arXiv subjects

Anatoly Klypin

Publications and source records attributed to Anatoly Klypin.

At least 91 records · Page 5Linked to original sources

arXiv2005

The role of resonances in barred galaxies

The dynamics of the inner disk of many galaxies is dominated by bars. As a result, resonances between the bar and the disk become an important factor. In order to detect resonances, we measure angular and radial frequencies of individual orbits in N-body models of barred galaxies. In our simulations with several million particles, stellar disks are immersed in live halos of dark matter. A bar forms spontaneously and interacts with the disk through resonances. We detect 9 of them. The corotation (CR) and the inner Lindblad resonance (ILR) are the most significant. The spatial distributions of particles trapped at these resonances show that ILR is not localized at a given radius. Particles trapped at ILR lie along the bar spanning a wide range of radii. On the other hand, particles at the corotation resonance form a ring around the corotation radius. We study how orbits evolve near corotation and how they are trapped. When an orbit is getting trapped, it evolves strongly, but after that there is little evolution. Those trapped orbits circulate along the corotation ring or librate around a stable lagragian point. The same technique is applied to study resonances in the halo. The bar interacts strongly with the halo particles that stay close to the disk. We find the corotation resonance and the inner Lindblad resonance for halo particles.

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arXiv2004

The rotation curves of dwarf galaxies: a problem for Cold Dark Matter?

We address the issue of accuracy in recovering density profiles from observations of rotation curves of galaxies. We ``observe'' and analyze our models in much the same way as observers do the real galaxies. We find that the tilted ring model analysis produces an underestimate of the central rotational velocity. In some cases the galaxy halo density profile seems to have a flat core, while in reality it does not. We identify three effects, which explain the systematic biases: (1) inclination (2), small bulge, and (3) bar. The presence of even a small non-rotating bulge component reduces the rotation velocity. In the case of a disk with a bar, the underestimate of the circular velocity is larger due to a combination of non-circular motions and random velocities. Signatures of bars can be difficult to detect in the surface brightness profiles of the model galaxies. The variation of inclination angle and isophote position angle with radius are more reliable indicators of bar presence than the surface brightness profiles. The systematic biases in the central ~ 1 kpc of galaxies are not large. Each effect separately gives typically a few kms error, but the effects add up. In some cases the error in circular velocity was a factor of two, but typically we get about 20 percent. The result is the false inference that the density profile of the halo flattens in the central parts. Our observations of real galaxies show that for a large fraction of galaxies the velocity of gas rotation (as measured by emission lines) is very close to the rotation of stellar component (as measured by absorption lines). This implies that the systematic effects discussed in this paper are also applicable both for the stars and emission-line gas.

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arXiv2004

Astrophysical inputs on the SUSY dark matter annihilation detectability

If dark matter (DM), which is considered to constitute most of the mass of galaxies, is made of supersymmetric (SUSY) particles, the centers of galaxies should emit gamma-rays produced by their self-annihilation. We present accurate estimates of continuum gamma-ray fluxes due to neutralino annihilation in the central regions of the Milky Way. We use detailed models of our Galaxy, which satisfy available observational data, and include some important physical processes, which were previously neglected. Our models predict that spatially extended annihilation signal should be detected at high confidence levels by incoming experiments assuming that neutralinos make up most of the DM in the Universe and that they annihilate according to current SUSY models.

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arXiv2003

The structure of voids

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.

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arXiv2003

Secular bar formation in galaxies with significant amount of dark matter

Using high resolution N-body simulations of stellar disks embedded in cosmologically motivated dark matter halos, we study the evolution of bars and the transfer of angular momentum between halos and bars. We find that dynamical friction results in some transfer of angular momentum to the halo, but the effect is much smaller than previously found in low resolution simulations and is incompatible with early analytical estimates. In simulations with millions of particles reaching a resolution of 20-40 pc, the pattern speed may not change over billions of years. Our high resolution models produce bars which are fast rotators, where the ratio of the corotation radius to the bar major semi-axis lies in the range R =1.2-1.7, marginally compatible with observational results. In contrast to many previous simulations, we find that bars are relatively short. As in many observed cases, the bar major semi-axis is close to the exponential length of the disk. The transfer of angular momentum between inner and outer parts of the disk plays a very important role in the secular evolution of the disk and the bar. The bar formation increases the exponential length of the disk by a factor of 1.2 -1.5. The transfer substantially increases the stellar mass in the centre of the galaxy and decreases the dark matter-to-baryons ratio. As the result, the central 2 kpc region is always strongly dominated by the baryonic component. At intermediate 3-10kpc scales the disk is sub-dominant. In summary, realistic models produce bar structure in striking agreement with observational results.

