arXiv subjects

A. Klypin

Publications and source records attributed to A. Klypin.

At least 37 records · Page 2Linked to original sources

arXiv2004

Phase-Space Structure of Dark-Matter Haloes: Scale-Invariant PDF Driven by Substructure

We present a method for computing the 6-dimensional coarse-grained phase-space density $f(x,v)$ in an N-body system, and derive its distribution function $v(f)$. The method is based on Delaunay tessellation, where $v(f)$ is obtained with an effective fixed smoothing window over a wide $f$ range. The errors are estimated, and $v(f)$ is found to be insensitive to the sampling resolution or the simulation technique. We find that in gravitationally relaxed haloes built by hierarchical clustering, $v(f)$ is well approximated by a robust power law, $v(f) \propto f^{-2.5 \pm 0.05}$, over more than 4 decades in $f$, from its virial level to the numerical resolution limit. This is tested to be valid in the $Λ$CDM cosmology for haloes with masses $10^9-10^{15}\msun$, indicating insensitivity to the slope of the initial fluctuation power spectrum. By mapping the phase-space density in position space, we find that the high-$f$ end of $v(f)$ is dominated by the ``cold'' subhaloes rather than the parent-halo central region and its global spherical profile. The value of $f$ in subhaloes near the virial radius is typically $>100$ times higher than its value at the halo centre, and it decreases gradually from outside in toward its value at the halo centre. This seems to reflect phase mixing due to mergers and tidal effects involving puffing up and heating. The phase-space density can thus provide a sensitive tool for studying the evolution of subhaloes during the hierarchical buildup of haloes. It remains to be understood why the evolved substructure adds up to the actual universal power law of $v(f) \propto f^{-2.5}$. It seems that this behaviour results from the hierarchical clustering process and is not a general result of violent relaxation.

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arXiv2003

Structure Formation in Dynamical Dark Energy Models

We perform N-body simulations for models with a DE component. Besides of DE with constant negative state parameter w, we consider DE due to scalar fields, self-interacting through RP or SUGRA potentials. According to our post-linear analysis, at z=0, DM power spectra and halo mass functions do not depend on DE nature. This is welcome, as LCDM fits observations. Halo profiles, instead, are denser than LCDM. For example, the density at 15 kpc of a DE halo with M=10^13 exceeds LCDM by ~45%. Differences, therefore, are small but, however, DE does not ease the problem with cuspy DM profiles. We study also subhalos and find that, at $z=0$, the number of satellites coincides in all DE models. At higher z, DE models show increasing differences from LCDM and among themselves (i.e. in the mass function evolution); this is the obvious pattern to distinguish between different DE state equations.

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arXiv2003

Modeling Dynamical Dark Energy

Cosmological models with different types of Dark Energy are becoming viable alternatives for standard models with the cosmological constant. Yet, such models are more difficult to analyze and to simulate. We present analytical approximations and discuss ways of making simulations for two families of models, which cover a wide range of possibilities and include models with both slow and fast changing ratio w=pρ. More specifically, we give analytical expressions for the evolution of the matter density parameter Omega_m(z) and the virial density contrast Delta_c at any redshift z. The latter is used to identify halos and to find their virial masses. We also provide an approximation for the linear growth factor of linear fluctuations between redshift z=40 and z=0. This is needed to set the normalization of the spectrum of fluctuations. Finally, we discuss the expected behavior of the halo mass function and its time evolution.

