Open Cold Dark Matter Models Research Articles

Constrained simulations of the local universe – I. Mass and motion in the local volume

It has been recently claimed that there is no correlation between the distribution of galaxies and their peculiar velocities within the local volume (LV), namely a sphere of R = 7 h −1 Mpc around the Local Group (LG). It has been then stated that this implies that either locally dark matter is not distributed in the same way as luminous matter or peculiar velocities are not due to fluctuations in mass. To test this statement, a set of constrained N-body cosmological simulations, designed to reproduce the main observed large-scale structure, have been analysed. The simulations were performed within the flat-� , open and flat matter only cold dark matter (CDM) cosmogonies. Two unconstrained simulations of the flat-� and open CDM models were performed for comparison. LG-like objects have been selected so as to mimic the real LG environment. The local gravitational field due to all haloes found within each LV is compared with the exact gravitational field induced by all matter in the simulation. We conclude that there is no correlation between the exact and local gravitational field obtained by pairwise Newtonian forces between haloes. Moreover, the local gravitational field is uncorrelated with the peculiar velocities of haloes. The exact gravitational field has a linear correlation with the peculiar velocities but the proportionality constant relating the velocity with gravitational field falls below the prediction of the linear theory. Upon considering all matter inside the LVs, the exact and local gravitational accelerations show a much better correlation, but with a considerable scatter independent of the cosmological models. The main conclusion is that the lack of correlation between the local gravitation and the peculiar velocity fields around LG-like objects is naturally expected in the CDM cosmologies.

Dark Energy and the Epoch of Galaxy Formation

The influence of a dark component on the first epoch of galaxy formation is analyzed by using the ages of the three oldest high-redshift galaxies known in the literature. Our results, based on a spatially flat accelerated universe driven by a component (px = ωρx), show that, if the inferred ages of these objects are correct, the first formation era is pushed back to extremely high redshifts. For the present best-fit quintessence model (Ωx = 0.7, ω < -0.6), we find a lower bound of zf ≥ 7.7, whereas in the extreme case of the Λ + cold dark matter model (ω = -1) the limit is slightly smaller (zf ≥ 5.8). The case for open cold dark matter models has also been discussed. For Ωm 0.3, the formation redshift is restricted by zf ≥ 18. As a general result, if Ωm ≥ 0.37, these galaxies are not formed in FRW cosmologies with no dark energy since for all these cases zf → ∞.

Properties of galaxy clusters: mass and correlation functions

We analyse parallel N-body simulations of three Cold Dark Matter (CDM) universes to study the abundance and clustering of galaxy clusters. The simulation boxes are 500h 1 Mpc on a side and cover a volume comparable to that of the forthcoming Sloan Digital Sky Survey. The use of a treecode algorithm and 47 million particles allows us at the same time to achieve high mass and force resolution. We are thus able to make robust measurements of cluster properties with good number statistics up to a redshift larger than unity. We extract halos using two independent, public domain group finders designed to identify virialised objects – ‘Friends-of-Friends’ (Davis et al. 1985) and ‘HOP’ (Eisenstein & Hut 1998) – and find consistent results. The correlation function of clusters as a function of mass in the simulations is in very good agreement with a simple analytic prescription based upon a Lagrangian biasing scheme developed by Mo & White (1996) and the Press-Schechter (PS) formalism for the mass function. The correlation length of clusters as a function of their number density, the R0–Dc relation, is in good agreement with the APM Cluster Survey in our open CDM model. The critical density CDM model (SCDM) shows much smaller correlation lengths than are observed. We also find that the correlation length does not grow as rapidly with cluster separation in any of the simulations as suggested by the analysis of very rich Abell clusters. Our SCDM simulation shows a robust deviation in the shape and evolution of the mass function when compared with that predicted by the PS formalism. Critical models with a low �8 normalization or small shape parameter have an excess of massive clusters compared with the PS prediction. When cluster normalized, the SCDM universe at z = 1 contains 10 times more clusters with temperatures greater than 7keV, compared with the Press & Schechter prediction. The agreement between the analytic and N-body mass functions can be improved, for clusters hotter than 3

