with each other. At 10 Mpc/h separation the amplitude of this signal is ~10% above the value expected from random orientations, and it vanishes on scales larger than 15 Mpc/h. The ``filamentary'' alignment between clusters' major elongation axes and the lines pointing towards neighboring clusters shows even stronger deviations from random orientation, which can be detected out to scales of 100 Mpc/h, both in 2D and 3D analyses. Similarly, strong correlations of the angular momentum were seen. Also a clear signal in the scalar correlation of the absolute value of the angular momentum, the spin parameter and the mass was found. They extend up to 50 Mpc/h and have an amplitude of 40%, 15%, and 10% above a random distribution at 10 Mpc/h separation, respectively., Comment: 10 pages, 10 figures, A&A, accepted 28/08/02

The theory of structure formation predicts that galaxies form within extended massive dark matter halos built from smaller pieces that collided and merged, resulting in the hierarchy of galaxies, groups, and clusters observed today. Here we present constrained numerical simulations designed to match the observed local universe as well as possible and to study the formation, evolution and present day properties of such dark matter halos in different environments. Simulations have been done with different versions of the Adaptive Refinement Tree (ART) N-body code, some of them including eulerian gasdynamics., Comment: 10 pages, 6 figures, to appear in the proceedings of the "First Joint HLRB and KONWIHR Result and Reviewing Workshop" (Munich, October 2002). Version with high resolution figures available at http://www.aip.de/People/SGottloeber/papers/h009za.ps.gz (gzipped 5.3 Mb)

Using numerical simulations and the Sheth-Tormen approximation we study the mass function of dark matter halos in voids. We find that the void mass function is significantly lower and its shape is different than that of the field halos. We predict that in the standard LCDM model a void with radius 10 h^-1 Mpc should have 50 halos with circular velocity v_c > 50 km/s and 600 halos with v_c > 20 km/s., Comment: 2 pages, 1 figure, to appear in "Galaxy Evolution: Theory and Observations (2002)"

We use a novel statistical tool, the mark correlation functions (MCFs), to study clustering of galaxy-size halos as a function of their properties and environment in a high-resolution numerical simulation of the LambdaCDM cosmology. We applied MCFs using several types of continuous and discrete marks: maximum circular velocity of halos, merger mark indicating whether halos experienced or not a major merger in their evolution history, and a stripping mark indicating whether the halo underwent a tidal stripping. We find that halos which experienced a relatively early (z>1) major merger or mass loss (due to tidal stripping) in their evolution histories are over-abundant in halo pairs with separations < 3 Mpc/h. This result can be interpreted as spatial segregation of halos with different merger histories, qualitatively similar to the morphological segregation in the observed galaxy distribution. The analysis presented in this paper demonstrate that MCFs provide powerful, yet algorithmically simple, quantitative measures of segregation in the spatial distribution of objects with respect to their various properties (marks)., Comment: 10 pages, 7 figures, A&A accepted 05/03/02

We present cosmological simulations which closely mimic the real Universe within 100Mpc of the Local Group. The simulations, called Constrained Simulations, reproduce the large-scale density field with major nearby structures, including the Local Group, the Coma and Virgo clusters, the Great Attractor, the Perseus-Pices, and the Local Supercluster, in approximately correct locations. The MARK III survey of peculiar velocities of the observed structures inside 80Mpc/h sphere is used to constrain the initial conditions. Fourier modes on scales larger then 5Mpc/h are dominated by the constraints, while small scale waves are random. The main aim of this paper is the structure of the Local Supercluster (LSC; 30Mpc/h around the Virgo cluster) and the Local Group environment. We find that at the current epoch most of the mass (7.5e14Msun/h) of the LSC is located in a filament roughly centered on the Virgo cluster and extending over 40Mpc/h. The simulated Local Group (LG) is located in an adjacent smaller filament, which is not a part of the main body of the LSC, and has a peculiar velocity of 250kms toward the Virgo cluster. The peculiar velocity field in the LSC region is complicated and is drastically different from the field assumed in the Virgocentric infall models. The peculiar velocity flow in the vicinity of the LG in the simulation is ``cold'': the peculiar line-of-sight velocity dispersion within 7Mpc/h of the LG is less than 60km/s, comparable to the observed velocity dispersion of nearby galaxies., Comment: 22 pages, 9 figures, submitted to ApJ, high resolution version is available at http://astro.nmsu.edu/~aklypin/HOFFMAN

According to the hierarchical scenario, galaxies form via merging and accretion of small objects. Using N-body simulations, we study the frequency of merging events in the history of the halos. We find that at z<~2 the merging rate of the overall halo population can be described by a simple power law (1+z)^3. The main emphasis of the paper is on the effects of environment of halos at the present epoch (z=0). We find that the halos located inside clusters have formed earlier (dz \approx 1) than isolated halos of the same mass. At low redshifts (z<1), the merger rate of cluster halos is 3 times lower than that of isolated halos and 2 times lower than merger rate of halos that end up in groups by z=0. At higher redshifts (z~1-4), progenitors of cluster and group halos have 3--5 times higher merger rates than isolated halos. We briefly discuss implications of our results for galaxy evolution in different environments., Comment: submitted to the Astrophys. Journal; 11 pages, 9 figs., LaTeX (uses emulateapj.sty)

