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We measure the clustering of dark matter halos in a large set of collisionless cosmological simulations of the flat LCDM cosmology. Halos are identified using the spherical overdensity algorithm, which finds the mass around isolated peaks in the density field such that the mean density is D times the background. We calibrate fitting functions for the large-scale bias that are adaptable to any value of D we examine. We find a [?]6% scatter about our best-fit bias relation. Our fitting functions couple to the halo mass functions of Tinker et al. such that the bias of all dark matter is normalized to unity. We demonstrate that the bias of massive, rare halos is higher than that predicted in the modified ellipsoidal collapse model of Sheth et al. and approaches the predictions of the spherical collapse model for the rarest halos. Halo bias results based on friends-of-friends halos identified with linking length 0.2 are systematically lower than for halos with the canonical D = 200 overdensity by [?]10%. In contrast to our previous results on the mass function, we find that the universal bias function evolves very weakly with redshift, if at all. We use our numerical results, both for the mass function and the bias relation, to test the peak-background split model for halo bias. We find that the peak-background split achieves a reasonable agreement with the numerical results, but [?]20% residuals remain, both at high and low masses.

We obtained ESI/Keck rotation curves of 10 Mg II absorption-selected galaxies (0.3 [?] z [?] 1.0) for which we have WFPC-2/HST images and high-resolution HIRES/Keck and UVES/VLT quasar spectra of the Mg II absorption profiles. We perform a kinematic comparison of these galaxies and their associated halo Mg II absorption. For all 10 galaxies, the majority of the absorption velocities lie in the range of the observed galaxy rotation velocities. In 7/10 cases, the absorption velocities reside fully to one side of the galaxy systemic velocity and usually align with one arm of the rotation curve. In all cases, a constant rotating thick-disk model poorly reproduces the full spread of observed Mg II absorption velocities when reasonably realistic parameters are employed. In 2/10 cases, the galaxy kinematics, star formation surface densities, and absorption kinematics have a resemblance to those of high-redshift galaxies showing strong outflows. We find that Mg II absorption velocity spread and optical depth distribution may be dependent on galaxy inclination. To further aid in the spatial-kinematic relationships of the data, we apply quasar absorption-line techniques to a galaxy (vc = 180 km s-1) embedded in LCDM simulations. In the simulations, Mg II absorption selects metal-enriched "halo" gas out to [?]100 kpc from the galaxy, tidal streams, filaments, and small satellite galaxies. Within the limitations inherent in the simulations, the majority of the simulated Mg II absorption arises in the filaments and tidal streams and is infalling toward the galaxy with velocities between -200 km s-1 [?] vr [?] -180 km s-1. The Mg II absorption velocity offset distribution (relative to the simulated galaxy) spans [?]200 km s-1 with the lowest frequency of detecting Mg II at the galaxy systematic velocity.

Although supernova (SN) explosions and stellar winds happen at very small scales, they affect the interstellar medium (ISM) at galactic scales and regulate the formation of a whole galaxy. Previous attempts of mimicking these effects in simulations of galaxy formation use very simplified assumptions. We develop a much more realistic prescription for modeling the feedback, which minimizes any ad hoc subgrid physics. We start with developing high-resolution models of the ISM and formulate the conditions required for its realistic functionality: formation of multiphase medium with hot chimneys, superbubbles, cold molecular phase, and very slow consumption of gas. We find that this can be achieved only by doing what the real universe does: formation of dense (>10 H atoms cm-3), cold (T [?] 100 K) molecular phase, where the star formation happens, and which young stars disrup. Another important ingredient is the effect of runaway stars: massive binary stars ejected from molecular clouds when one of the companions becomes an SN. Those stars can move to 10-100 pc away from molecular clouds before exploding themselves as SNe. This greatly facilitates the feedback. Once those effects are implemented into cosmological simulations, galaxy formation proceeds more realistically. For example, we do not have the overcooling problem. The angular momentum problem (resulting in a too massive bulge) is also reduced substantially: the rotation curves are nearly flat. The galaxy formation also becomes more violent. Just as often observed in quasar absorption lines, there are substantial outflows from forming and active galaxies. At high redshifts we routinely find gas with few hundred km s-1 and occasionally 1000-2000 km s-1. The gas has high metallicity, which may exceed the solar metallicity. The temperature of the gas in the outflows and in chimneys can be very high: T = 107-108 K. The density profile of dark matter is still consistent with a cuspy profile. The simulations reproduce this picture only if the resolution is very high: better than 50 pc, which is 10 times better than the typical resolution in previous cosmological simulations. Our simulations of galaxy formation reach the resolution of 35 pc.

The motion of satellite galaxies around normal galaxies at distances 50-500 kpc provides a sensitive test for theories. We study the surface density and velocities of satellites around isolated red galaxies in the Sloan Digital Sky Survey. We find that the surface number density of satellites declines with the projected distance as a power law with the slope -1.5 to 2. The rms velocities gradually decline: observations exclude constant velocities at a [?]10s level. We show that observational data strongly favor the standard model; all three major statistics of satellites--the number-density profile, the line-of-sight velocity dispersion, and the distribution function of the velocities--agree remarkably well with the predictions of the standard cosmological model. Thus, the success of the standard model extends to scales (50-500) kpc, much lower than what was previously considered. Modified Newtonian dynamics (MOND) fails on these scales for models which assume any single power-law number-density profile of satellites and any constant velocity anisotropy by predicting nearly constant rms velocities of satellites. Satellite data can be fit by fine-tuned models, which require (1) specific non-power-law density profile, (2) very radial orbits at large distances (velocity anisotropy b = 0.60.7 at R = 200-300 kpc), and (3) 2-2.5 times more stellar mass than what is found in the galaxies. The external gravity force--a necessary component for MOND--makes the situation even worse. We argue that a combination of satellite data and observational constraints on stellar masses make these models very problematic.

