arXiv subjects

Andrey V. Kravtsov

Publications and source records attributed to Andrey V. Kravtsov.

At least 91 records · Page 5Linked to original sources

arXiv2006

Modeling Galaxy Clustering with Cosmological Simulations

I review recent progress in understanding and modeling galaxy clustering in cosmological simulations, with emphasis on models based on high-resolution dissipationless simulations. During the last decade, significant advances in our understanding of abundance and clustering of dark matter halos allowed construction of accurate, quantitative models of galaxy clustering both in linear and non-linear regimes. Results of several recent studies show that dissipationless simulations with a simple, non-parametric model for the relation between halo circular velocity and luminosity of the galaxy they host predict the shape, amplitude, and luminosity dependence of the two-point correlation function in excellent agreement with the observed galaxy clustering in the SDSS data at z~0 and in the DEEP2 samples at z~1 over the entire probed range of projected separations. In particular, the small-scale upturn of the correlation function from the power-law form in the SDSS and DEEP2 luminosity-selected samples is reproduced very well. At z~3-5, predictions also match the observed shape and amplitude of the angular two-point correlation function of Lyman-break galaxies (LBGs) on both large and small scales, including the theoretically predicted strong upturn at small scales. This suggests that, like galaxies in lower redshift samples, the LBGs are fair tracers of the overall halo population and that their luminosity is tightly correlated with the circular velocity (and hence mass) of their dark matter halos.

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arXiv2006

The Dependence of Halo Clustering on Halo Formation History, Concentration, and Occupation

We investigate the dependence of dark matter halo clustering on halo formation time, density profile concentration, and subhalo occupation number, using high-resolution numerical simulations of a LCDM cosmology. We confirm results that halo clustering is a function of halo formation time, and that this trend depends on halo mass. For the first time, we show unequivocally that halo clustering is a function of halo concentration and show that the dependence of halo bias on concentration, mass, and redshift can be accurately parameterized in a simple way: b(c,M|z) = b(M|z) b(c|M/M*). The scaling between bias and concentration changes sign with the value of M/M*: high concentration (early forming) objects cluster more strongly for M <~ M* while low concentration (late forming) objects cluster more strongly for rare high-mass halos, M >~ M*. We show the first explicit demonstration that host dark halo clustering depends on the halo occupation number (of dark matter subhalos) and discuss implications for halo model calculations of dark matter power spectra and galaxy clustering statistics. The effect of these halo properties on clustering is strongest for early-forming dwarf-mass halos, which are significantly more clustered than typical halos of their mass. Our results suggest that isolated low-mass galaxies (e.g. low surface-brightness dwarfs) should have more slowly-rising rotation curves than their clustered counterparts, and may have consequences for the dearth of dwarf galaxies in voids. They also imply that self calibrating richness-selected cluster samples with their clustering properties might overestimate cluster masses and bias cosmological parameter estimation.

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arXiv2006

A Large Dark Matter Core in the Fornax Dwarf Spheroidal Galaxy?

We use measurements of the stellar velocity dispersion profile of the Fornax dwarf spheroidal galaxy to derive constraints on its dark matter distribution. Though the data are unable to distinguish between models with small cores and those with cusps, we show that a large > 1 kpc dark matter core in Fornax is highly implausible. Irrespective of the origin of the core, reasonable dynamical limits on the mass of the Fornax halo constrain its core radius to be no larger than 700 pc. We derive an upper limit core radius of 300 pc by demanding that the central phase space density of Fornax not exceed that directly inferred from the rotation curves of low-mass spiral galaxies. Further, if the halo is composed of warm dark matter then phase-space constraints force the core to be quite small in order to avoid conservative limits from the Ly alpha forest power spectrum, implying a core radius < 85 pc. We discuss our results in the context of the idea that the extended globular cluster distribution in Fornax can be explained by the presence of a large 1.5 kpc core. A self-consistent core of this size would be drastically inconsistent with the expectations of standard warm or cold dark matter models, and would also require an unreasonably massive dark matter halo, with a maximum circular velocity of 200 km/s.