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arXiv2003

Observing the dark matter density profile of isolated galaxies

Using the Sloan Digital Sky Survey (SDSS), we probe the halo mass distribution by studying the velocities of satellites orbiting isolated galaxies. In a subsample that covers 2500 sq. degrees on the sky, we detect about 3000 satellites with absolute blue magnitudes going down to M_B = -14; most of the satellites have M_B=-16 to -18, comparable to the magnitudes of M32 and the Magellanic Clouds. After a careful, model-independent removal of interlopers, we find that the line-of-sight velocity dispersion of satellites declines with distance to the primary. For an L* galaxy the r.m.s. line-of-sight velocity changes from ~120 km/s at 20 kpc to ~60 km/s at 350 kpc. This decline agrees remarkably well with theoretical expectations, as all modern cosmological models predict that the density of dark matter in the peripheral parts of galaxies declines as rho_DM propto r^{-3}. Thus, for the first time we find direct observational evidence of the density decline predicted by cosmological models; we also note that this result contradicts alternative theories of gravity such as MOND. We also find that the velocity dispersion of satellites within 100 kpc scales with the absolute magnitude of the central galaxy as sigma propto L^{0.3}; this is very close to the Tully-Fisher relation for normal spiral galaxies.

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arXiv2002

Simulations of the Local Universe

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.

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arXiv2002

Low mass dark matter halos in voids

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.

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arXiv2002

Spatial distribution of galactic halos and their merger histories

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).

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arXiv2001

LCDM-based models for the Milky Way and M31 I: Dynamical Models

We apply standard disk formation theory with adiabatic contraction within cuspy halo models predicted by the standard LCDM cosmology. The resulting models score remarkably well when confronted with the broad range of observational data available for the Milky Way and M31 galaxies, giving a Milky Way virial mass of 1-2x10^12Msun and concentration C=12-17. We consider two types of models, in which: (A) baryons conserve angular momentum and (B) some of the angular momentum of the baryons is transferred to the dark matter. Type-A models produce good agreement with observed rotation curves and obey constraints in the solar neighborhood, but may have too much dark matter in the center to allow a fast rotating bar. The type-B models with angular momentum transport have a slightly more massive disk and less dark matter in the central part, allowing a fast rotating bar to persist. Both classes of models probably have sufficient baryonic mass in the central 3.5kpc to reproduce recent observational values of the optical depth to microlensing events towards the Galactic center. All models require that about 1/2 of all baryons expected inside the virial radius must not be in the disk or bulge. We investigate whether the range of virial masses allowed by our dynamical models is compatible with constraints from the galaxy luminosity function, and find a range of parameter space that is allowed by this constraint. We conclude that rotation curves and dynamical properties of ``normal'' high surface brightness spiral galaxies appear to be consistent with standard LCDM.

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arXiv2001

Constrained Simulations of the Real Universe: the Local Supercluster

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.

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arXiv2001

Resolving the Structure of Cold Dark Matter Halos II

We study the effects of mass and force resolution on the density profiles of galaxy-size Cold Dark Matter (CDM) halos in a flat, low-density cosmological model with vacuum energy. We show thatalthough increasing the mass and force resolution allows us to probe deeper into the inner halo regions, it does not lead to steeper inner density profiles. Instead, the halo profiles converge at scales larger than four times the formal resolution or the radius containing more than 200 particles, whichever is larger. In the simulations presented in this paper, we are able to probe density profile of a relaxed isolated galaxy-size halo at scales r=(0.005-1)r_vir. We find that the density distribution can be well approximated by the profile suggested by Moore etal (1998): rho= x^{-1.5}(1+x^{1.5})^-1, where x=r/r_s and r_s is the characteristic radius. The analytical profile proposed by Navarro et al. (1996) rho= x^{-1}(1+x)^-2, also provides a good fit, with the same relative errors of about 10% for radii larger than 1% of the virial radius. Both analytical profiles fit equally well because for high-concentration galaxy-size halos the differences between these profiles become significant only at scales well below 0.01r_vir. We also find that halos of similar mass may have somewhat different parameters (characteristic radius, maximum rotation velocity, etc.) and shapes of their density profiles. We associate this scatter in properties with differences in halo merger histories and the amount of substructure present in the analyzed halos.

