Standard Cold Dark Matter Universe Research Articles

Value of H(0) in the inhomogeneous universe.

Local measurements of the Hubble expansion rate are affected by structures like galaxy clusters or voids. Here we present a fully relativistic treatment of this effect, studying how clustering modifies the mean distance- (modulus-)redshift relation and its dispersion in a standard cold dark matter universe with a cosmological constant. The best estimates of the local expansion rate stem from supernova observations at small redshifts (0.01<z<0.1). It is interesting to compare these local measurements with global fits to data from cosmic microwave background anisotropies. In particular, we argue that cosmic variance (i.e., the effects of the local structure) is of the same order of magnitude as the current observational errors and must be taken into account in local measurements of the Hubble expansion rate.

Simulating the mass-metallicity relation from z $\mathsf{\sim}$ 1

We use 112 N-body/hydrodynamical simulations in the standard Cold Dark Matter universe, to follow the formation of galaxy-sized halos and investigate the chemical enrichment of both the stellar component and the interstellar medium of galaxies, with stellar masses larger than 1e9 Msun. The resulting chemical properties of the simulated galaxies are broadly consistent with the observations. The predicted relationship between the mean metallicity and the galaxy stellar mass for both the stellar and the gaseous components at z=0 are in agreement with the relationships observed locally. The predicted scatter about these relationships, which is traced to the differing merging histories amongst the simulated galaxies with similar final masses, is similar to that observed. The predicted correlations between the total mass and the stellar mass of galaxies in our simulated sample from the present epoch up to z=1 agree with observed ones. The stellar mass versus metallicity relation and its associated scatter are reproduced by the simulations as consequences of the increasing efficiency of the conversion of gas into stars with stellar mass, and the differing merging histories amongst the galaxies with similar masses. The old ages of simulated low mass galaxies at z=0, and the weak level of chemical evolution for massive galaxies suggest however that our modeling of the supernova feedback may be incomplete, or that other feedback processes have been neglected. (Abridged)

Galaxy Number Counts in the Subaru Deep Field: Multiband Analysis in a Hierarchical Galaxy Formation Model

Number counts of galaxies are reanalyzed using a semianalytic model (SAM) of galaxy formation based on the hierarchical clustering scenario. Faint galaxies in the Subaru Deep Field (SDF; near-infrared J and K') and the Hubble Deep Field (HDF; ultraviolet/optical U, B, V, and I) are compared with our model galaxies. We have determined the astrophysical parameters in the SAM that reproduce observations of nearby galaxies and used them to predict the number counts and redshifts of faint galaxies for three cosmological models, the standard cold dark matter (CDM) universe, a low-density flat universe with nonzero cosmological constant, and a low-density open universe with zero cosmological constant. The novelty of our SAM analysis is the inclusion of selection effects arising from the cosmological dimming of surface brightness of high-redshift galaxies, and from the absorption of visible light by internal dust and intergalactic H I clouds. As was found in our previous work, in which the ultraviolet/optical HDF galaxies were compared with our model galaxies, we find that our SAM reproduces counts of near-infrared SDF galaxies in a low-density universe either with or without a cosmological constant, and that the standard CDM universe is not preferred, as suggested by other recent studies. Moreover, we find that simple prescriptions for (1) the timescale of star formation being proportional to the dynamical timescale of the formation of galactic disks, (2) the size of galactic disks being rotationally supported with the same specific angular momentum as that of surrounding dark halo, and (3) the dust optical depth being proportional to the metallicity of cold gas, cannot completely explain all of the observed data. Improved prescriptions incorporating mild redshift-dependence for those are suggested from our SAM analysis.

Galaxy Number Counts in the Hubble Deep Field as a Strong Constraint on a Hierarchical Galaxy Formation Model

Number counts of galaxies are reanalyzed using a semianalytic model (SAM) of galaxy formation based on the hierarchical clustering scenario. We have determined the astrophysical parameters in the SAM that reproduce observations of nearby galaxies and used them to predict the number counts and redshifts of faint galaxies for three cosmological models: (1) the standard cold dark matter (CDM) universe, (2) a low-density flat universe with nonzero cosmological constant, and (3) a low-density open universe with zero cosmological constant. The novelty of our SAM analysis is the inclusion of selection effects arising from the cosmological dimming of the surface brightness of high-redshift galaxies and also from the absorption of visible light by internal dust and intergalactic H I clouds. Contrary to previous SAM analyses that do not take into account such selection effects, we find from comparison with observed counts and redshifts of faint galaxies in the Hubble Deep Field (HDF) that the standard CDM universe is not preferred, and a low-density universe either with or without a cosmological constant is favorable, as suggested by other recent studies. Moreover, we find that a simple prescription for the timescale of star formation (SF), being proportional to the dynamical timescale of the formation of the galactic disk, is unable to reproduce the observed number-redshift relation for HDF galaxies, and that the SF timescale should be nearly independent of redshift, as suggested by other SAM analyses for the formation of quasars and the evolution of damped Lyα systems.