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arXiv2006

Constraining the Projected Radial Distribution of Galactic Satellites with the Sloan Digital Sky Survey

We use the Sloan Digital Sky Survey (SDSS) spectroscopic sample to constrain the projected radial distribution of satellites around isolated ~ L* galaxies. We employ mock galaxy catalogs derived from high-resolution cosmological simulations to investigate the effects of interloper contamination and show that interlopers significantly bias the estimated slope of the projected radial distribution of satellites. We also show that the distribution of interlopers around galaxies is expected to be non-uniform in velocity space because galaxies are clustered and reside in crowded environments. Successful methods of interloper contamination correction should therefore take into account environments of the host galaxies. Two such new methods are presented and the most reliable of them is used to correct for interloper contamination in analyses of the SDSS galaxy sample. The best fit power-law slope of the interloper-corrected surface density distribution of satellites, Sigma(R) ~ R^alpha, in the volume-limited SDSS sample is alpha ~ -1.7 +/- 0.1, independent of the galaxy and satellite luminosities. Comparison with Lambda-CDM simulations shows that the radial distribution of the SDSS satellites is more concentrated than that of subhalos around galaxy-sized halos, especially at R < 100 kpc/h. The predicted dark matter radial distribution is somewhat more concentrated than the profile of the SDSS satellites, but the difference is not statistically significant for our sample.

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arXiv2006

A New Robust Low-Scatter X-ray Mass Indicator for Clusters of Galaxies

We present comparison of X-ray proxies for the total cluster mass, including the spectral temperature (Tx), gas mass measured within r500 (Mg), and the new proxy, Yx, which is a simple product of Tx and Mg and is related to the total thermal energy of the ICM. We use mock Chandra images constructed for a sample of clusters simulated with the eulerian N-body+gasdynamics adaptive mesh refinement ART code in the concordance LCDM cosmology. The simulations achieve high spatial and mass resolution and include radiative cooling, star formation, and other processes accompanying galaxy formation. Our analysis shows that simulated clusters exhibit a high degree of regularity and tight correlations between the considered observables and total mass. The normalizations of the M-Tx, Mg-Tx, and M-Yx relations agree to better than 10-15% with the current observational measurements of these relations. Our results show that Yx is the best mass proxy with a remarkably low scatter of only ~5-7% in M500 for a fixed Yx, at both low and high redshifts and regardless of whether clusters are relaxed or not. In addition, we show that redshift evolution of the Yx-M500 relation is close to the self-similar prediction, which makes Yx a very attractive mass indicator for measurements of the cluster mass function from X-ray selected samples.

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arXiv2006

Modeling Luminosity-Dependent Galaxy Clustering Through Cosmic Time

We employ high-resolution dissipationless simulations of the concordance LCDM cosmology to model the observed luminosity dependence and evolution of galaxy clustering through most of the age of the universe, from z~5 to z~0. We use a simple, non-parametric model which monotonically relates galaxy luminosities to the maximum circular velocity of dark matter halos (V_max) by preserving the observed galaxy luminosity function in order to match the halos in simulations with observed galaxies. The novel feature of the model is the use of the maximum circular velocity at the time of accretion, V_max,acc, for subhalos, the halos located within virial regions of larger halos. We argue that for subhalos in dissipationless simulations, V_max,acc reflects the luminosity and stellar mass of the associated galaxies better than the circular velocity at the epoch of observation, V_max,now. The simulations and our model L-V_max relation predict the shape, amplitude, and luminosity dependence of the two-point correlation function in excellent agreement with the observed galaxy clustering in the SDSS data at z~0 and in the DEEP2 samples at z~1 over the entire probed range of projected separations, 0.1<r_p/(Mpc/h)<10.0. In particular, the small-scale upturn of the correlation function from the power-law form in the SDSS and DEEP2 luminosity-selected samples is reproduced very well. At z~3-5, our predictions also match the observed shape and amplitude of the angular two-point correlation function of Lyman-break galaxies (LBGs) on both large and small scales, including the small-scale upturn.

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arXiv2006

Fossils of Reionization in the Local Group

We use a combination of high-resolution gasdynamics simulations of high-redshift dwarf galaxies and dissipationless simulations of a Milky Way sized halo to estimate the expected abundance and spatial distribution of the dwarf satellite galaxies that formed most of their stars around z~8 and evolved only little since then. Such galaxies can be considered as fossils of the reionization era, and studying their properties could provide a direct window into the early, pre-reionization stages of galaxy formation. We show that 5-15% of the objects existing at z~8 do indeed survive until the present in the MW like environment without significant evolution. This implies that it is plausible that the fossil dwarf galaxies do exist in the Local Group. Because such galaxies form their stellar systems early during the period of active merging and accretion, they should have spheroidal morphology regardless of their current distance from the host galaxy. We show that both the expected luminosity function and spatial distribution of dark matter halos which are likely to host fossil galaxies agree reasonably well with the observed distributions of the luminous (L_V>10^6 Lsun) Local Group fossil candidates near the host galaxy (d<200 kpc). However, the predicted abundance is substantially larger (by a factor of 2-3) for fainter galaxies (L_V<10^6 Lsun) at larger distances (d>300 kpc). We discuss several possible explanations for this discrepancy.

