arXiv subjects

David H. Weinberg

Publications and source records attributed to David H. Weinberg.

At least 307 records · Page 17Linked to original sources

arXiv1998

The Clustering of High Redshift Galaxies in the Cold Dark Matter Scenario

We investigate the clustering of high redshift galaxies in five variants of the cold dark matter (CDM) scenario, using hydrodynamic cosmological simulations that resolve the formation of systems with circular velocities $v_c \geq 100 \kms$ ($Ω=1$) or $v_c \geq 70 \kms$ ($Ω=0.4$). Although the five models differ in their cosmological parameters and in the shapes and amplitudes of their mass power spectra, they predict remarkably similar galaxy clustering at $z=2$, 3, and 4. The galaxy correlation functions show almost no evolution over this redshift range, even though the mass correlation functions grow steadily in time. Despite the fairly low circular velocity threshold of the simulations, the high redshift galaxies are usually highly biased tracers of the underlying mass distribution; the bias factor evolves with redshift and varies from model to model. Predicted correlation lengths for the resolved galaxy population are $2-3\hmpc$ (comoving) at $z=3$. More massive galaxies tend to be more strongly clustered. These CDM models have no difficulty in explaining the strong observed clustering of Lyman-break galaxies, and some may even predict excessive clustering. Because the effects of bias obscure differences in mass clustering, it appears that Lyman-break galaxy clustering will not be a good test of cosmological models but will instead provide a tool for constraining the physics of galaxy formation.

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arXiv1998

Reconstruction Analysis of Galaxy Redshift Surveys: A Hybrid Reconstruction Method

In reconstruction analysis of galaxy redshift surveys, one works backwards from the observed galaxy distribution to the primordial density field in the same region, then evolves the primordial fluctuations forward in time with an N-body code. This incorporates assumptions about the cosmological parameters, the properties of primordial fluctuations, and the biasing relation between galaxies and mass. These can be tested by comparing the reconstruction to the observed galaxy distribution, and to peculiar velocity data. This paper presents a hybrid reconstruction method that combines the `Gaussianization'' technique of Weinberg(1992) with the dynamical schemes of Nusser & Dekel(1992) and Gramann(1993). We test the method on N-body simulations and on N-body mock catalogs that mimic the depth and geometry of the Point Source Catalog Redshift Survey and the Optical Redshift Survey. This method is more accurate than Gaussianization or dynamical reconstruction alone. Matching the observed morphology of clustering can limit the bias factor b, independent of Omega. Matching the cluster velocity dispersions and z-space distortions of the correlation function xi(s,mu) constrains the parameter beta=Omega^{0.6}/b. Relative to linear or quasi-linear approximations, a fully non-linear reconstruction makes more accurate predictions of xi(s,mu) for a given beta, thus reducing the systematic biases of beta measurements and offering further scope for breaking the degeneracy between Omega and b. It also circumvents the cosmic variance noise that limits conventional analyses of xi(s,mu). It can also improve the determination of Omega and b from joint analyses of redshift & peculiar velocity surveys as it predicts the fully non-linear peculiar velocity distribution at each point in z-space.

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arXiv1998

Cosmological Simulations with Scale-Free Initial Conditions II: Radiative Cooling

The growth of structure from scale-free initial conditions is one of the most important tests of cosmological simulation methods, providing a realistically complex problem in which numerical results can be compared to rigorous analytic scaling laws. Previous studies of this problem have incorporated gravitational dynamics and adiabatic gas dynamics, but radiative cooling, an essential element of the physics of galaxy formation, normally violates the conditions necessary for self-similar evolution. We show that for any specified value of the initial power spectrum index n [where P(k)~k^n], there is a family of power-law cooling functions that preserves self-similarity by ensuring that the cooling time in an object of the characteristic mass M_* is a fixed fraction t_C of the Hubble time. We perform hydrodynamic numerical simulations with an Einstein-de Sitter cosmology, a baryon fraction of 5%, Gaussian initial conditions, two different power spectrum indices, and four values of t_C for each index, ranging from no cooling to strong cooling. We restrict the numerical simulations to two dimensions in order to allow exploration of a wide parameter space with adequate dynamic range. In all cases, the simulations are remarkably successful at reproducing the analytically predicted scalings of the mass function of dissipated objects and the gas temperature distributions and cooled gas fractions in collapsed systems. While similar success with 3-D simulations must still be demonstrated, our results have encouraging implications for numerical studies of galaxy formation, indicating that simulations with resolution comparable to that in many current studies can accurately follow the collapse and dissipation of baryons into the dense, cold systems where star formation is likely to occur.