The Evolution Of The Star Formation Rate In Galaxies

The aim of this work is to provide clues for the understanding of the star formation history of the universe. We study the process of star formation in galactic objects in hierarchical clustering scenarios using hydrodynamical cosmological simulations of standard Cold Dark Matter Universe. We estimate the specific star formation rate (sSFR) of galactic haloes, which is defined as the SFR per unit luminosity; their luminosities are obtained from the evolutionary synthesis population models of Charlot and Bruzual (1991). To confront the results of the simulations with observations, we consider the sample of field galaxies at high redshift up to z = 1.3 of the Canada-France Redshift Survey (CFRS, Hammer et al.,1997). The sSFR for these galaxies were estimated from their spectral properties. From the confrontation of models and observations, we see that galactic objects formed in hierarchical clustering scenarios seem to reproduce the specific SFR observed at intermediate (0.35 < z < 0.7) and high (0.7 < z < 1.3) redshifts in the CFRS and also their dependence with mass and redshift. Our results suggest that in both simulations and observations, the mechanisms controlling the transformation of gas into stars is more effective at high redshift and in smaller objects producing larger sSFR in low lµ-luminosity galaxies. We find that the sSFR depends on the redshift (Cora et al.,2000, in prep.). Starbursts tidally induced during mergers might be responsible for this trend.

Semianalytical Models for the Formation of Disk Galaxies. I. Constraints from the Tully‐Fisher Relation

We present new semianalytical models for the formation of disk galaxies with the purpose of investigating the origin of the near-infrared Tully-Fisher (TF) relation. The models assume that disks are formed by cooling of the baryons inside dark halos with realistic density profiles and that the baryons conserve their specific angular momentum. Adiabatic contraction of the dark halo is taken into account, as well as a recipe for bulge formation based on a self-regulating mechanism that ensures disks to be stable. Only gas with densities above the critical density given by Toomre's stability criterion is considered eligible for star formation. A Schmidt law is assumed to prescribe the rate at which this gas is transformed into stars. The introduction of the star formation threshold density proves an essential ingredient of our models and yields gas mass fractions that are in excellent agreement with observations. Finally, a simple recipe for supernovae feedback is included. We emphasize the importance of extracting the proper luminosity and velocity measures from the models, something that has often been ignored in the past. We use the zero point of the K-band TF relation to place stringent constraints on cosmological parameters. In particular, we rule out a standard cold dark matter universe, in which disk galaxies are too faint to be consistent with observations. The TF zero point, in combination with nucleosynthesis constraints on the baryon density, and with constraints on the normalization of the power spectrum, requires a matter density Ω0 0.3. The observed K-band TF relation has a slope that is steeper than simple predictions based on dynamical arguments suggest. Taking the stability related star formation threshold densities into account steepens the TF relation and decreases its scatter. However, in order for the slope to be as steep as observed, further physics are required. We argue that the characteristics of the observed near-infrared TF relation do not reflect systematic variations in stellar populations, or cosmological initial conditions. In fact, feedback seems an essential ingredient in order to explain the observed slope of the K-band TF relation. Finally, we show that our models provide a natural explanation for the small amount of scatter that makes the TF relation useful as a cosmological distance indicator.