We measure the mass function of dark matter halos in a large set of collisionless cosmological simulations of flat LCDM cosmology and investigate its evolution at z[?] 2. Halos are identified as isolated density peaks, and their masses are measured within a series of radii enclosing specific overdensities. We argue that these spherical overdensity masses are more directly linked to cluster observables than masses measured using the friends-of-friends algorithm (FOF), and are therefore preferable for accurate forecasts of halo abundances. Our simulation set allows us to calibrate the mass function at z = 0 for virial masses in the range 1011 h[?]1 M [?] M[?] 1015 h[?]1 M to [?]5%, improving on previous results by a factor of 2-3. We derive fitting functions for the halo mass function in this mass range for a wide range of overdensities, both at z = 0 and earlier epochs. Earlier studies have sought to calibrate a universal mass function, in the sense that the same functional form and parameters can be used for different cosmologies and redshifts when expressed in appropriate variables. In addition to our fitting formulae, our main finding is that the mass function cannot be represented by a universal function at this level or accuracy. The amplitude of the "universal" function decreases monotonically by [?]20%-50%, depending on the mass definition, from z = 0 to 2.5. We also find evidence for redshift evolution in the overall shape of the mass function.

Two well-studied dwarf galaxies, NGC 3109 and NGC 6822, present some of the strongest observational support for a flat core at the center of galactic dark matter (DM) halos. We use detailed, cosmologically motivated numerical models to investigate the systematic effects and the accuracy of recovering parameters of the galaxies. Some of our models match the observed structure of the two galaxies remarkably well. Our analysis shows that the rotation curves of these two galaxies are instead quite compatible with their DM halos having steep cuspy density profiles. The rotation curves in our models are measured using standard observational techniques, projecting velocities along the line of sight of an imaginary observer and performing a tilted-ring analysis. The models reproduce the rotation curves of both galaxies and the disk surface brightness profiles, as well as the profile of isophotal ellipticity and position angle. The models are centrally dominated by baryons; however, the DM component is globally dominant. The simulated disk mass is marginally consistent with a stellar mass-to-light ratio, in agreement with the observed colors and the detected gaseous mass. We show that noncircular motions, combined with gas pressure support and projection effects, result in a large underestimation of the circular velocity in the central ~1 kpc region, creating the illusion of a constant-density core. Although the systematic effects mentioned above are stronger in barred systems, they are also present in axisymmetric disks. Our results strongly suggest that there is no contradiction between the observed rotation curves in dwarf galaxies and the cuspy central DM density profiles predicted by cold dark matter models.

We measure the evolution in the virial mass-to-light ratio (M200/LB) and virial-to-stellar mass ratio (M200/M*) for isolated ~L* galaxies between z ~ 1 and z ~ 0 by combining data from the DEEP2 Galaxy Redshift Survey and the Sloan Digital Sky Survey. Utilizing the motions of satellite galaxies around isolated galaxies, we measure line-of-sight velocity dispersions and derive dark matter halo virial masses for these host galaxies. At both epochs the velocity dispersion of satellites correlates with host galaxy stellar mass, s [?] Mimg1.gif, while the relation between satellite velocity dispersion and host galaxy B-band luminosity may grow somewhat shallower from s [?] Limg2.gif at z ~ 1 to s [?] Limg3.gif at z ~ 0. The evolution in M200/M* from z ~ 1 to z ~ 0 displays a bimodality insofar as host galaxies with stellar mass below M* ~ 1011 h-1 M maintain a constant ratio (the intrinsic increase is constrained to a factor of 1.1 +- 0.5) while host galaxies above M* ~ 1011 h-1 M experience a factor of 3.3 +- 2.2 increase in their virial-to-stellar mass ratio. This result can be easily understood if galaxies below this stellar mass scale continue to form stars while star formation in galaxies above this scale is quenched and the dark matter halos of galaxies both above and below this scale grow in accordance with LCDM cosmological simulations. Host galaxies that are red in U - B color have larger satellite dispersions and hence reside on average in more massive halos than blue galaxies at both z ~ 1 and z ~ 0. The satellite population of host galaxies varies little between these epochs. The redshift and host galaxy stellar mass dependence of M200/M* agrees qualitatively with the Millennium Run semianalytic model of galaxy formation.

We use measurements of the stellar velocity dispersion profile of the Fornax dwarf spheroidal galaxy to derive constraints on its dark matter distribution. Although 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 of rcore [?] 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. Furthermore, 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 Lya forest power spectrum, rcore [?] 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 Vmax [?] 200 km s-1.

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 nonuniform 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, S(R) [?] Ra, in the volume-limited SDSS sample is a [?] -1.7 +- 0.1, independent of the galaxy and satellite luminosities. Comparison with LCDM 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 h-1 kpc. 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.

We study the density profiles of collapsed galaxy-size dark matter halos with masses 1011 to 5 x 1012 M focusing mostly on the halo outer regions from the formal virial radius Rvir up to 5Rvir-7Rvir. We find that isolated halos in this mass range extend well beyond Rvir exhibiting all properties of virialized objects up to 2Rvir-3Rvir: relatively smooth density profiles and no systematic infall velocities. The dark matter halos in this mass range do not grow as one naively may expect through a steady accretion of satellites; i.e., on average there is no mass infall. This is strikingly different from more massive halos, which have large infall velocities outside the virial radius. We provide an accurate fit for the density profile of these isolated galaxy-size halos. For a wide range 0.01Rvir-2Rvir of radii the halo density profiles are fitted with the approximation r = rs exp img1.gif + [?]rm[?], where x [?] r/rs, [?]rm[?] is the mean matter density of the universe, and the index n is in the range n = 6-7.5. These profiles do not show a sudden change of behavior beyond the virial radius. For larger radii we combine the statistics of the initial fluctuations with the spherical collapse model to obtain predictions for the mean and most probable density profiles for halos of several masses. The model gives excellent results beyond 2-3 formal virial radii for the most probable profile and qualitatively correct predictions for the mean profile.