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arXiv1998

Cosmological Simulations with Scale-Free Initial Conditions I: Adiabatic Hydrodynamics

We analyze hierarchical structure formation based on scale-free initial conditions in an Einstein-de Sitter universe, including a baryonic component. We present three independent, smoothed particle hydrodynamics (SPH) simulations, performed with two different SPH codes (TreeSPH and P3MSPH) at two resolutions. Each simulation is based upon identical initial conditions, which consist of Gaussian distributed initial density fluctuations that have an n=-1 power spectrum. The baryonic material is modeled as an ideal gas subject only to shock heating and adiabatic heating and cooling. The evolution is expected to be self-similar in time, and under certain restrictions we identify the expected scalings for many properties of the distribution of collapsed objects in all three realizations. The distributions of dark matter masses, baryon masses, and mass and emission weighted temperatures scale quite reliably. However, the density estimates in the central regions of these structures are determined by the degree of numerical resolution. As a result, mean gas densities and luminosities obey the expected scalings only when calculated within a limited dynamic range in density contrast. The temperatures and luminosities of the groups show tight correlations with the baryon masses, which can be well-represented by power-laws. The Press-Schechter (PS) approximation predicts the distribution of group dark matter and baryon masses fairly well, though it tends to overestimate the baryon masses. Combining the PS mass distribution with the measured relations for T(M) and L(M) predicts the temperature and luminosity distributions reasonably, though there are some discrepancies at high temperatures/luminosities. The three simulations agree well for the properties of groups that are resolved by 32 or more particles.

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arXiv1998

Simulating Gas at High Redshift

We discuss simulations of gas at high redshift. We briefly review the methods used and the results for quasar absorption lines. We present gas mass functions and galaxy correlation functions for 5 different cosmological models. Galaxies should be detectable at redshifts greater than 2 by SKAI, and measurements of the gas mass functions and galaxy correlation functions could be used to discriminate between different cosmological models.

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arXiv1997

Constraints on the Effects of Locally-biased Galaxy Formation

While it is well-known that "biased galaxy formation" can increase the strength of galaxy clustering, it is less clear whether straightforward biasing schemes can change the shape of the galaxy correlation function on large scales. Here we consider "local" biasing models, in which the galaxy density field $δ_g$ at a point is a function of the matter density field $δ$ at that point: $δ_g = f(δ)$. We consider both deterministic biasing, where $f$ is simply a function, and stochastic biasing, in which the galaxy density is a random variable whose distribution depends on the matter density: $δ_g=X(δ)$. We show that even when this mapping is performed on a highly nonlinear density field with a hierarchical correlation structure, the correlation function $ξ$ is simply scaled up by a constant, as long as $ξ<< 1$. In stochastic biasing models, the galaxy autocorrelation function behaves exactly as in deterministic models, with the mean value of $X$ for a given value of $δ$ taking the role of the deterministic bias function. We extend our results to the power spectrum P(k), showing that for sufficiently small k, the effect of local biasing is equivalent to the multiplication of P(k) by a constant, with the addition of a constant term. If a cosmological model predicts a large-scale mass correlation function in conflict with the shape of the observed galaxy correlation function, then the model cannot be rescued by appealing to a complicated but local relation between galaxies and mass.