A Comparison of Simple Mass Estimators for Galaxy Clusters

High-resolution N-body simulations are used to investigate systematic trends in the mass profiles and total masses of clusters as derived from three simple estimators: (1) the weak gravitational lensing shear field, under the assumption of an isothermal cluster potential; (2) the dynamical mass obtained from the measured velocity dispersion, under the assumption of an isothermal cluster potential; and (3) the classical virial estimator. The clusters used for the analysis were obtained from simulations of a standard cold dark matter universe at z = 0.5 and consist of order ? 105 particles of mass mp 1010 M?. The clusters are not smooth and spherically symmetric but, rather, the mass distribution is triaxial, and significant substructure exists within the clusters. Not surprisingly, the level of agreement between the cluster mass profiles obtained from the various estimators and the actual mass profiles is found to be scale-dependent. We define the total cluster mass to be the mass contained within a three-dimensional radius r200 of the cluster centers, where r200 is the radius inside which the mean interior overdensity is equal to 200. Under this definition the classical virial estimator yields a good measurement of the total cluster mass, although it is systematically underestimated by ~10%. This result suggests that, at least in the limit of ideal data, the virial estimator is quite robust to deviations from pure spherical symmetry and the presence of substructure within a cluster. The dynamical mass estimate based upon a measurement of the cluster velocity dispersion and an assumption of an isothermal potential yields a poor measurement of the total cluster mass, underestimating it by ~40% for the case in which ?v is computed from an average over the entire cluster. The weak lensing estimate yields a very good measurement of the total cluster mass, provided that the mean shear used to determine the equivalent cluster velocity dispersion is computed from an average of the lensing signal over the entire cluster (i.e., the mean shear is computed interior to a projected cluster radius of R200).

P3M-SPH simulations of the Lyα forest

We investigate the importance of several numerical artefacts such as lack of resolution on spectral properties of the Lyα forest as computed from cosmological hydrodynamic simulations in a standard cold dark matter universe. We use a new simulation code which is based on a combination of a hierarchical particle–particle–particle–mesh (P3M) scheme for gravity and smoothed particle hydrodynamics (SPH) for gas dynamics. We have performed extensive comparisons between this new code and a modified version of the HYDRA code of Couchman et al. and find excellent agreement. We have also rerun the TREESPH simulations of Hernquist et al. using our new codes and find very good agreement with their published results. This shows that results from hydrodynamical simulations that include cooling are reproducible with different numerical algorithms. We then use our new code to investigate several numerical effects, such as resolution, on spectral statistics deduced from Voigt profile fitting of lines by running simulations with gas particle masses of 1.4 × 108, 1.8 × 107, 2.2 × 106 and 2.1 × 105 M⊙. When we increase the numerical resolution the mean effective hydrogen optical depth converges and so does the column density distribution. However, higher resolution simulations produce narrower lines and consequently the b parameter (velocity width) distribution has only marginally converged in our highest resolution run. Obtaining numerical convergence for the mean He II transmission is demanding. When progressively smaller haloes are resolved at better resolution, a larger fraction of low-density gas contracts to moderate overdensities in which He ii is already optically thick, and this increases the net transmission, making it difficult to simulate He ii reliably. Our highest resolution simulation gives a mean effective optical depth in He ii 5 per cent lower than the simulation with eight times lower mass resolution, illustrating the degree to which the He ii optical depth has converged. In contrast, the hydrogen mean optical depth for these runs is identical. As many properties of the simulated Lyα forest depend on resolution, one should be careful when deducing physical parameters from a comparison of the simulated forest with the observed one. We compare predictions from our highest resolution simulation in a cold dark matter universe, with a photoionizing background inferred from quasars as computed by Haardt &38; Madau, with observations. The simulation reproduces both the H I column density and b parameter distribution when we assume a high baryon density, ΩB h2 ≳ 0.028. In addition we need to impose a higher intergalactic medium (IGM) temperature than predicted within our basic set of assumptions. We argue that such a higher temperature could be caused by differences between the assumed and true reionization history. The simulated H I optical depth is in good agreement with observations, but the He ii optical depth is lower than observed. Fitting the He ii optical depth requires a larger jump, ∼ 14, between the photon flux at the H I and He ii edge than is present in the Haardt &38; Madau spectrum.

Testing Cosmological Models by Gravitational Lensing. I. Method and First Applications

Gravitational lensing directly measures mass density fluctuations along the lines of sight to very distant objects. No assumptions need to be made concerning bias, the ratio of fluctuations in galaxy density to mass density. Hence, lensing is a very useful tool for studying the universe at low to moderate redshifts. We describe in detail a new method for tracing light rays from redshift zero through a three-dimensional mass distribution to high redshift. As an example, this method is applied here to a standard cold dark matter universe. We obtain a variety of results, some of them statistical in nature, others from rather detailed case studies of individual lines of sight. Among the statistical results are the frequency of multiply imaged quasars, the distribution of separation of multiple quasars, and the redshift distribution of lenses, all as functions of quasar redshift. We find effects ranging from very weak lensing to highly magnified multiple images of high-redshift objects, which for extended background sources (i.e., galaxies) range from slight deformation of shape to tangentially aligned arclets to giant luminous arcs. Different cosmological models differ increasingly with redshift in their predictions of mass (and thus gravitational potential) distributions. Our ultimate goal is to apply this method to a number of cosmogonic models and to eliminate some models for which the gravitational lensing properties are inconsistent with those observed.