We present a study of numerical effects in dissipationless cosmological simulations. The numerical effects are evaluated and studied by comparing results of a series of 64^3-particle simulations of varying force resolution and number of time steps, performed using three different N-body techniques: the Particle Mesh (PM), the adaptive P3M (AP3M) code, and the Adaptive Refinement Tree (ART) code. This study can therefore be interesting both as an analysis of numerical effects and as a systematic comparison of different codes. We find that the AP3M and the ART codes produce similar results, given that convergence is reached within the code type. We also find that numerical effects may affect the high-resolution simulations in ways that have not been discussed before. In particular, our study revealed the presence of two-body scattering, effects of which can be greatly amplified by inaccuracies of time integration. This process appears to affect the correlation function of matter, mass function and inner density of dark matter halos and other statistics at scales much larger than the force resolution, although different statistics may be affected in different fashion. We discuss the conditions at which strong two-body scattering is possible and discuss the choice of the force resolution and integration time step. Furthermore, we discuss recent claims that simulations with force softening smaller than the mean interparticle separation are not trustworthy and argue that this claim is incorrect in general and applies only to the phase-sensitive statistics. Our conclusion is that, depending on the choice of mass and force resolution and integration time step, a force resolution as small as 0.01 of the mean interparticle separation can be justified., Comment: submitted to MNRAS, LaTeX (uses mn.sty), 19 pages, 15 figs, high-res figures can be found at http://www-thphys.physics.ox.ac.uk/users/AlexanderKnebe/publications.html

We discuss the impact of the cosmological environment on the evolution of dark matter halos using a high-resolution simulation within a spatially flat LCDM cosmology., Comment: 4 pages, 1 figure, contribution to the Morphology Conference in Johannesburg, September 1999

We use a high-resolution dissipationless simulation to study the evolution of the dark matter and halo distributions in a spatially flat cosmological model dominated by a cosmological constant $\Lambda$ and cold dark matter ($\Lambda$CDM). In order to quantify the evolution of structure, we calculate the Minkowski functionals of the halos and the dark matter component at various redshifts. A comparison of Minkowski functionals and the more standard correlation function analysis shows that the Minkowski functionals contain information about correlation functions of arbitrary order. While little evolution of the Minkowski functionals of halos between $z=4$ and $z=0$ is observed, we find that the Minkowski functionals of the dark matter evolve rapidly with time. The difference of the Minkowski functionals of halos and dark matter can be interpreted as a scale dependent bias. This implies that scale-dependent bias is a property of not only the two-point halo correlation function, but also of correlation functions of higher order., Comment: 10 pages including 6 figures. Accepted for publication in Monthly Notices

A significant fraction of mass in the universe is believed to be in the form of dark matter (DM). Due to gravitational instability, the DM collapses hierarchically into DM halos. In this contribution we present a study of the formation and evolution of such DM halos in a COBE-normalized spatially flat LCDM model (Omega_0=1-Omega_L=0.3; h=0.7) using high-resolution N-body simulations. The novelty of this study is use of the newly developed halo-finding algorithms to study the evolution of both isolated and satellite (located inside virial radii of larger group- and cluster-size systems) halos. The force and mass resolution required for a simulated halo to survive in the high-density environments typical of groups and clusters is high: ~1-3 kpc and \~10^9 M_sun, respectively. We use the high-resolution Adaptive Refinement Tree (ART) N-body code to follow the evolution of 256^3 dark matter particles with dynamic range in spatial resolution of 32,000 in a box of 60/h Mpc. We show that the correlation function of these halos is anti-biased with respect to the dark matter correlation function and is high and steeper than the correlation function of the isolated virialized objects. The correlation function evolves only mildly between z=3 and z=1. The mass evolution of isolated virialized objects determined from the simulation is in good agreement with prediction of semi-analytical models. The differences exist, however, if we include satellite halos in the halo catalogs., Comment: Contributions to the Proceedings of the MPA/ESO Conference "Evolution of LSS: from Recombination to Garching", Garching, August 2-7, 1998. LaTeX (uses tp.sty, psfig.sty), 5 pages, 5 figs

The large scale gravitational potential distribution and its influence on the large-scale matter clustering is considered on the basis of six simulations. It is found that the mean separation between zero levels of the potential along random straight lines coincides with the theoretical expectations, but it scatters largely. A strong link of the initial potential and the structure evolution is shown. It is found that the under-dense and over-dense regions correlate with regions of positive and negative gravitational potential at large redshifts. The over-dense regions arise due to a slow matter flow into the negative potential regions, where more pronounced non-linear structures appear. Such regions are related to the formation of huge super-large scale structures seen in the galaxy distribution., Comment: 14 pages LaTeX with 14 postscript figures included. To be publishes in Astron. Astroph. (accepted 17 July 1997)