We analyze the angular distribution and the orbital rotation directions of a sample of carefully selected satellite galaxies about disk galaxy primaries extracted from the Sloan Digital Sky Survey (SDSS). We complement this analysis with a theoretical study of these statistics in an N-body simulation of cosmological structure formation set within the LCDM paradigm under various assumptions for the orientations of disk angular momenta. Under the assumption that the angular momenta of the disks are aligned with the angular momenta of the inner regions of their host dark matter halos, we find that the fraction of simulated satellite halos that exhibit prograde motion is fprog [?] 0.55-0.60, with larger satellites more likely to be prograde. In our observational sample, approximately 60% of the satellites exhibit prograde motion, a result that is broadly consistent with the simulated sample. Our observational sample of satellite galaxies shows no evidence for an anisotropic satellite distribution relative to disk primaries; however, the small sample size does not yet support a statistically significant comparison to previous studies of satellite anisotropy. Again, this result is broadly consistent with our simulated sample of satellites under the assumption that disk and halo angular momenta are aligned. However, the small size of our observational sample does not yet allow us to distinguish between various assumptions regarding the orientations of disks in their halos. Finally, we present an assessment of the importance of contamination by interlopers on the measured prograde and retrograde statistics.

We present a study of the spatial distribution of dwarf satellites (or subhalos) in galactic dark matter halos using dissipationless cosmological simulations of the concordance flat cold dark matter (CDM) model with vacuum energy. 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 PKS [?] 1.5 x 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 [?]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 that is 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) the preferential accretion of satellites 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.

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. Our models include stellar disks, disks with bars, and small bulges. 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 that explain the systematic biases: inclination, small bulge, and bar. Inclination effects are due to the finite thickness of the disk, bar, or bulge. Admixture of a nonrotating bulge component reduces the rotational velocity. A small (200-500 pc) bulge may be overlooked, leading to systematic bias even on relatively large (~1 kpc) distances. In the case of a disk with a bar, the underestimate of the circular velocity is larger because of a combination of noncircular motions and random velocities. The effect of the bar depends on the angle that the bar makes with the line of sight. Signatures of bars can be difficult to detect in the surface brightness profiles of the model galaxies. The variations 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 km s-1 error, but the effects add up. In some cases the error in circular velocity was a factor of 2, but typically we get about a 20% effect. 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 the 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.

The cooling of gas in the centers of dark matter halos is expected to lead to a more concentrated dark matter distribution. The response of dark matter to the condensation of baryons is usually calculated using the model of adiabatic contraction, which assumes spherical symmetry and circular orbits. In contrast, halos in the hierarchical structure formation scenarios grow via multiple violent mergers and accretion along filaments, and particle orbits in the halos are highly eccentric. We study the effects of the cooling of gas in the inner regions of halos using high-resolution cosmological simulations that include gas dynamics, radiative cooling, and star formation. We find that the dissipation of gas indeed increases the density of dark matter and steepens its radial profile in the inner regions of halos compared to the case without cooling. For the first time, we test the adiabatic contraction model in cosmological simulations and find that the standard model systematically overpredicts the increase of dark matter density in the inner 5% of the virial radius. We show that the model can be improved by a simple modification of the assumed invariant from M(r)r to M(img1.gif)r, where r and img1.gif are the current and orbit-averaged particle positions. This modification approximately accounts for orbital eccentricities of particles and reproduces simulation profiles to within 10%-20%. We present analytical fitting functions that accurately describe the transformation of the dark matter profile in the modified model and can be used for interpretation of observations.

Hierarchical cold dark matter (CDM) models predict that Milky Way-sized halos contain several hundred dense low-mass dark matter satellites (the substructure), an order of magnitude more than the number of observed satellites in the Local Group. If the CDM paradigm is correct, this prediction implies that the Milky Way and Andromeda are filled with numerous dark halos. To understand why these halos failed to form stars and become galaxies, we need to understand their history. We analyze the dynamical evolution of the substructure halos in a high-resolution cosmological simulation of Milky Way-sized halos in the LCDM cosmology. We find that about 10% of the substructure halos with the present masses [?]108-109 M (circular velocities Vm [?] 30 km s-1) had considerably larger masses and circular velocities when they formed at redshifts z [?] 2. After the initial period of mass accretion in isolation, these objects experience dramatic mass loss because of tidal stripping. Our analysis shows that strong tidal interaction is often caused by actively merging massive neighboring halos, even before the satellites are accreted by their host halo. These results can explain how the smallest dwarf spheroidal galaxies of the Local Group were able to build up a sizable stellar mass in their seemingly shallow potential wells. We propose a new model in which all the luminous dwarf spheroidals in the Local Group are descendants of the relatively massive ([?]109 M) high-redshift systems, in which the gas could cool efficiently by atomic line emission, and which were not significantly affected by the extragalactic ultraviolet radiation. We present a simple galaxy formation model based on the trajectories extracted from the simulation, which accounts for the bursts of star formation after strong tidal shocks and the inefficiency of gas cooling in halos with virial temperatures Tvir [?] 104 K. Our model reproduces the abundance, spatial distribution, and morphological segregation of the observed Galactic satellites. The results are insensitive to the redshift of reionization.

We analyze the halo occupation distribution (HOD) 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 HOD, the probability distribution for a halo of mass M to host a number of subhalos N, is similar to that found in semianalytic and N-body+gasdynamics studies. Its first moment, [?]N[?]M, has a complicated shape consisting of a step, a shoulder, and a power-law high-mass tail. The HOD can be described by Poisson statistics at high halo masses but becomes sub-Poisson for [?]N[?]M [?] 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 steplike function, while the latter can be well approximated by a Poisson distribution, fully specified by its first moment. The first moment of the satellite HOD can be well described by a simple power law [?]Ns[?] [?] Mb 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 fitted by a power law down to ~100 h-1 kpc with an amplitude and slope similar to those of observed galaxies. The dependence of correlation amplitude on the number density of objects is in general agreement with results from the Sloan Digital Sky Survey. At redshifts z [?] 1, we find significant departures from the power-law shape of the CF at small scales, where the CF steepens because of a more pronounced one-halo component. The departures from the power law may thus be easier to detect in high-redshift galaxy surveys than at the present-day epoch. They can be used to put useful constraints on the environments and formation of galaxies. 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 dangerous and is likely to bias the estimates of the correlation length and slope of the correlation function.