Spectral Analysis of the Lyα Forest in a Cold Dark Matter Cosmology

We simulate the Lyα forest in a standard cold dark matter universe using a two-level hierarchical grid code to evolve the dark and baryonic matter components self-consistently. We solve the time-dependent ionization equations for hydrogen and helium, adopting Haardt & Madau's recent estimate for the metagalactic UV radiation background. We compare our simulation results with the measured properties of the Lyα forest by constructing synthetic spectra and analyzing them using an automated procedure to identify, deblend, and fit Voigt line profiles to the absorption features. The H I column density and Doppler parameter distributions that we obtain agree closely with those measured by the Keck Observatory's High Resolution Eschelle Spectrograph (HIRES) and earlier high spectral resolution observations over the column density range 1012 cm-2 3.5). We also compare with measured values of the intergalactic He II opacity. Our results require an He II ionizing background lower than the Haardt & Madau estimate by a factor of 4, corresponding to a soft intrinsic QSO spectrum, with αQ ≈ 1.8-2.

Projection effects in cluster catalogues

We investigate the importance of projection effects in the identification of galaxy clusters in 2D galaxy maps and their effect on the estimation of cluster velocity dispersions. From large N-body simulations of a standard cold dark matter universe, we construct volume-limited galaxy catalogues that have similar low-order clustering properties to those of the observed galaxy distribution. We then select clusters using criteria tailored to match those employed in the construction of real cluster catalogues such as Abell's. We find that our mock Abell cluster catalogues are heavily contaminated and incomplete. Over one third (34 ± 6 per cent) of clusters of richness class R ≥ 1 are misclassifications arising from the projection of one or more subclumps on to an intrinsically poor cluster. Conversely, 32±5 per cent of intrinsically rich clusters are missed altogether from the R ± 1 catalogues, mostly because of statistical fluctuations in the background count. Selection by X-ray luminosity rather than by optical richness reduces, but does not completely eliminate, these problems. Contamination by unvirialized subclumps near a cluster leads to an overestimation of the cluster velocity dispersion which can be very substantial even if the analysis is restricted only to galaxies close to the cluster centre. Thus, the distribution of cluster masses — often used to test cosmological models — is a highly unreliable statistic. The median value of the distribution, however, is considerably more robust because the main effect of contamination is to create an artificial tail of high velocity dispersion clusters. Improved estimates of the cluster velocity dispersion distribution require construction of new cluster catalogues in which clusters are defined according to the number of galaxies within a radius about three times smaller than the Abell radius.