Testing cold dark matter models using Hubble flow variations

COBE-normalized flat (matter plus cosmological constant) and open cold dark matter (CDM) models are tested by comparing their expected Hubble flow variations and the observed variations in a Type Ia supernova sample and a Tully--Fisher cluster sample. The test provides a probe of the CDM power spectrum on scales of 0.02h Mpc-1≲ k≲ 0.2h Mpc-1, free of the bias factor b. The results favour a low matter content universe, or a flat matter-dominated universe with a very low Hubble constant and/or a very small spectral index nps, with the best fits having Ο0∼ 0.3 to 0.4. The test is found to be more discriminative to the open CDM models than to the flat CDM models. For example, the test results are found to be compatible with those from the X-ray cluster abundance measurements at smaller length-scales, and consistent with the galaxy and cluster correlation analysis of Peacock & Dodds at similar length-scales, if our universe is flat; but the results are marginally incompatible with the X-ray cluster abundance measurements if our universe is open. The open CDM results are consistent with that of Peacock & Dodds only if the matter density of the universe is less than about 60 per cent of the critical density. The shortcoming of the test is discussed, so are ways to minimize it.

Cosmic Microwave Background Anisotropy Correlation Function and Topology from Simulated Maps forMAP

We have simulated cosmic microwave background (CMB) anisotropy maps for several COBE-Differential Microwave Radiometer experiment normalized cold dark matter (CDM) cosmogonies, to make predictions for the MAP experiment, an upcoming whole-sky CMB anisotropy space mission. We have studied the sensitivity of the simulated MAP data to cosmology, sky coverage, and instrumental noise. Given an accurate knowledge of instrumental noise, MAP data will ably discriminate among the cosmogonies considered, and superbly determine the topology of the initial fluctuations. A correlation function analysis of the simulated MAP data results in a very accurate measurement of the acoustic Hubble radius at decoupling. A low-density open CDM model with Ω0 = 0.4 can be distinguished from the Ω0 = 1 fiducial CDM model or a spatially flat CDM model with a cosmological constant and Ω0 = 0.4 with more than 99% confidence from the location of the acoustic valley in the correlation function. A genus analysis of the simulated MAP data indicates that in cosmogonies with Gaussian random-phase initial conditions, the horizontal shift of the zero-crossing point of the genus curve near the mean temperature threshold level ν = 0 should not exceed |Δν| = 0.01 (0.04) when the total effective FWHM smoothing is 03 (10). The asymmetry of the genus curve at the positive and negative threshold levels should not exceed |Δg/g(ν = ±1)| = 0.8% (4%) at 03 (1°) FWHM smoothing. Deviations of the observed MAP data in excess of these small values will be evidence for non-Gaussian behavior. The amplitude of the genus curve is a measure of the shape of the power spectrum at the smoothing scale. Even with the expected amount of instrumental noise and partial sky coverage (due to the Galaxy), the MAP data should allow discrimination among the cosmogonies considered at more than 99% confidence solely from a genus amplitude analysis.

Testing Cosmological Models against the Abundance of Damped Lyman‐Alpha Absorbers