We analyze the mass accretion histories (MAHs) and density profiles of cluster-size halos with virial masses of img1.gif x 1014 h-1 M in a flat LCDM cosmology. We find that most MAHs have a similar shape: an early merger-dominated mass increase followed by a more gradual accretion-dominated growth. For some clusters the intense merger activity and rapid mass growth continue until the present-day epoch. In agreement with previous studies, we find that the concentration of the density distribution is tightly correlated with the halo's MAH and with its formation redshift. During the period of fast mass growth, the concentration remains approximately constant and low, cv [?] 3-4, while during the slow accretion stages the concentration increases with decreasing redshift as cv [?] (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. At the same time, if a cluster is best fitted 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. Interestingly, for all the studied clusters the logarithmic slope down to the smallest resolved scale ([?]1% of the virial radius) becomes shallower with decreasing radius without reaching an asymptotic value. 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 r(r) [?] r-g with g ~ 1.5-2. The relatively shallow power-law slopes result in low concentrations at these stages of evolution, as the scale radius at which the density profiles reach the slope of -2 is at large radii. 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 the outer steeper profile is formed when the mass accretion slows down. To check the convergence and robustness of our conclusions, we resimulate one of our clusters by using 8 times as many particles and twice as good force resolution. We find good agreement between the two simulations in all the results discussed in our study.

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 deg2 on the sky, we detect about 3000 satellites with absolute blue magnitudes going down to MB = -14; most of the satellites have MB = -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 rms line-of-sight velocity changes from [?]120 km s-1 at 20 kpc to [?]60 km s-1 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 rDM [?] 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 modified Newtonian dynamics (MOND). We also find that the velocity dispersion of satellites within 100 kpc scales with the absolute magnitude of the central galaxy as s [?] L0.3; this is very close to the Tully-Fisher relation for normal spiral galaxies.

We present cosmological simulations that closely mimic the real uiverse within ~100 Mpc 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-Pisces Supercluster, and the Local Supercluster, in approximately correct locations. The MARK III survey of peculiar velocities of the observed structures inside an 80 h-1 Mpc sphere is used to constrain the initial conditions. Fourier modes on scales larger than a (Gaussian) resolution of [?]Rg = 5 h-1 Mpc are dominated by the constraints, while small-scale waves are essentially random. The main aim of this paper is the structure of the Local Supercluster region (LSC; ~30 h-1 Mpc around the Virgo Cluster) and the Local Group environment. We find that at the current epoch most of the mass ([?]7.5 x 1014 h-1 M) of the LSC is located in a filament roughly centered on the Virgo Cluster and extending over ~40 h-1 Mpc. 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 [?]250 km s-1 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. We find that the peculiar velocity flow in the vicinity of the LG in the simulation is relatively "cold": the peculiar line-of-sight velocity dispersion within 7 h-1 Mpc of the LG is [?]60 km s-1, comparable to the observed velocity dispersion of nearby galaxies.

We propose a new explanation for the origin of angular momentum in galaxies and their dark halos, in which the halos obtain their spin through the cumulative acquisition of angular momentum from satellite accretion. In our model, the buildup of angular momentum is a random walk process associated with the mass assembly history of the halo's major progenitor. We assume no correlation between the angular momenta of accreted objects. The main role of tidal torques in this approach is to produce the random tangential velocities of merging satellites. Using the extended Press-Schechter approximation, we calculate the growth of mass, angular momentum, and spin parameter l for many halos. Our random walk model reproduces the key features of the angular momentum of halos found in LCDM N-body simulations: a lognormal distribution in l with an average ofimg1.gif [?] 0.045 and dispersion sl = 0.56, independent of mass and redshift. The evolution of the spin parameter in individual halos in this model is quite different from the steady increase with time of angular momentum in the tidal torque picture. We find both in N-body simulations and in our random walk model that the value of l changes significantly with time for a halo's major progenitor. It typically has a sharp increase due to major mergers and a steady decline during periods of gradual accretion of small satellites. The model predicts that, on average, the l of ~1012 M halos that had major mergers after redshift z = 3 should be substantially larger than the l of those that did not. Perhaps surprisingly, this suggests that halos that host later forming elliptical galaxies should rotate faster than halos of spiral galaxies.

We apply standard disk formation theory with adiabatic contraction within cuspy halo models predicted by the standard cold dark matter (LCDM) cosmology. The resulting models are confronted with the broad range of observational data available for the Milky Way and M31 galaxies. We find that there is a narrow range of parameters that can satisfy the observational constraints, but within this range, the models score remarkably well. Our favored models have virial masses of 1012 and 1.6 x 1012 M for the Galaxy and for M31, respectively, average spin parameters l [?] 0.03-0.05, and concentrations Cvir = 10-17, typical for halos of this mass in the standard LCDM cosmology. The models require neither dark matter modifications nor flat cores to fit the observational data. We explore two types of models, with and without the exchange of angular momentum between the dark matter and the baryons. The models without exchange give reasonable rotation curves, fulfill constraints in the solar neighborhood, and satisfy constraints at larger radii, but they may be problematic for fast rotating central bars. We explore models in which the baryons experience additional contraction due to loss of angular momentum to the surrounding dark matter. These models produce similar global properties, but the dark matter is only a 25% of the total mass in the central 3 kpc region, allowing a fast rotating bar to persist. According to preliminary calculations, our model galaxies probably have sufficient baryonic mass in the central ~3.5 kpc to reproduce recent observational values of the optical depth to microlensing events toward the Galactic center. Our dynamical models unequivocally require that about 50% of all the gas inside the virial radius must not be in the disk or in the bulge, a result that is obtained naturally in standard semianalytic models. Assuming that the Milky Way is "typical," we investigate whether the range of virial masses allowed by our dynamical models is compatible with constraints from the galaxy luminosity function. We find that if the Milky Way has a luminosity MK = -24.0, then these constraints are satisfied, but if it is more luminous (as expected if it lies on the Tully-Fisher relation), then the predicted space density is larger than the observed space density of galaxies of the corresponding luminosity by a factor of 1.5-2. We conclude that observed rotation curves and dynamical properties of "normal" spiral galaxies appear to be consistent with standard LCDM.