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arXiv2003

Halo properties in models with dynamical Dark Energy

We study properties of dark matter halos in a variety of models which include Dark Energy (DE). We consider both DE due to a scalar field self--interacting through Ratra-Peebles or SUGRA potentials, and DE with constant negative w=pρ>-1. We find that at redshift zero the nonlinear power spectrum of the dark matter, and the mass function of halos, practically do not depend on DE state equation and are almost indistinguishable from predictions of the LCDM model. This is consistent with the nonlinear analysis presented in the accompanying paper. It is also a welcome feature because LCDM models fit a large variety of data. On the other hand, at high redshifts DE models show substantial differences from LCDM and substantial differences among themselves. Halo profiles differ even at z=0. DE halos are denser than LCDM in their central parts because the DE halos collapse earlier. Nevertheless, differences between the models are not so large. For example, the density at 10 kpc of a DE \~10^{13}Msun halo deviates from LCDM by not more than 50%. This, however, means that DE is not a way to ease the problem with cuspy dark matter profiles. Addressing another cosmological problem - abundance of subhalos -- we find that the number of satellites of halos in various DE models does not change relative to the LCDM, when normalized to the same circular velocity of the parent halo. To summarize, the best way to find which DE model fits the observed Universe is to look for evolution of halo properties. For example, the abundance of galaxy groups with mass larger than 10^{13}\Msun at z> 2 can be used to discriminate between the models, and, thus, to constrain the nature of DE.

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arXiv2000

Formation and evolution of disk galaxies within cold dark matter halos

We present results of extensive model calculations of disk galaxy evolution within an hierarchical inside-out formation scenario. We first compare properties of the dark halos identified in a cosmological N-body simulation with predictions of a seminumerical method based on an extended collapse model and find a good agreement. We also study the properties of the halos in dependence on their environment. We then describe detailed modelling of the formation and evolution of disks within the growing isolated cold dark matter halos and predictions for the main properties, correlations and evolutionary features of normal disk galaxies. The possible shortcomings of the scenario are discussed.

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arXiv1999

Release of data from high-resolution cosmological N-body simulations

We make available to the public 4 dark matter simulations that cover 4 CDM cosmological models and were run with the AP^3M code (Couchamn 1991). These simulations are described in Colin et al. (ApJ, 523, 32, 1999). There are about 20 epochs available per model. They were run in a 30 Mpc/h box and have 128^3 particles in a 128^3 mesh. To download the binary data please contact me at: colin@astroscu.unam.mx

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arXiv1999

The evolution of groups and clusters

Using high resolution N-body simulations we have studied the formation and evolution of clusters and groups in a LCDM cosmological model. Groups of galaxies have been formed already before z=4. The total number of small bound systems and the total number of galaxies in these small systems rapidly decreases after z=1.5. The fraction of isolated galaxies remains approximately constant after z =1, whereas the fraction of galaxies in groups decreases. Merging of groups and accretion leads to cluster formation at z < 2. Some of the groups merge into large isolated halos.

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arXiv1999

Density profiles of dark matter haloes: diversity and dependence on environment

(Abridged) We study the outer density profiles of dark matter haloes predicted by a generalized secondary infall model and observed in a N-body cosmological simulation of a ΛCDM model. We find substantial systematic variations in shapes and concentrations of the halo profiles as well as a strong correlation of the profiles with the environment. In the N-body simulation, the average outer slope of the density profiles, β(ρ\propto r^{-β}), of isolated haloes is \approx 2.9; 68% of these haloes have values of βbetween 2.5 and 3.8. Haloes in dense environments of clusters are more concentrated and exhibit a broad distribution of βwith values larger than for isolated haloes . Contrary to what one may expect, the haloes contained within groups and galaxy systems are less concentrated and have flatter outer density profiles than the isolated haloes. The concentration decreases with M_h, but its scatter for a given mass is substantial. The mass and circular velocity of the haloes are strongly correlated: M_h \propto V_m^α with α~ 3.3 (isolated) and ~3.5 (haloes in clusters). For M_h=10^12M_sun the rms deviations from these relations are ΔlogM_h=0.12 and 0.18, respectively. Approximately 30% of the haloes are contained within larger haloes or have massive companions (larger than ~0.3 the mass of the current halo) within 3 virial radii. The remaining 70% of the haloes are isolated objects. The distribution of βas well as the concentration-mass and M_h-V_m relations for the isolated haloes agree very well with the predictions of our seminumerical approach which is based on a generalization of the secondary infall model and on the extended Press-Schechter formalism.