We calculate the number of damped Lyman-alpha absorbers expected in various popular cosmological models as a function of redshift and compare our predictions with observed abundances. The Press-Schechter formalism is used to obtain the distribution of halos with circular velocity in different cosmologies, and we calibrate the relation between circular velocity and absorption cross-section using detailed gas dynamical simulations of a ``standard'' cold dark matter (CDM) model. Because of this calibration, our approach makes more realistic assumptions about the absorption properties of collapsed objects than previous, analytic calculations of the damped Lyman-alpha abundance. CDM models with Omega_0=1, H_0=50, baryon density Omega_b=0.05, and scale-invariant primeval fluctuations reproduce the observed incidence and redshift evolution of damped Lyman-alpha absorption to within observational uncertainty, for both COBE normalization (sigma_8=1.2) and a lower normalization (sigma_8=0.7) that better matches the observed cluster abundance at z=0. A tilted (n=0.8, sigma_8=0.7) CDM model tends to underproduce absorption, especially at z=4. With COBE normalization, a CDM model with Omega_0=0.4, Omega_{Lambda}=0.6 gives an acceptable fit to the observed absorption; an open CDM model is marginally acceptable if Omega_0 is at least 0.4 and strongly inconsistent with the z=4 data if Omega_0=0.3. Mixed dark matter models tend not to produce sufficient absorption, being roughly comparable to tilted CDM models if Omega_{nu} = 0.2 and failing drastically if Omega_{nu} = 0.3.

The Average Mass Profile of Galaxy Clusters

The average mass density profile measured in the Canadian Network for Observational Cosmology cluster survey is well described with the analytic form ρ(r) = Ar-1(r + aρ)-2, as advocated on the basis of n-body simulations by Navarro, Frenk, & White. The predicted core radii are aρ = 0.20 (in units of the radius where the mean interior density is 200 times the critical density) for an Ω = 0.2 open cold dark matter model and aρ = 0.26 for a flat Ω = 0.2 model, with little dependence on other cosmological parameters for simulations normalized to the observed cluster abundance. The dynamically derived local mass-to-light ratio, which has little radial variation, converts the observed light profile to a mass profile. We find that the scale radius of the mass distribution, 0.20 ≤ aρ ≤ 0.30 (depending on modeling details, with a 95% confidence range of 0.12-0.50), is completely consistent with the predicted values. Moreover, the profiles and total masses of the clusters as individuals can be acceptably predicted from the cluster rms line-of-sight velocity dispersion alone. This is strong support for the hierarchical clustering theory for the formation of galaxy clusters in a cool, collisionless, dark-matter-dominated universe.

Peculiar Velocities of Clusters in Cold Dark Matter Models

Recently, peculiar velocity measurements became available for a new sample of galaxy clusters, hereafter the SCI sample. From an accurately calibrated Tully-Fisher relation for spiral galaxies, we compute the rms peculiar velocity, Vrms, and compare it with the linear theory predictions of COBE-normalized low-density and open cold dark matter models (?CDM and OCDM, respectively). Confidence levels for model rejection are estimated using a Monte Carlo procedure in order to generate a large ensemble of artificial data sets for each model. Following Zaroubi et al., we express our results in terms of constraints on the (?0, npr, h) parameter space. Such constraints turn into ?8? -->00.6 = 0.50 -->?0.14+0.25 at the 90% confidence level, thus in agreement with results from cluster abundance. We show that our constraints are also consistent with those implied by the shape of the galaxy power spectrum within a rather wide range for the values of the model parameters. Finally, we point out that our findings disagree at about the 3 ? level with respect to those by Zaroubi et al., based on the Mark III catalog, which tend to prefer larger ?0 values within the CDM class of models.

Peaks in the Cosmic Microwave Background: Flat versus Open Models

We present properties of the peaks (maxima) of the microwave background anisotropies expected in flat and open cold dark matter models. We obtain analytical expressions of several topological descriptors: mean number of maxima and the probability distribution of the Gaussian curvature and the eccentricity of the peaks. These quantities are calculated as functions of the radiation power spectrum, assuming a Gaussian distribution of temperature anisotropies. We present results for angular resolutions ranging from 5' to 20' (antenna FWHM), scales that are relevant for the MAP and COBRAS/SAMBA space missions and the ground-based interferometer experiments. Our analysis also includes the effects of noise. We find that the number of peaks can discriminate between standard cold dark matter models and that the Gaussian curvature distribution provides a useful test for these various models, whereas the eccentricity distribution cannot distinguish between them.