We present results of gasdynamics+N-body constrained cosmological simulations of the Local Supercluster region (LSC; about 30 h-1 Mpc around the Virgo cluster), which closely mimic the real universe within 100 Mpc, by imposing constraints from the MARK III catalog of galaxy peculiar velocities. The simulations are used to study the properties and possible observational signatures of intergalactic medium in the LSC region. We find that in agreement with previous unconstrained simulations, [?]30% of the gas in this region is in the warm/hot phase at T ~ 105-107 K and [?]40% in the diffuse phase at T < 105 K in low-density regions. The X-ray emission from the warm/hot gas may represent a small (~5%-10%) but important contribution to the X-ray background observed by the ROSAT All-Sky Survey at energies around 1 keV. The best prospects for detection of the warm/hot intergalactic medium of the LSC located in filaments and in the vicinity of virialized regions of groups and clusters are through absorption in resonant lines of O VII and O VIII in soft X-rays and in the O VI doublet in UV. If intergalactic gas in filaments (r/[?]r[?] ~ 1-10) is enriched to typical metallicities of [?]0.05, the column densities of O VI, O VII, and O VIII along a random line of sight near the north Galactic pole, especially near the supergalactic plane, have a significant probability to be in the range detectable by current (FUSE, XMM) and future (Constellation-X) instruments.

We present results of a convergence study in which we compare the density profiles of cold dark matter halos simulated with varying mass and force resolutions. We show that although increasing the mass and force resolution allows one to probe deeper into the inner halo regions, the halo profiles converge on scales larger than the "effective" spatial resolution of the simulation. This resolution is defined by both the force softening and the mass resolution. On radii larger than the "effective" spatial resolution, density profiles do not experience any systematical trends when the number of particles or the force resolution increases further. In the simulations presented in this paper, we are able to probe the density profile of a relaxed isolated galaxy-size halo on scales r = (0.005-1)rvir. We find that the density distribution at resolved scales can be well approximated by the profile suggested by Moore and coworkers: r [?] x-1.5(1 + x1.5)-1, where x = r/rs and rs is the characteristic radius. The analytical profile proposed by Navarro and coworkers, r [?] 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. For this limit, both analytical profiles fit well because for high-concentration galaxy-size halos, the differences between these profiles become significant only on scales well below 0.01rvir. We also find that halos of similar mass may have density profiles with somewhat different parameters (characteristic radius, maximum rotation velocity, etc.) and shapes. We associate this scatter in properties with differences in halo merger histories and with the amount of substructure present in the analyzed halos.

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 this 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 [?] 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 twice as low as that 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 the implications of our results for galaxy evolution in different environments.

We use a high-resolution N-body simulation to study the velocity bias of dark matter halos, the difference in the velocity fields of dark matter and halos, in a flat low-density cold dark matter (LCDM) model. The high force, 2 h-1 kpc, and mass, 109 h-1 M, resolution allows dark matter halos to survive in very dense environments of groups and clusters, making it possible to use halos as galaxy tracers. We find that the velocity bias, bv,12, measured as a ratio of pairwise velocities of the halos to that of the dark matter, evolves with time and depends on scale. At high redshifts (z ~ 5), halos generally move faster than the dark matter on almost all scales: bv,12(r) [?] 1.2, r > 0.5 h-1 Mpc. At later moments, the bias decreases and gets below unity on scales less than r [?] 5 h-1 Mpc: bv,12(r) [?] (0.6-0.8) at z = 0. We find that the evolution of the pairwise velocity bias follows and probably is defined by the spatial antibias of the dark matter halos at small scales. The one-point velocity bias, bv, defined as the ratio of the rms velocities of halos and dark matter, provides a more direct measure of the difference in velocities, because it is less sensitive to the spatial bias. We analyze bv in clusters of galaxies and find that halos are "hotter" than the dark matter: bv = 1.2-1.3 for r = (0.2-0.8)rvir, where rvir is the virial radius. At larger radii, bv decreases and approaches unity at r = (1-2)rvir. We argue that dynamical friction may be responsible for this small positive velocity bias (bv > 1) found in the central parts of clusters. We do not find significant systematic difference in the velocity anisotropy of halos and the dark matter. The velocity anisotropy function, b, of dark matter particles can be approximated as b(x) = 0.15 + 2x/(x2 + 4), where the distance x is measured in units of the virial radius.

Abstract: We discuss the impact of the cosmological environment on the evolution of dark matter halos using a high-resolution simulation within a spatially flat λCDM cosmology.

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 108-109 M, respectively. We use the high-resolution Adaptive Refinement Tree (ART) N-body code to run cosmological simulations with particle mass [?]2 x 108 h-1 M and spatial resolution [?]1-2 h-1 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 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, which can be compared with those of observed galaxies. Particularly, we find that, on average, halos in groups have the same velocity dispersion as the dark matter particles; i.e., they do not exhibit significant velocity bias. The small-scale (100 kpc to 1 Mpc) halo correlation function in both models is well described by the power law x [?] r-1.7 and is in good agreement with observations. It is slightly antibiased (b[?]0.7-0.9) relative to the dark matter. To test other galaxy statistics, we use the maximum of the halo rotation velocity and the Tully-Fisher relation to assign luminosity to the halos. For two cosmological models, a flat model with the cosmological constant and O0=1-OL=0.3,h=0.7 and a model with a mixture of cold and hot dark matter and O0=1.0,On=0.2,h=0.5, we construct luminosity functions and evaluate mass-to-light ratios in groups. Both models produce luminosity functions and mass-to-light ratios ( ~200-400) that are in reasonable agreement with observations. The latter implies that the mass-to-light ratio in galaxy groups (at least for Mvir [?] 3 x 1013 h-1 M analyzed here) is not a good indicator of O0.