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arXiv1999

The Santa Barbara cluster comparison project: a comparison of cosmological hydrodynamics solutions

We have simulated the formation of an X-ray cluster in a cold dark matter universe using 12 different codes. The codes span the range of numerical techniques and implementations currently in use, including SPH and grid methods with fixed, deformable or multilevel meshes. The goal of this comparison is to assess the reliability of cosmological gas dynamical simulations of clusters in the simplest astrophysically relevant case, that in which the gas is assumed to be non-radiative. We compare images of the cluster at different epochs, global properties such as mass, temperature and X-ray luminosity, and radial profiles of various dynamical and thermodynamical quantities. On the whole, the agreement among the various simulations is gratifying although a number of discrepancies exist. Agreement is best for properties of the dark matter and worst for the total X-ray luminosity. Even in this case, simulations that adequately resolve the core radius of the gas distribution predict total X-ray luminosities that agree to within a factor of two. Other quantities are reproduced to much higher accuracy. For example, the temperature and gas mass fraction within the virial radius agree to about 10%, and the ratio of specific kinetic to thermal energies of the gas agree to about 5%. Various factors contribute to the spread in calculated cluster properties, including differences in the internal timing of the simulations. Based on the overall consistency of results, we discuss a number of general properties of the cluster we have modelled.

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arXiv1998

Constraining cosmological models with cluster power spectra

Using extensive N-body simulations we estimate redshift space power spectra of clusters of galaxies for different cosmological models (SCDM, TCDM, CHDM, Lambda-CDM, OCDM, BSI, tau-CDM) and compare the results with observational data for Abell-ACO clusters. Our mock samples of galaxy clusters have the same geometry and selection functions as the observational sample which contains 417 clusters of galaxies in a double cone of galactic latitude |b| > 30 degrees up to a depth of 240 Mpc/h. The power spectrum has been estimated for wave numbers k in the range 0.03 < k < 0.2 h/Mpc. For k > k_max ~ 0.05 h/Mpc the power spectrum of the Abell-ACO clusters has a power-law shape, P(k)\propto k^n, with n ~ -1.9, while it changes sharply to a positive slope at k < k_max. By comparison with the mock catalogues SCDM, TCDM (n=0.9), and also OCDM with Omega_0 = 0.35 are rejected. Better agreement with observation can be found for the Lambda-CDM model with Omega_0 = 0.35 and h = 0.7 and the CHDM model with two degenerate neutrinos and Omega_HDM = 0.2 as well as for a CDM model with broken scale invariance (BSI) and the tau-CDM model. As for the peak in the Abell-ACO cluster power spectrum, we find that it does not represent a very unusual finding within the set of mock samples extracted from our simulations.

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arXiv1998

Hydrodynamical simulations of galaxy properties: Environmental effects

Using N-body+hydro simulations we study relations between the local environments of galaxies on 0.5 Mpc scale and properties of the luminous components of galaxies. Our numerical simulations include effects of star formation and supernova feedback in different cosmological scenarios: the standard Cold Dark Matter model, the Broken Scale Invariance model (BSI), and a model with cosmological constant (LCDM). In this paper, we concentrate on the effects of environment on colors and morphologies of galaxies, on the star formation rate and on the relation between the total luminosity of a galaxy and its circular velocity. We demonstrate a statistically significant theoretical relationship between morphology and environment. In particular, there is a strong tendency for high-mass galaxies and for elliptical galaxies to form in denser environments, in agreement with observations. We find that in models with denser environments (CDM scenario) ~ 13 % of the galactic halos can be identified as field ellipticals, according to their colors. In simulations with less clustering (BSI and LCDM), the fraction of ellipticals is considerably lower (~ 2-3 %). The strong sensitivity of morphological type to environment is rather remarkable because our results are applicable to ``field'' galaxies and small groups. If all galaxies in our simulations are included, we find a statistically significant dependence of the galaxy luminosity - circular velocity relation on dark matter overdensity within spheres of radius 0.5 Mpc, for the CDM simulations. But if we remove ``elliptical'' galaxies from our analysis to mimic the Tully-Fisher relation for spirals, then no dependence is found in any model.