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arXiv2005

The Robustness of Dark Matter Density Profiles in Dissipationless Mergers

We present a comprehensive series of dissipationless N-body simulations to investigate the evolution of density distribution in equal-mass mergers between dark matter (DM) halos and multicomponent galaxies. The DM halo models are constructed with various asymptotic power-law indices ranging from steep cusps to core-like profiles and the structural properties of the galaxy models are motivated by the LCDM paradigm of structure formation. The adopted force resolution allows robust density profile estimates in the inner ~1% of the virial radii of the simulated systems. We demonstrate that the central slopes and overall shapes of the remnant density profiles are virtually identical to those of the initial systems suggesting that the remnants retain a remarkable memory of the density structure of their progenitors, despite the relaxation that accompanies merger activity. We also find that halo concentrations remain approximately constant through hierarchical merging involving identical systems and show that remnants contain significant fractions of their bound mass well beyond their formal virial radii. These conclusions hold for a wide variety of initial asymptotic density slopes, orbital energies, and encounter configurations, including sequences of consecutive merger events, simultaneous mergers of severals ystems, and mergers of halos with embedded cold baryonic components in the form of disks, spheroids, or both. As an immediate consequence, the net effect of gas cooling, which contracts and steepens the inner density profiles of DM halos, should be preserved through a period of dissipationless major merging. Our results imply that the characteristic universal shape of DM density profiles may be set early in the evolution of halos.

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arXiv2005

Dark Energy Studies: Challenges to Computational Cosmology

The ability to test the nature of dark mass-energy components in the universe through large-scale structure studies hinges on accurate predictions of sky survey expectations within a given world model. Numerical simulations predict key survey signatures with varying degrees of confidence, limited mainly by the complex astrophysics of galaxy formation. As surveys grow in size and scale, systematic uncertainties in theoretical modeling can become dominant. Dark energy studies will challenge the computational cosmology community to critically assess current techniques, develop new approaches to maximize accuracy, and establish new tools and practices to efficiently employ globally networked computing resources.

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arXiv2005

Constraining Dark Energy with the Dark Energy Survey: Theoretical Challenges

The Dark Energy Survey (DES) will use a new imaging camera on the Blanco 4-m telescope at CTIO to image 5000 square degrees of sky in the South Galactic Cap in four optical bands, and to carry out repeat imaging over a smaller area to identify and measure lightcurves of Type Ia supernovae. The main imaging area overlaps the planned Sunyaev-Zel'dovich survey of the South Pole Telescope. The idea behind DES is to use four distinct and largely independent methods to probe the properties of dark energy: baryon oscillations of the power spectrum, abundance and spatial distribution of clusters, weak gravitational lensing, and Type Ia supernovae. This white paper outlines, in broad terms, some of the theoretical issues associated with the first three of these probes (the issues for supernovae are mostly different in character), and with the general task of characterizing dark energy and distinguishing it from alternative explanations for cosmic acceleration. A companion white paper discusses the kind of numerical simulations and other theoretical tools that will be needed to address the these issues and to create mock catalogs that allow end-to-end tests of analysis procedures. Although we have been thinking about these problems in the specific context of DES, many of them are also relevant to other planned dark energy studies.

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arXiv2005

Galaxy-galaxy Lensing: Dissipationless Simulations Versus the Halo Model

Galaxy-galaxy lensing is a powerful probe of the relation between galaxies and dark matter halos, but its theoretical interpretation requires a careful modeling of various contributions, such as the contribution from central and satellite galaxies. For this purpose, a phenomenological approach based on the halo model has been developed, allowing for fast exploration of the parameter space of models. In this paper, we investigate the ability of the halo model to extract information from the g-g weak lensing signal by comparing it to high-resolution dissipationless simulations that resolve subhalos. We find that the halo model reliably determines parameters such as the host halo mass of central galaxies, the fraction of galaxies that are satellites, and their radial distribution inside larger halos. If there is a significant scatter present in the central galaxy host halo mass distribution, then the mean and median mass of that distribution can differ significantly from one another, and the halo model mass determination lies between the two. This result suggests that when analyzing the data, galaxy subsamples with a narrow central galaxy halo mass distribution, such as those based on stellar mass, should be chosen for a simpler interpretation of the results.