We study the evolution of the halo-halo correlation function and small-scale ([?]0.2-7 h-1 Mpc) bias in four cosmological models (LCDM, OCDM, tCDM, and SCDM) using very high resolution n-body simulations with a dynamical range of ~10,000-32,000 (force resolution of [?]2-4 h-1 kpc and particle mass of [?]109 h-1 M). The high force and mass resolution allows dark matter halos to survive in the tidal fields of high-density regions and thus prevents the ambiguities related with the "overmerging problem." This allows us to estimate for the first time the evolution of the correlation function and bias at small (down to ~100 h-1 kpc) scales. We find that at all epochs the two-point correlation function of galaxy-size halos xhh is well approximated by a power law with slope [?]1.6-1.8. The difference between the shape of xhh and the shape of the correlation function of matter results in the scale-dependent bias at scales [?]7 h-1 Mpc, which we find to be a generic prediction of the hierarchical models, independent of the epoch and of the model details. The bias evolves rapidly from a high value of ~2-5 at z ~ 3-7 to the antibias of b ~ 0.5-1 at small [?]5 h-1 Mpc scales at z = 0. Another generic prediction is that the comoving amplitude of the correlation function for halos above a certain mass evolves nonmonotonically: it decreases from an initially high value at z ~ 3-7, and very slowly increases at z [?] 1. We find that our results agree well with existing clustering data at different redshifts, indicating the general success of the hierarchical models of structure formation in which galaxies form inside the host DM halos. Particularly, we find an excellent agreement in both slope and the amplitude between xhh(z = 0) in our LCDM60 simulation and the galaxy correlation function measured using the Automatic Plate Measuring Facility galaxy survey. At high redshifts, the observed clustering of the Lyman-break galaxies is also well reproduced by the models. We find good agreement at z [?] 2 between our results and predictions of the analytical models of bias evolution. This indicates that we have a solid understanding of the nature of the bias and of the processes that drive its evolution at these epochs. We argue, however, that at lower redshifts the evolution of the bias is driven by dynamical processes inside the nonlinear high-density regions such as galaxy clusters and groups. These processes do not depend on cosmology and tend to erase the differences in clustering properties of halos that exist between cosmological models at high z.

According to the hierarchical clustering scenario, galaxies are assembled by merging and accretion of numerous satellites of different sizes and masses. This ongoing process is not 100% efficient in destroying all of the accreted satellites, as evidenced by the satellites of our Galaxy and of M31. Using published data, we have compiled the circular velocity (Vcirc) distribution function (VDF) of galaxy satellites in the Local Group. We find that within the volumes of radius of 570 kpc (400 h-1 kpc assuming the Hubble constant h = 0.7) centered on the Milky Way and Andromeda, the average VDF is roughly approximated as n(> Vcirc) [?] 55 +- 11(Vcirc/10 km s-1)-1.4+-0.4 h3 Mpc-3 for Vcirc in the range [?]10-70 km s-1. The observed VDF is compared with results of high-resolution cosmological simulations. We find that the VDF in models is very different from the observed one: n(> Vcirc) [?] 1200(Vcirc/10 km s-1)-2.75 h3 Mpc-3. Cosmological models thus predict that a halo the size of our Galaxy should have about 50 dark matter satellites with circular velocity greater than 20 km s-1 and mass greater than 3 x 108 M within a 570 kpc radius. This number is significantly higher than the approximately dozen satellites actually observed around our Galaxy. The difference is even larger if we consider the abundance of satellites in simulated galaxy groups similar to the Local Group. The models predict ~300 satellites inside a 1.5 Mpc radius, while only ~40 satellites are observed in the Local Group. The observed and predicted VDFs cross at [?]50 km s-1, indicating that the predicted abundance of satellites with Vcirc [?] 50 km s-1 is in reasonably good agreement with observations. We conclude, therefore, that unless a large fraction of the Local Group satellites has been missed in observations, there is a dramatic discrepancy between observations and hierarchical models, regardless of the model parameters. We discuss several possible explanations for this discrepancy including identification of some satellites with the high-velocity clouds observed in the Local Group and the existence of dark satellites that failed to accrete gas and form stars either because of the expulsion of gas in the supernovae-driven winds or because of gas heating by the intergalactic ionizing background.

We present results from a study of bias and its evolution for galaxy-size halos in a large, high-resolution simulation of a low-density, cold dark matter model with a cosmological constant. In addition to the previous studies of the halo two-point correlation function, we consider the evolution of bias estimated using two different statistics: power spectrum bP and a direct correlation of smoothed halo and matter overdensity fields bd. We present accurate estimates of the evolution of the matter power spectrum probed deep into the stable clustering regime [k ~ (0.1-200) h Mpc-1 at z = 0] and find that its shape and evolution can be well described, with only a minor modification, by the fitting formula of Peacock & Dodds. The halo power spectrum evolves much slower than the power spectrum of matter and has a different shape which indicates that the bias is time and scale dependent. At z = 0, the halo power spectrum is antibiased (bP < 1) with respect to the matter power spectrum at wavenumbers k ~ (0.15-30) h Mpc-1 and provides an excellent match to the power spectrum of the Automatic Plate Measuring Facility (APM) galaxies at all probed k. In particular, both the halo and matter power spectra show an inflection at k [?] 0.15 h Mpc-1, which corresponds to the present-day scale of nonlinearity and nicely matches the inflection observed in the APM power spectrum. We complement the power spectrum analysis with a direct estimate of bias using smoothed halo and matter overdensity fields and show that the evolution observed in the simulation in linear and mildly nonlinear regimes can be well described by the analytical model of Mo & White, if the distinction between formation redshift of halos and observation epoch is introduced into the model. We present arguments and evidence that at higher overdensities the evolution of bias is significantly affected by dynamical friction and tidal stripping operating on the satellite halos in high-density regions of clusters and groups; we attribute the strong antibias observed in the halo correlation function and power spectrum to these effects. The results of this study show that despite the apparent complexity, the origin and evolution of bias can be understood in terms of the processes that drive the formation and evolution of dark matter halos. These processes conspire to produce a halo distribution quite different from the overall distribution of matter, yet remarkably similar to the observed distribution of galaxies.