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arXiv1998

Self-regulating galaxy formation as an explanation for the Tully-Fisher relation

Using 3D hydrodynamical simulations of galaxy formation with supernova feedback and a multiphase medium, we derive theoretical relations analogous to the observed Tully-Fisher (TF) relations in various photometric bands. This paper examines the influence of self-regulation mechanisms including supernova feedback on galaxy luminosities and the TF relation in three cosmological scenarios (CDM, ΛCDM and BSI (broken scale invariance)). The galaxy catalogs derived from our hydrodynamical simulations lead to an acceptably small scatter in the theoretical TF relation amounting to ΔM =0.2-0.4 in the I band, and increasing by 0.1 magnitude from the I-band to the B-band. Our results give strong evidence that the tightness of the TF relation cannot be attributed to supernova feedback alone. However, although eliminating supernova feedback hardly affects the scatter, it does influence the slope of the TF relation quite sensitively. With supernova feedback, L \propto V_c^{3-3.5} (depending on the strength of supernova feedback). Without it, L \propto V_c^{2} as predicted by the virial theorem with constant M/L. The luminosity functions in the B and K bands are quite sensitive to supernova feedback at the faint end studied here. We find that the faint end of the B-band luminosity function (-18 \leq M_B \leq -15) has a slope that is steeper than the Stromlo-APM estimate, but in rough agreement with the recent ESO Slice Project estimates.

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arXiv1996

Constraining the Power Spectrum using Clusters

(Shortened Abstract). We analyze a redshift sample of Abell/ACO clusters and compare them with numerical simulations based on the truncated Zel'dovich approximation (TZA), for a list of eleven dark matter (DM) models. For each model we run several realizations, on which we estimate cosmic variance effects. We analyse correlation statistics, the probability density function, and supercluster properties from percolation analysis. As a general result, we find that the distribution of galaxy clusters provides a constraint only on the shape of the power spectrum, but not on its amplitude: a shape parameter 0.18 < Γ< 0.25 and an effective spectral index at 20Mpc/h in the range [-1.1,-0.9] are required by the Abell/ACO data. In order to obtain complementary constraints on the spectrum amplitude, we consider the cluster abundance as estimated using the Press--Schechter approach, whose reliability is explicitly tested against N--body simulations. We conclude that, of the cosmological models considered here, the only viable models are either Cold+Hot DM ones with Ω_ν= [0.2-0.3], better if shared between two massive neutrinos, and flat low-density CDM models with Ω_0 = [0.3-0.5].

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arXiv1996

Statistical Tests for CHDM and \LambdaCDM Cosmologies

We apply several statistical estimators to high-resolution N-body simulations of two currently viable cosmological models: a mixed dark matter model, having $Ω_ν=0.2$ contributed by two massive neutrinos (C+2\nuDM), and a Cold Dark Matter model with Cosmological Constant (\LambdaCDM) with $Ω_0=0.3$ and h=0.7. Our aim is to compare simulated galaxy samples with the Perseus-Pisces redshift survey (PPS). We consider the n-point correlation functions (n=2-4), the N-count probability functions P_N, including the void probability function P_0, and the underdensity probability function U_ε(where εfixes the underdensity threshold in percentage of the average). We find that P_0 (for which PPS and CfA2 data agree) and P_1 distinguish efficiently between the models, while U_εis only marginally discriminatory. On the contrary, the reduced skewness and kurtosis are, respectively, S_3\simeq 2.2 and S_4\simeq 6-7 in all cases, quite independent of the scale, in agreement with hierarchical scaling predictions and estimates based on redshift surveys. Among our results, we emphasize the remarkable agreement between PPS data and C+2\nuDM in all the tests performed. In contrast, the above \LambdaCDM model has serious difficulties in reproducing observational data if galaxies and matter overdensities are related in a simple way.

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arXiv1996

Numerical Simulations in Cosmology I

The purpose of these lectures is to give a short introduction into a very vast field of numerical simulations for cosmological applications. I focus on major features of the simulations: the equations, main numerical techniques, effects of resolution, and methods of halo identification.