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arXiv2005

The Shape of Galaxy Cluster Dark Matter Haloes: Systematics of Its Imprint on Cluster Gas, and Comparison to Observations

(Abridged) We study predictions for galaxy cluster observables that can test the statistics of dark matter halo shapes expected in a flat LCDM universe. We present a simple analytical model for the prediction of cluster-scale X-ray observations, approximating clusters as isothermal systems in hydrostatic equilibrium, and dark matter haloes as ellipsoids with uniform axial ratios. We test the model against high-resolution, hydrodynamic cluster simulations to gauge its reliability. We find that this simple prescription does a good job of predicting the distribution of cluster X-ray ellipticities compared to the simulations as long as one focuses on cluster regions that are less sensitive to recent mergers. Based on this simple model, the distribution of cluster-size halo shapes expected in the concordance LCDM cosmology implies an X-ray ellipticity distribution with a mean of 0.32 +- 0.01 and a scatter of 0.14 +- 0.01 for the mass range (1-4)x10^{14} Msun/h. We find it important to include the mass dependence of halo shape to make comparisons to observational samples that contain many, very massive clusters. We analyse the systematics of four observational samples of cluster ellipticities and find that our results are statistically compatible with observations. In particular, we find remarkably good agreement between two recent ROSAT samples and LCDM predictions that DO NOT include gas cooling. We also test how well our analytical model can predict Sunyaev-Zel'dovich decrement maps and find that it is less successful although still useful; the model does not perform as well as a function of flux level in this case because of the changing triaxiality of dark matter haloes as a function of radial distance. Both this effect and the changing alignment of isodensity shells of dark matter haloes leave an imprint on cluster gas...

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arXiv2005

The Shape of Dark Matter Halos: Dependence on Mass, Redshift, Radius, and Formation

Using six high resolution dissipationless simulations with a varying box size in a flat LCDM universe, we study the mass and redshift dependence of dark matter halo shapes for M_vir = 9.0e11 - 2.0e14, over the redshift range z=0-3, and for two values of sigma_8=0.75 and 0.9. Remarkably, we find that the redshift, mass, and sigma_8 dependence of the mean smallest-to-largest axis ratio of halos is well described by the simple power-law relation = (0.54 +- 0.02)(M_vir/M_*)^(-0.050 +- 0.003), where s is measured at 0.3 R_vir and the z and sigma_8 dependences are governed by the characteristic nonlinear mass, M_*=M_*(z,sigma_8). We find that the scatter about the mean s is well described by a Gaussian with sigma ~ 0.1, for all masses and redshifts. We compare our results to a variety of previous works on halo shapes and find that reported differences between studies are primarily explained by differences in their methodologies. We address the evolutionary aspects of individual halo shapes by following the shapes of the halos through ~100 snapshots in time. We determine the formation scalefactor a_c as defined by Wechsler et al. (2002) and find that it can be related to the halo shape at z = 0 and its evolution over time.

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arXiv2005

Impact of Dark Matter Substructure on the Matter and Weak Lensing Power Spectra

We explore the effect of substructure in dark matter halos on the power spectrum and bispectrum of matter fluctuations and weak lensing shear. By experimenting with substructure in a cosmological N = 512^3 simulation, we find that when a larger fraction of the host halo mass is in subhalos, the resulting power spectrum has less power at 1 < k < 100 h Mpc^{-1} and more power at k > 100 h Mpc^{-1}. We explain this effect using an analytic halo model including subhalos, which shows that the 1 < k < 100 h Mpc^{-1} regime depends sensitively on the radial distribution of subhalo centers while the interior structure of subhalos is important at k > 100 h Mpc^{-1}. The corresponding effect due to substructures on the weak lensing power spectrum is up to about 11% at angular scale l < 10^4. Predicting the nonlinear power spectrum to a few percent accuracy for future surveys would therefore require large cosmological simulations that also have exquisite numerical resolution to model accurately the survivals of dark matter subhalos in the tidal fields of their hosts.