We use the rotation curves of a sample of dark matter-dominated dwarf and low surface brightness (LSB) late-type galaxies to study their radial mass distributions. We find that the shapes of the rotation curves are remarkably similar for all (both dwarf and LSB) galaxies in the sample, suggesting a self-similar density distribution of their dark matter (DM) halos. This shape can be reproduced well by a density profile with a shallow central cusp [r(r) [?] 1/rg, g [?] 0.2-0.4] corresponding to a steeply rising velocity curve [v(r) [?] rg, g [?] 0.9-0.8]. We further show that the observed shapes of the rotation curves are well matched by the average density profiles of dark matter halos formed in very high resolution simulations of the standard cold dark matter model (CDM), the low-density CDM model with cosmological constant (LCDM), and the cold + hot dark matter model with two types of neutrino (CHDM). This is surprising in light of several previous studies, which suggested that the structure of simulated dark matter halos is inconsistent with the dynamics of dwarf galaxies. We discuss possible explanations for this discrepancy and show that it is most likely caused by the systematic differences at small radii between the analytic model proposed by Navarro, Frenk, & White, with gNFW = 1, and the actual central density profiles of the dark matter halos. We also show that the mass distributions in the hierarchically formed halos are on average consistent with the shape of rotation curves of dark matter-dominated galaxies. However, the scatter of the individual profiles around the average is substantial. Finally, we show that the dark matter halos in our hierarchical simulations and the real galaxies in our sample exhibit a similar decrease in their characteristic densities with increasing characteristic radial scales and show increase in their maximum rotation velocities with increase in the radii at which their maximum velocities occur.

We apply several statistical estimators to high-resolution N-body simulations of two currently viable cosmological models: a mixed dark matter model, having On = 0.2 contributed by two massive neutrinos (C + 2nDM), and a cold dark matter model with cosmological constant (LCDM) with O0 = 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 PN, including the void probability function P0; and the underdensity probability function U[?] (where [?] fixes the underdensity threshold in percentage of the average). We find that P0 (for which PPS and CfA2 data agree) and P1 distinguish efficiently between the models, while U[?] is only marginally discriminatory. On the contrary, the reduced skewness and kurtosis are, respectively, S3 [?] 2.2 and S4 [?] 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 + 2nDM in all the tests performed. In contrast, the above LCDM model has serious difficulties in reproducing observational data if galaxies and matter overdensities are related in a simple way.

We present techniques for identifying and analyzing galaxy groups and apply them to large-scale particle-mesh (PM) N-body simulations of structure formation in three O0 = 1 cosmological models: cold plus hot dark matter (CHDM), with Ocold = 0.6, On = 0.3, and Obaryon = 0.1 at bias b [?] s [?]18=1.5; and two cold dark matter (CDM) models, at bias b = 1.5 and b = 1.0. Groups are identified with the adaptive friends-of-friends algorithm of Nolthenius. Our most important conclusions follow. The standard group M/L method gives O0 [?] 0.08 for the CfA1 survey (for redshift link parameter V5 = 350), and, applied to our O0 = 1 simulations, it gives O0 [?] 0.12 for CHDM (V5 = 350) and O0 [?] 0.35 for CDM (V5 = 600). This O bias appears to be even stronger at higher resolution. We show quantitatively how three different effects conspire to produce this large discrepancy, and we conclude that low observed O values need not argue for a low-O universe. Our preferred statistics of groups show promise in becoming powerful discriminators between Gaussian cosmological models, whose On differ and are robust against several methods for assigning luminosity to dark matter halos, and for merging CfA1 data. However, our latest results at higher resolution show such strong sensitivity to how massive overmergers are broken up that more reliable ways of identifying luminous galaxies within large-scale simulations will be necessary before these statistics can provide reliable discrimination. When overmergers are broken up, the median virial-to-DM mass Mvir/MDM of three-dimensional-selected groups is ~1 for all simulations. Groups with MDM > 1014 M appear virialized in all simulations. We measure global (not pairwise) velocity biases bv, similar to previous studies. Within three-dimensional-selected groups, CHDM and CDM with b = 1.5 show a stronger bias of bv = 0.7-0.8, while CDM with b = 1.0 shows groups of bv [?] 1.

We present a series of four simulations of cold dark matter (CDM) and cold + hot dark matter (CHDM) cosmologies, which we analyze together in this and subsequent papers. These dissipationless simulations were done using the particle mesh method with a 5123mesh, corresponding to a resolution of approximately 200 kpc for an assumed Hubble parameter of H0= 50 km s-1Mpc-1, and with approximately 17 × 106cold and (for CHDM) an additional 34 × 106hot particles. In this paper we discuss the power spectrum and correlation functions in real and redshift space, with comparisons to the CfA2 and IRASredshift data, the pairwise velocity of galaxies in real space, and the distribution of hot and cold particles in CHDM simulations. We confirm that CHDM with cold/hot/baryon density ratios ?c/??/?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 from our CHDM simulations agrees rather well with both the CfA2 and IRASpower spectra in both the nonlinear and linear regimes.