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arXiv1996

Hydrodynamical simulations of galaxy formation: effects of supernova feedback

We numerically simulate some of the most critical physical processes in galaxy formation: The supernova feedback, in conjunction with gasdynamics and gravity, plays a crucial role in determining how galaxies arise within the context of a model for large-scale structure. Our treatment incorporates a multi-phase model of the interstellar medium and includes the effects of cooling, heating and metal enrichment by supernovae, and evaporation of cold clouds. The star formation happens inside the clouds of cold gas, which are produced via thermal instability. We simulate the galaxy formation in standard biased CDM model for a variety of parameters and for several resolutions in the range 2--20$h^{-1}$kpc. In our picture, supernova feedback regulates the evolution of the gas components and star formation. The efficiency of cloud evaporation by supernova strongly influences star formation rates. This feedback results in a steady rate of star formation in large galaxies (mass larger than $2-3x10^{11}\Msun$) at a level of $(1-10)\Msun\yr$ for $z<3$. Supernova feedback has an even stronger effect on the evolution of dwarf galaxies, most of which have a small fraction of stars and extremely low luminosities: $M_R>-15$. In the case of both large and small galaxies, the distribution of luminous matter (stars) is strongly BIASED with respect to the dark matter. We find an approximate biasing measure of the form $ρ_{lum}= (ρ_{dm}/133)^{1.7}$ for z=0 and overdensities exceeding 1000. Deviations from this relation (a factor 2-3) depend on the environment. For halo masses exceeding $2x10^{10}\Msun$, the dependence of the absolute magnitude on the total mass can be approximated as $M_V=-18.5-4\log(M_{tot}/10^{11}\Msun)$, with a scatter of less than 0.5mag.

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arXiv1995

COLD + HOT AND COLD DARK MATTER COSMOLOGIES: ANALYSIS OF NUMERICAL SIMULATIONS

We present a series of four simulations of Cold Dark Matter (CDM) and Cold + Hot Dark Matter (CHDM) cosmologies. We discuss the power spectrum and correlation functions in real and redshift space, with comparisons to the CfA2 and IRAS redshift data, the pairwise velocity of galaxies, and the distribution of hot and cold particles in CHDM simulations. We confirm that CHDM with cold/hot/baryon density ratios $Ω_c/Ω_ν/Ω_b= 0.6/0.3/0.1$ is a good fit to a wide variety of present-epoch data, much better than CDM. In particular, with reasonable assumptions about identification of galaxies and biasing, we find that the power spectrum agrees rather well with both the CfA2 and IRAS power specta in both the nonlinear and linear regimes. New CHDM models (e.g., with $Ω_ν=0.2$ or with two massive neutrinos) predict a significantly larger rate of formation of galaxies at high redshift, which may be needed to explain some observational data. At the same time, the difference between the variants is rather small at $z=0$. The results presented in this paper are interesting for two purposes: (i) For a comparison with other classes of models (like CDM and $Λ$CDM) at $z=0$.(ii) As a reference point for comparison between different variants of the CHDM model.

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arXiv1995

MASS SEGREGATION IN DARK MATTER MODELS.

We use the moments of counts of neighbors as given by the Generalized Correlation Integrals, to study the clustering properties of Dark Matter Halos (DH) in Cold Dark Matter (CDM) and Cold+Hot Dark Matter (CHDM) models. We compare the results with those found in the CfA and SSRS galaxy catalogs. We show that if we apply the analysis in redshift space, both models reproduce equally well the observed clustering of galaxies. Mass segregation is also found in the models: more massive DHs are more clustered compared with less massive ones. In redshift space, this mass segregation is reduced by a factor 2-3 due to the peculiar velocities. Observational catalogs give an indication of luminosity and size segregation, which is consistent with the predictions of the models. Because the mass segregation is smaller in redshift space, it is suggestive that the real luminosity or size segregation of galaxies could be significantly larger than what it is found in redshift catalogs.

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