The Power Spectrum of Galaxy Clustering in the Las Campanas Redshift Survey

The Las Campanas Redshift Survey (LCRS) contains 23, 697 galaxies, with an average redshift z = 0.1, distributed over six 15 × 80° slices in the north and south galactic caps. We have computed the power spectrum P(k) for magnitude-limited samples of LCRS galaxies over wavelengths λ = 2π/k = 5–400 h−1 Mpc. The LCRS P(k) may be approximated as ∝ k−1.8±0.1 for small scales λ = 5–30 h−1 Mpc, changing to ∝k1 ± 1 for large scales λ ≈ 200-400 h−1 Mpc. The overall amplitude corresponds to σ8 = 1.0 ± 0.1 in redshift space. Comparisons to the power spectra of other redshift surveys will be presented; the LCRS results agree best with those from the combined Center for Astrophysics (CfA2) and Southern Sky redshift surveys (SSRS2). For λ 100 h−1 Mpc, the LCRS results are consistent with those of other surveys, given the large errors among all the surveys on these scales. For λ 100 h−1 Mpc, the LCRS P(k) is well determined and similar in shape to the P(k) of other surveys, but with an amplitude differing from some of the other samples, possibly because of inherent clustering differences among different types of galaxies. In particular, power spectrum measurements for volume-limited LCRS samples show that galaxies brighter than about M* − 1 appear about 50% more strongly clustered than those fainter. Also, a sample of LCRS emission galaxies shows 30% weaker clustering than the full LCRS sample. Comparisons to N-body models show that the LCRS power spectrum lies intermediate between that of a standard flat Ω0h = 0.5 cold dark matter (CDM) model and an open Ω0h = 0.2 model, both normalized to σ8 = 1 for galaxies. On large scales λ 40 h−1 Mpc, we have fit the LCRS results to various linear CDM models, and find that a number of them could meet the constraints set by the LCRS power spectrum, the Hubble constant range 0.5 h 0.8, the abundance of galaxy clusters, and the reasonable assumption that LCRS galaxies are roughly unbiased tracers of the mass, relative to the normalization provided by the 4 year COBE DMR data. The possibilities include open CDM or flat nonzero cosmological-constant CDM models with Ω0 ≈ 0.4–0.6 and shape parameter Γ ≈ Ω0h ≈ 0.2–0.3, as well as flat Ω0 = 1 models with massive neutrino density Ωv ≈ 0.2–0.3 or a spectral tilt n ≈ 0.7–0.8.

Observational Tests of One-Bubble Open Inflationary Cosmological Models

Motivated by recent studies of the one-bubble inflationary scenario, simple open cold dark matter models are tested for consistency with cosmological observations. The initial perturbation spectrum is derived by solving for the evolution of fluctuations in an open inflationary stage. A likelihood analysis is performed for the Cosmic Microwave Background anisotropies using the two-year COBE DMR data and considering models based on both the Bunch-Davies and conformal vacua. Having normalized the perturbation spectrum to fit the COBE data, we reconsider the validity of the open model from the view point of cosmic structure formation. Open models may be severely constrained by the COBE likelihood analysis. In particular, small values of $\Omega_0$ are ruled out in the Bunch-Davies case: we find that $\Omega_0\ge 0.34$ at 95\% confidence for this model.