Semi‐analytic treatment of the power spectrum with the approximation of constant linear bias provides a way to compare cosmological models to a large amount of data, as Peacock &38; Dodds have shown. By applying the appropriate corrections to the observational power spectrum it is possible to recover the underlying linear power spectrum for any given cosmological model. Using extensive N‐body simulations we carefully test and calibrate all important corrections. To demonstrate that the method is applicable to a wide range of cosmological models, we test the standard Ω = 1 cold dark matter (CDM) model, Ω < 1 models that include a cosmological constant (ΛCDM), and Ω = 1 models with a mixture of cold and hot dark matter (CHDM), both with one massive neutrino and two equal mass neutrinos. The ΛCDM and CHDM models are normalized to COBEand to cluster abundances. Our tests indicate that the improved linear–non‐linear mapping recently suggested by Peacock &38; Dodds for treating CDM‐type power spectra works well for a wide range of scale‐dependent power spectra. However, we find that the Peacock &38; Dodds prescription for the recovery of the linear power spectrum from observations, which is often used to test cosmological models, can be misleading because the corrections are model‐dependent. A model should not be judged based on the linear spectrum recovered by that procedure, but must be compared with the recovered spectrum for that particular model. When we apply the proper corrections for a given model to the observational power spectrum, we find that no model in our test group recovers the linear power spectrum well for the bias values suggested by Peacock &38; Dodds between Abell, radio, optical, and IRAScatalogues: bA: bR: bO: bI= 4.5 : 1.9 : 1.3 : 1.0, with bI= 1.0. The recovered linear ΛCDM and CHDM power spectra were systematically below their respective linear power spectra using bI= 1.0. When we allow blto vary (keeping the same bias ratios) we find that: (i) CHDM models give very good fits to observations if optically selected galaxies are slightly biased (bO∼ 1.1). (ii) Most ΛCDM models give worse but acceptable fits if blue galaxies are considerably antibiased (0.6 ≲ bO≲ 0.9) and fail if optical galaxies are biased. (iii) There is a universal shape of the recovered linear power spectrum of all ΛCDM models over their entire range of explored wavenumbers, 0.01 ≲ k≲ 0.6 hMpc−1. For a given bias, recovered linear power spectra of CDM and CHDM models are nearly the same as that of ΛCDM in the region 0.01 ≲ k≲ 0.2 hMpc−1but diverge from this spectrum at higher k. We tabulate the recovered linear spectra, and also the initial linear spectra, for all the models considered.

We use the rotation curves of a sample of dark matter-dominated dwarf and low surface brightness (LSB) late-type galaxies to study their radial mass distributions. We find that the shapes of the rotation curves are remarkably similar for all (both dwarf and LSB) galaxies in the sample, suggesting a self-similar density distribution of their dark matter (DM) halos. This shape can be reproduced well by a density profile with a shallow central cusp [?(r) 1/r?, ? ? 0.2-0.4] corresponding to a steeply rising velocity curve [v(r) rg, g? 0.9-0.8]. We further show that the observed shapes of the rotation curves are well matched by the average density profiles of dark matter halos formed in very high resolution simulations of the standard cold dark matter model (CDM), the low-density CDM model with cosmological constant (?CDM), and the cold + hot dark matter model with two types of neutrino (CHDM). This is surprising in light of several previous studies, which suggested that the structure of simulated dark matter halos is inconsistent with the dynamics of dwarf galaxies. We discuss possible explanations for this discrepancy and show that it is most likely caused by the systematic differences at small radii between the analytic model proposed by Navarro, Frenk, & White, with ?NFW= 1, and the actual central density profiles of the dark matter halos. We also show that the mass distributions in the hierarchically formed halos are onaverageconsistent with the shape of rotation curves of dark matter-dominated galaxies. However, the scatter of the individual profiles around the average is substantial. Finally, we show that the dark matter halos in our hierarchical simulations and the real galaxies in our sample exhibit a similar decrease in their characteristic densities with increasing characteristic radial scales and show increase in their maximum rotation velocities with increase in the radii at which their maximum velocities occur.

We present two matched sets of five dissipationless simulations each, including four presently favoured minimal modifications to the standard cold dark matter (CDM) scenario. One simulation suite, with a linear box size of 75 h−1Mpc, is designed for high resolution and good statistics on the group/poor cluster scale, and the other, with a box size of 300 h−1Mpc, is designed for good rich cluster statistics. All runs had 57 million cold particles, and models with massive neutrinos (CHDM‐2ν) had an additional 113 million hot particles. We consider separately models with massive neutrinos, tilt, curvature, and a non‐zero cosmological constant (Λ ≡ 3H20ΩΛ) in addition to the standard CDM model. We find that the dark matter in each of our tilted Ω0+ΩΛ= 1 (TΛCDM) model with Ω0= 0.4, our tilted Ω01 model (TCDM), and our open Λ = 0 (OCDM) model with Ω = 0.5 has too much small‐scale power by a factor of ∼2, while CHDM‐2ν and SCDM are acceptable fits. In addition, we take advantage of the large dynamic range in detectable halo masses afforded by the combination of the two sets of simulations to test the Press–Schechter approximation. We find good fits at cluster masses for δc,g= 1.27–1.35 for a Gaussian filter and δc,t= 1.57 − 1.73 for a top hat filter. However, when we adjust δcto obtain a good fit at cluster mass scales, we find that the Press–Schechter model overpredicts the number density of haloes compared to the simulations by a weakly cosmology‐dependent factor of 1.5–2 at galaxy and group masses. It is impossible to obtain a good fit over the entire range of masses simulated by adjusting δcwithin reasonable bounds.

We use state-of-the-art scientific visualization techniques to explore cold + hot (CHDM) and cold dark matter (CDM) cosmologies and to contrast them to each other (in real and redshift space) and to the CfA1 data (in redshift space). We show dark matter halo locations and particle distributions at the present epoch, using animations and lighting or cutting planes to show three-dimensional structure; we also show isodensity contours for both cold and hot particles in one of the CHDM simulations and halo minimal spanning trees corresponding to halo grouping. Presenting the data visually provides valuable insights that can enhance analyses. Visually, CHDM exhibits considerable more filamentary structure and larger voids compared to CDM. Included in the accompanying videotape is an animation of the surprisingly rich dark matter substructure we discovered in the CHDM simulations.