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arXiv2000

Numerical Simulations in Cosmology I: Methods

This lecture gives a short introduction to different methods used in cosmology. The focus is on major features of N-body simulations: equations, main numerical techniques, effects of resolution, and methods of halo identification.

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arXiv2000

Numerical Simulations in Cosmology II: Spatial and Velocity Biases

We give a summary of recent results on spatial and velocity biases in cosmological models. Progress in numerical techniques made it possible to simulate halos in large volumes with a such accuracy that halos survive in dense environments of groups and clusters of galaxies. Halos in simulations look like real galaxies, and, thus, can be used to study the biases -- differences between galaxies and the dark matter. The biases depend on scale, redshift, and circular velocities of selected halos. Two processes seem to define the evolution of the spatial bias: (1) statistical bias and (2) merger bias (merging of galaxies, which happens preferentially in groups, reduces the number of galaxies, but does not affect the clustering of the dark matter). There are two kinds of velocity bias. The pair-wise velocity bias is b_12=0.6-0.8 at r< 5Mpc/h, z=0. This bias mostly reflects the spatial bias and provides almost no information on the relative velocities of the galaxies and the dark matter. One-point velocity bias is a better measure of the velocities. Inside clusters the galaxies should move slightly faster (b_v =1.1-1.3) than the dark matter. Qualitatively this result can be understood using the Jeans equations of the stellar dynamics. For the standard LCDM model we find that the correlation function and the power spectrum of galaxy-size halos at z=0 are antibiased on scales r<5Mpc/h and k=(0.15-30)h/Mpc.

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arXiv2000

Numerical Simulations in Cosmology III: Dark Matter Halos

Properties of dark matter halos are reviewed. Taken from different publications, we present results on (1) the mass and velocity functions, (2) density and velocity profiles, and (3) concentration of halos. In the range of radii r=(0.005-1)rvir the density profile for a quiet isolated halo is very accurately approximated by a fit suggested by Moore etal (1997): rho=1/x^1.5(1+x^1.5), where x=r/rs and rs is a characteristic radius. The fit suggested by Navarro et al (1995) rho= 1/x(1+x)^2, also gives a very satisfactory approximation with relative errors of about 10% for radii not smaller than 1% of the virial radius. The mass function of z=0 halos with mass below 10^{13}Msun/h is approximated by a power law with slope alpha =-1.85. The slope increases with the redshift. The velocity function of halos with Vmax< 500km/s is also a power law with the slope beta= -3.8-4. The power-law extends to halos at least down to 10km/s. It is also valid for halos inside larger virialized halos. The concentration of halos depends on mass (more massive halos are less concentrated) and environment, with isolated halos being less concentrated than halos of the same mass inside clusters. Halos have intrinsic scatter of concentration: at 1sigma level halos with the same mass have Delta(log{c_vir})=0.18 or, equivalently, DeltaVmax/Vmax =0.12. Velocity anisotropy for both subhalos and the dark matter is approximated by beta(r) =0.15+2x/[x^2+4], where x is radius in units of the virial radius.

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arXiv2000

Simulated Cluster Archive: A Computational Catalog of X-Ray Clusters in a Lambda-CDM Universe

We have simulated the evolution of a large sample of X-ray clusters in a Lambda-CDM universe at high spatial resolution using adaptive mesh refinement and placed the results in an online archive for public access. The Simulated Cluster Archive website http://sca.ncsa.uiuc.edu provides tools for interactive 2D and 3D analysis of gas and dark matter fields, X-ray and SZ imaging, and data export. We encourage community use and solicit their feedback.

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arXiv2000

Merging history as a function of halo environment

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.

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arXiv1999

On effects of resolution in dissipationless cosmological simulations

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.

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