Voids and constraints on nonlinear clustering of galaxies

Void statistics of the galaxy distribution in the Center for Astrophysics Redshift Survey provide strong constraints on galaxy clustering in the nonlinear regime, i.e., on scales R equal to or less than 10/h Mpc. Computation of high-order moments of the galaxy distribution requires a sample that (1) densely traces the large-scale structure and (2) covers sufficient volume to obtain good statistics. The CfA redshift survey densely samples structure on scales equal to or less than 10/h Mpc and has sufficient depth and angular coverage to approach a fair sample on these scales. In the nonlinear regime, the void probability function (VPF) for CfA samples exhibits apparent agreement with hierarchical scaling (such scaling implies that the N-point correlation functions for N greater than 2 depend only on pairwise products of the two-point function xi(r)) However, simulations of cosmological models show that this scaling in redshift space does not necessarily imply such scaling in real space, even in the nonlinear regime; peculiar velocities cause distortions which can yield erroneous agreement with hierarchical scaling. The underdensity probability measures the frequency of 'voids' with density rho less than 0.2 -/rho. This statistic reveals a paucity of very bright galaxies (L greater than L asterisk) in the 'voids.' Underdensities are equal to or greater than 2 sigma more frequent in bright galaxy samples than in samples that include fainter galaxies. Comparison of void statistics of CfA samples with simulations of a range of cosmological models favors models with Gaussian primordial fluctuations and Cold Dark Matter (CDM)-like initial power spectra. Biased models tend to produce voids that are too empty. We also compare these data with three specific models of the Cold Dark Matter cosmogony: an unbiased, open universe CDM model (omega = 0.4, h = 0.5) provides a good match to the VPF of the CfA samples. Biasing of the galaxy distribution in the 'standard' CDM model (omega = 1, b = 1.5; see below for definitions) and nonzero cosmological constant CDM model (omega = 0.4, h = 0.6 lambda(sub 0) = 0.6, b = 1.3) produce voids that are too empty. All three simulations match the observed VPF and underdensity probability for samples of very bright (M less than M asterisk = -19.2) galaxies, but produce voids that are too empty when compared with samples that include fainter galaxies.

AN OBSERVATIONAL STUDY OF GALAXIES AND THEIR ENVIRONMENT ON LARGE SCALES

ABSTRACT The Las Campanas Deep Redshift Survey (LCDRS) is a survey of the galaxy distribution, out to a redshift of 0.2, which will eventually cover ~1000 square degrees over the Northern and Southern Galactic Caps in alternating 1.5 degrees-thick slices. The goals of the LCDRS are to sample a large enough volume of the nearby Universe in order to constrain the scale of the largest features in the galaxy distribution, and to use this sample for the accurate measurement of galaxy clustering on a wide variety of scales. Upon completion, 25,000 - 30,000 galaxy redshifts will have been compiled. This dissertation describes results obtained by the analysis of the first ~5,400 reduced redshifts. Three main topics are discussed. First, the distribution of galaxies in the LCDRS is examined. Although visual inspection of the slices indicates an upper limit of ~100h-1 Mpc to the scale of high-contrast features, quantitative study of the galaxy distribution suggests the presence of low-contrast density gradients on scales comparable to the LCDRS volume itself. The galaxy-galaxy spatial auto-correlation function is also determined; it shows more power on large scales than is compatible for a standard Cold Dark Matter (CDM) model, although it is consistent with results expected from an open CDM model. Second, a group catalog is extracted from the nearly complete LCDRS -6 degree Slice, and its properties are compared with those of other group and cluster catalogs. A group-group spatial auto-correlation function is determined, and its amplitude is compared with the correlation amplitudes for other galaxy, group, and cluster catalogs. Finally, galaxy segregation is studied as a function of color. Overall, evidence from both visual impressions and quantitative tests points toward color segregation only on scales on the order of or less than the width of a typical void wall (<~10-20 h-1 Mpc). Arguments are made for galaxy evolution as a major driver of color-segregation, with primordial effects playing at most a minor role in the observed color-environment effects.