We present analysis of the evolution of dark matter halos in dense environments of groups and clusters in dissipationless cosmological simulations. The premature destruction of halos in such environments, known as the overmerging, reduces the predictive power of N-body simulations and makes difficult any comparison between models and observations. We analyze the possible processes that cause the overmerging and assess the extent to which this problem can be cured with current computer resources and codes. Using both analytic estimates and high resolution numerical simulations, we argue that the overmerging is mainly due to the lack of numerical resolution. We find that the force and mass resolution required for a simulated halo to survive in galaxy groups and clusters is extremely high and was almost never reached before: ~1-3 kpc and 10^8-10^9 Msun, respectively. We use the high-resolution Adaptive Refinement Tree (ART) N-body code to run cosmological simulations with the particle mass of \approx 2x10^8/h Msun} and the spatial resolution of \approx 1-2/h kpc, and show that in these simulations the halos do survive in regions that would appear overmerged with lower force resolution. Nevertheless, the halo identification in very dense environments remains a challenge even with the resolution this high. We present two new halo finding algorithms developed to identify both isolated and satellite halos that are stable (existed at previous moments) and gravitationally bound. To illustrate the use of the satellite halos that survive the overmerging, we present a series of halo statistics, that can be compared with those of observed galaxies. (Abridged), Comment: Accepted for publication in ApJ, substantional revisions after the first version, LaTeX 23 pages, 18 figs. (uses emulateapj.sty), Full-resolution version of Fig.9 is available upon request

The large-scale matter distribution represents a complex network of structure elements such as voids, clusters, filaments, and sheets. This network is spanned by a point distribution. The global properties of the point process can be measured by different statistical methods, which, however, do not describe directly the structure elements. The morphology of structure elements is an important property of the point distribution. Here we apply the core-sampling method to various Voronoi tessellations. Using the core-sampling method we identify one- and two-dimensional structure elements (filaments and sheets) in these Voronoi tessellations and reconstruct their mean separation along random straight lines. We compare the results of the core-sampling method with the a priori known structure elements of the Voronoi tessellations under consideration and find good agreement between the expected and found structure parameters, even in the presence of substantial noise. We conclude that the core-sampling method is a potentially powerful tool to investigate the distribution of such structure elements like filaments and walls of galaxies., Comment: 14 pages (Latex) with 6 figures, the complete paper with 8 figures is available at http://kosmos.aip.de/~got/projects.html {Characteristical scales in point distributions}, Astronomy and Astrophysics Supplement Series, accepted

We investigate the performance of Lagrangian perturbation theory up to the second order for two scenarios of cosmological large-scale structure formation, SCDM (standard cold dark matter) and BSI (broken scale invariance). The latter model we study as a representative of COBE-normalized CDM models which fit the small-scale power of galaxy surveys. In this context we optimize the performance of the Lagrangian perturbation schemes by smoothing the small-scale fluctuations in the initial data. The results of the so obtained Lagrangian mappings are computed for a set of COBE-normalized SCDM and BSI initial data of different sizes and at different times. We compare these results against those obtained with a numerical PM-code. We find an excellent performance of the optimized Lagrangian schemes down to scales close to the correlation length. This is explained by the counterintuitive fact that nonlinearities in the model can produce more small-scale power, if initially such power is removed. The optimization scheme can be expressed in a way which is independent of the type of fluctuation spectrum and of the size of the simulations., Comment: 19 pages uuencoded compressed postscript file (including 7 figures). Figure 8 (greyscale and color postcript files) are available by anonymous ftp from ftp://ibm-3.mpa-garching.mpg.de/pub/aow . See the directory with this preprint number

Observational constraints on standard CDM spectra and perturbation spectra with broken scale invariance are discussed., Comment: To appear in the proceedings of the 17th Texas Symposium, 4 pages uuencoded, including figures.

CDM models with non-scale-free step-like spectra of adiabatic perturbations produced in a realistic double inflationary model are compared with recent observational data. The model contains two additional free parameters relatively to the standard CDM model with the flat ($n=1$) initial spectrum. Results of the COBE experiment are used for the determination of a free overall spectrum normalization. Then predictions for the galaxy biasing parameter, the variance for "counts in cells", the galaxy angular correlation function, bulk flow peculiar velocities and the Mach number test are obtained. Also considered are conditions for galaxy and quasar formation. Observational data strongly restricts allowed values for the two remaining model parameters. However, a non-empty region for them satisfying all considered tests is found., Comment: 17 pages and 4 figures obtainable by request, LaTex, AIP 93-11