On the peculiar velocity field of CDM universe

view Abstract Citations (6) References (25) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Peculiar Velocity Field of a CDM Universe Brainerd, Tereasa G. ; Villumsen, Jens V. Abstract Using a large N-body simulation of a standard cold dark matter universe in which individual dark matter galaxy halos can be resolved, the peculiar velocity field of the halos and mass particles is investigated. The rms velocity (v_rms_), velocity correlation function [ξ_vv_(r)], and the three-dimensional pairwise velocity dispersion (σ_vv_) of the halos and mass are determined. The effects of halo mass and local environment on these functions are investigated. High-mass halos with overdensities of ~250 and ~70 are good tracers of the major mass motions in the simulation as defined by a volume-averaged velocity field. Low-mass halos are biased tracers of the same field. Assuming one luminous galaxy would reside in each of the halos and the mass-to-light ratio is a constant, this implies bright galaxies are fair tracers of the major mass motions in a CDM universe. The rms velocity of the halos is strongly affected by the local environment; the higher the background mass density the larger v_rms_. However, it is not straightforward to determine the magnitude of an enhancement/suppression in the local mass density given the local enhancement/suppression of v_rms_. At the end of the simulation, we obtain values of the velocity bias, b_v_ = σ_vv,h_/σ_vv,m_, as a function of halo mass and minimum overdensity similar to those found by Carlberg, Couchman, & Thomas and by Carlberg. For halos with overdensities of ~250 and ~70, b_v_ is a decreasing function of background mass density (i.e., the higher the background density, the larger the discrepancy between the velocity dispersions of the halos and mass particles), and for halos with overdensities of ~2000 it is an increasing function of background density (i.e., the lower the background density, the larger the discrepancy between the velocity dispersions of the halos and mass particles). The velocity bias as a function of scale, b_v_(r), is an increasing function of separation and even on large scales, r ~ 1400 km s^-1^, a noticeable velocity bias is present at the end of the simulation (~0.9 for halos with overdensities ~2000, ~0.8 for halos with overdensities ~250, and 0.7 for halos with overdensities ~70). Little difference is found between ξ_vv_(r) for halos and mass particles, and ξ_vv_(r) is similar for halos of high and low mass. Making the assumption that one luminous galaxy would reside in each of the halos and the mass-to-light ratio is a constant, this suggests the velocity correlation statistics of both faint and bright galaxies are representative of the velocity correlation statistics of the mass in a CDM universe. By the end of the simulation, ξ_vv_(r) is about 70% of the value predicted by linear theory over all scales investigated. Little environmental effect on ξ_vv_(r) of halos with overdensities of ~2000 is observed. On scales <~630 km s^-1^, the power in ξ_vv_(r) for halos with overdensities of ~200 and ~70 is due to halos in regions of high background mass density. Publication: The Astrophysical Journal Pub Date: December 1994 DOI: 10.1086/174926 Bibcode: 1994ApJ...436..528B Keywords: Astronomical Models; Cosmology; Dark Matter; Galactic Halos; Many Body Problem; Universe; Velocity Distribution; Computerized Simulation; Mathematical Models; Astrophysics; COSMOLOGY: DARK MATTER; COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; COSMOLOGY: THEORY; METHODS: NUMERICAL full text sources ADS |

The Redshift Space Power Spectrum of Galaxies in a Cold Dark Matter Universe

view Abstract Citations (5) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Redshift Space Power Spectrum of Galaxies in a Cold Dark Matter Universe Brainerd, Tereasa G. ; Villumsen, Jens V. Abstract The redshift space power spectrum of dark galaxy halos in a standard cold dark matter (CDM) universe on scales 0.1 Mpc^-1^ <~kh^-1^ <~ 2.5 Mpc^-1^ (2.5 Mpc <~ rh <~ 63 Mpc) is determined. An N-body simulation is evolved to a linear bias factor, b, of 1.0, and groups of particles that are overdense by a factor of ~250 are selected as halos. On the scales that can be investigated in this simulation, the redshift space power spectrum of all the halos has the same shape as the power spectra of the IRAS galaxies and the CfA galaxies and an amplitude that falls between that of the CfA and IRAS galaxies. The power spectrum of halos with masses >= 9.8 x 10^12^ M_sun_ agrees well with the power spectrum of IRAS galaxies. If it is assumed that one luminous galaxy would reside in each of the halos, a standard CDM universe that is normalized to the observed cosmic microwave background fluctuations does not predict excessive power in the galaxy distribution on small scales. Publication: The Astrophysical Journal Pub Date: October 1993 DOI: 10.1086/187034 Bibcode: 1993ApJ...415L..67B Keywords: COSMOLOGY: LARGE-SCALE STRUCTURE OF UNIVERSE; COSMOLOGY: THEORY; GALAXIES: CLUSTERING full text sources ADS |

Reionization and its imprint of the cosmic microwave background

Early reionization changes the pattern of anisotropies expected in the cosmic microwave backgrond. To explore these changes, we derive from first principles the equations governing anisotropies, focusing on the interactions of photons with electrons. Vishniac (1987) claimed that second-order terms can be large in a reionized universe, so we derive equations correct to second order in the perturbations. There are many more second-order terms than were considered by Vishniac. To understand the basic physics involved, we present a simple analytic approximation to the first-order equation. Then, turning to the second order equation, we show that the Vishniac term is indeed the only important one. We also present numerical results for a variety of ionization histories (in a standard cold dark matter universe) and show quantitatively how the signal in several experiments depends on the ionization history. The most pronounced indication of a reionized universe would be seen in very small scale experiments; the expected signal in the Owens Valley experiment is smaller by a factor of order 10 if the last scattering surface is at a redshift z approximately = 100 as it would be if the universe were reionized very early. On slightly larger scales, the expected signal in a reionized universe is smaller than it would be with standard recombination, but only a factor of 2 or so. The signal is even smaller in these experiments in the intermediate case where some photons last scattered at the standard recombination epoch.