Topological analysis of the CfA redshift survey

view Abstract Citations (108) References (49) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Topological Analysis of the CfA Redshift Survey Vogeley, Michael S. ; Park, Changbom ; Geller, Margaret J. ; Huchra, John P. ; Gott, J. Richard, III Abstract We study the topology of large-scale structure in the Center for Astrophysics Redshift Survey, which now includes ~12,000 galaxies with limiting magnitude m_B_ <= 15.5. The dense sampling and large volume of this survey allow us to compute the topology on smoothing scales from 6 to 20 h^-1^ Mpc; we thus examine the topology of structure in both the "nonlinear" and "linear" regimes. On smoothing scales <= 10 h^-1^ Mpc this sample has 3 times the number of resolution elements of samples examined in previous studies. Isodensity surfaces of the smoothed galaxy density field demonstrate that coherent high-density structures and large voids dominate the galaxy distribution. We compute the genus-threshold density relation for isodensity surfaces of the CfA survey. To quantify phase correlation in these data, we compare the CfA genus with the genus of realizations of Gaussian random fields with the power spectrum measured for the CfA survey. On scales <= 10 h^-1^ Mpc the observed genus amplitude is smaller than random phase (96% confidence level). This decrement reflects the degree of phase coherence in the observed galaxy distribution. In other words the genus amplitude on these scales is not a good measure of the power spectrum slope. On scales greater than 10 h^-1^ Mpc, where the galaxy distribution is roughly in the "linear" regime, the genus amplitude is consistent with the random phase amplitude. The shape of the genus curve reflects the strong coherence in the observed structure; the observed genus curve appears broader than random phase (94% confidence level for smoothing scales <= 10 h^-1^ Mpc) because the topology is spongelike over a very large range of density threshold. This departure from random phase is consistent with a distribution like a filamentary net or "walls with holes." On smoothing scales approaching ~20 h^-1^ Mpc the shape of the CfA genus curve is consistent with random phase. There is very weak evidence for a shift of the genus toward a "bubble-like" topology. To test cosmological models, we compute the genus for mock CfA surveys drawn from large (L ~> 400 h^-1^ Mpc) N-body simulations of three variants of the cold dark matter (CDM) cosmogony. The genus amplitude of the "standard" CDM model ({OMEGA}h = 0.5, b = 1.5) differs from the observations (96% confidence level) on smoothing scales <~ 10 h^-1^ Mpc. An open CDM model ({OMEGA}h = 0.2) and a CDM model with nonzero cosmological constant ({OMEGA}h = 0.24, λ_0_ = 0.6) are consistent with the observed genus amplitude over the full range of smoothing scales. All of these models fail (97% confidence level) to match the broadness of the observed genus curve on smoothing scales <= 10 h^-1^ Mpc. Publication: The Astrophysical Journal Pub Date: January 1994 DOI: 10.1086/173583 Bibcode: 1994ApJ...420..525V Keywords: Astronomical Models; Cosmology; Galactic Clusters; Red Shift; Topology; Dark Matter; Many Body Problem; Star Distribution; Astrophysics; COSMOLOGY: OBSERVATIONS; GALAXIES: CLUSTERING; GALAXIES: DISTANCES AND REDSHIFTS; SURVEYS full text sources ADS |

Cluster correlations in the Zel'dovich approximation

We show how to simulate the clustering of rich clusters of galaxies using a technique based on the Zel'dovich approximation. This method well reproduces the spatial distribution of clusters obtainable from full N-body simulations at a fraction of the computational cost. We use an ensemble of large--scale simulations to assess the level and statistical significance of cluster clustering in open, tilted and flat versions of the Cold Dark Matter (CDM) model, as well as a model comprising a mixture of Cold and Hot Dark Matter (CHDM). We find the open and flat CDM models are excluded by the data. The tilted CDM model with a slight tilt is in marginal agreement, while larger tilt produces the right amount of clustering; CHDM is the best of all our models at reproducing the observations of cluster clustering. We find that {\em all} our models display a systematically weaker relationship between clustering length and mean cluster separation than seems to be implied by observations. We also note that the cluster bias factor, is not constant in any of the models, showing that one needs to be very careful when relating cluster clustering statistics to primordial density fluctuations.

Large N-body simulations of a universe dominated by cold dark matter

We show that pure gravitational evolution from an initial random density field with the standard cold dark matter (CDM) power spectrum does occasionally lead to filaments, walls and voids remarkably like those seen in observed samples. A problem with the flat CDM model is the degree of streaming of galaxies relative to their velocity dispersion. A biased open CDM model seems to be more compatible with the observational data though it requires a higher amplitude of microwave background fluctuations.