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arXiv2005

Effects of cooling and star formation on the baryon fractions in clusters

We study the effects of dissipation on the baryon fractions in clusters using high-resolution cosmological simulations of nine clusters that resolve formation of cluster galaxies. The simulations of each cluster are performed with the shock-capturing eulerian adaptive mesh refinement N-body+gasdynamics ART code with and without radiative cooling. We show that dissipation and associated galaxy formation increase the total baryon fractions within radii as large as the virial radius. The effect is the strongest within cluster cores, where the simulations with cooling have baryon fractions larger than the universal value, while the fraction of baryons in adiabatic simulations are smaller than universal. At larger radii (r >~ r_500) the cumulative baryon fractions in simulations with cooling are close to, while those in the adiabatic runs remain below than, the universal value. The gas fractions in simulations with dissipation are reduced by ~20-40% at r<0.3r_vir and ~10% at larger radii compared to the adiabatic runs, because a fraction of gas is converted into stars. There is an indication that gas fractions within different radii increase with increasing cluster mass as f_gas ~ M_vir^0.2. We find that the total baryon fraction within the virial radius does not evolve with time in both adiabatic simulations and in simulations with cooling. Finally, to evaluate systematic uncertainties in the baryon fraction in cosmological simulations we present a comparison of gas fractions in our adiabatic simulations to re-simulations of the same objects with the entropy-conserving SPH code Gadget. The cumulative gas fraction profiles in the two sets of simulations on average agree to <~3% at r/r_vir>0.2, but differ systematically by up to 10% at small radii.

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arXiv2005

The Anisotropic Distribution of Galactic Satellites

We present a study of the spatial distribution of subhalos in galactic dark matter halos using dissipationless cosmological simulations of the concordance LCDM model. We find that subhalos are distributed anisotropically and are preferentially located along the major axes of the triaxial mass distributions of their hosts. The Kolmogorov-Smirnov probability for drawing our simulated subhalo sample from an isotropic distribution is P_KS \simeq 1.5 \times 10^{-4}. An isotropic distribution of subhalos is thus not the correct null hypothesis for testing the CDM paradigm. The nearly planar distribution of observed Milky Way (MW) satellites is marginally consistent (probability \simeq 0.02) with being drawn randomly from the subhalo distribution in our simulations. Furthermore, if we select the subhalos likely to be luminous, we find a distribution that is consistent with the observed MW satellites. In fact, we show that subsamples of the subhalo population with a centrally-concentrated radial distribution, similar to that of the MW dwarfs, typically exhibit a comparable degree of planarity. We explore the origin of the observed subhalo anisotropy and conclude that it is likely due to (1) preferential accretion of subhalos along filaments, often closely aligned with the major axis of the host halo, and (2) evolution of satellite orbits within the prolate, triaxial potentials typical of CDM halos. Agreement between predictions and observations requires the major axis of the outer dark matter halo of the Milky Way to be nearly perpendicular to the disk. We discuss possible observational tests of such disk-halo alignment with current large galaxy surveys.

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arXiv2005

Formation of globular clusters in hierarchical cosmology

We study the formation of globular clusters (GCs) in a Milky Way-size galaxy using a high-resolution cosmological simulation. The clusters in our model form in the dense cores of supergiant molecular clouds in the gaseous disks of high-redshift galaxies. The properties of clusters are estimated using a physically-motivated subgrid model of the isothermal cloud collapse. The first clusters in the simulation form at z ~ 12, while we conjecture that the best conditions for GC formation appear to be at z ~ 3-5. Most clusters form in the progenitor galaxies of the virial mass >10^9 Msun and the total mass of the cluster population is strongly correlated with the mass of its host galaxy with a fraction ~2x10^-4 of the galactic baryons being in the form of GCs. In addition, the mass of the GC population and the maximum cluster mass in a given region strongly correlate with the local average star formation rate. We find that the mass, size, and metallicity distributions of the cluster population identified in the simulation are remarkably similar to the corresponding distributions of the Milky Way globulars. We find no clear mass-metallicity or age-metallicity correlations for the old clusters. The zero-age cluster mass function can be approximated by a power-law, dN/dM ~ M^-alpha, with alpha ~ 2, in agreement with the mass function of young stellar clusters in starbursting galaxies. However, the shape of the zero-age mass function may be better described by the high-mass tail of a lognormal distribution which peaks at ~10^3 Msun. We discuss in detail the origin, universality, and dynamical evolution of the globular cluster mass function. Our results indicate that globular clusters with properties similar to those of observed clusters can form naturally within young dense gaseous disks at z >~ 3 in the LCDM cosmology.

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arXiv2005

Supersonic Motions of Galaxies in Clusters

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.

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