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arXiv1997

Hubble Flow Broadening of the Lyman-alpha Forest and its Implications

Lyman-alpha forest lines in QSO spectra have typical widths of 20-50 km/s. Low column density absorbers in cosmological simulations are large, diffuse structures, and the Hubble flow across the spatially extended absorber is usually the dominant contribution to the width of its associated absorption line. Thermal broadening is unimportant over most of the spectrum, and peculiar velocities tend to make absorption features narrower rather than broader. As a consequence of Hubble flow broadening, there is a close relation between local Lyman-alpha optical depth and local neutral hydrogen density, which is well approximated by the Gunn-Peterson formula. The physics that governs the unshocked intergalactic medium leads to a tight correlation between the neutral hydrogen density and the underlying gas and dark matter overdensity. For many purposes, it is simpler to regard a Lyman-alpha forest spectrum as a continuous, non-linear map of the density field rather than a collection of discrete lines. This continuous field view of the Lyman-alpha forest can be applied to measurement of the baryon density parameter, testing of cosmological models, and robust determination of the shape and amplitude of the primordial mass power spectrum.

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arXiv1997

A Study of Quasar Clustering at z>2.7 from the Palomar Transit Grism Survey

The quest for structure indicators at earlier and earlier times in the evolution of the universe has led to the search for objects with ever higher redshifts. The Palomar Transit Grism Survey has produced a large sample of high redshift quasars ($z>2.7$), allowing statistical analysis of correlation between quasar positions. In this study, clustering is identified through comparison with $100 000$ Monte Carlo generated, randomly populated volumes, which are identical to the observed region in spatial coordinates, redshift distribution, and number of quasars. Three pairs have been observed with comoving separations of 11.34, 12.97, and 24.13 h_{50}^{-1}Mpc (assuming $q_0=0.5$), smaller separations than would be expected to arise by chance in an unclustered distribution. Selection effects are ruled out as a false source of clustering by scrambling the observed quasar coordinates and redshifts, which gives a pair separation distribution nearly identical to that of the Monte Carlo distribution. Tests using the distribution of pair separations and nearest neighbor distances show that the observed pairs have a probability less than 0.1% of arising in an unclustered distribution. Using a maximum likelihood technique to estimate the correlation length $r_0$, assuming $ξ(r) = (r/r_0)^{-1.8}$, we find $r_0 = 35\pm15 h_{50}^{-1}Mpc$ (comoving, $q_0=0.5$, 1$σ$ errors), a value much larger than the correlation length of present-day galaxies.

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arXiv1997

Simulating Cosmic Structure Formation

We describe cosmological simulation techniques and their application to studies of cosmic structure formation, with particular attention to recent hydrodynamic simulations of structure in the high redshift universe. Collisionless N-body simulations with Gaussian initial conditions produce a pattern of sheets, filaments, tunnels, and voids that resembles the observed large scale galaxy distribution. Simulations that incorporate gas dynamics and dissipation form dense clumps of cold gas with sizes and masses similar to the luminous parts of galaxies. Models based on inflation and cold dark matter predict a healthy population of high redshift galaxies, including systems with star formation rates of 20 M_{\sun}/year at z=6. At z~3, most of the baryons in these models reside in the low density intergalactic medium, which produces fluctuating Lyman-alpha absorption in the spectra of background quasars. The physical description of this ``Lyman-alpha forest'' is particularly simple if the absorption spectrum is viewed as a 1-dimensional map of a continuous medium instead of a collection of lines. The combination of superb observational data and robust numerical predictions makes the Lyman-alpha forest a promising tool for testing cosmological models.

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arXiv1997

Recovery of the Power Spectrum of Mass Fluctuations from Observations of the Lyman-alpha Forest

We present a method to recover the shape and amplitude of the power spectrum of mass fluctuations, P(k), from observations of the high redshift \lya forest. The method is motivated by the physical picture of the \lya forest that has emerged from hydrodynamic cosmological simulations and related semi-analytic models, which predicts a tight correlation between the \lya optical depth and the underlying matter density. We monotonically map the QSO spectrum to a Gaussian density field, measure its 3-d P(k), and normalize by evolving cosmological simulations with this P(k) until they reproduce the observed power spectrum of the QSO flux. Imposing the observed mean \lya opacity as a constraint makes the derived P(k) normalization insensitive to the choice of cosmological parameters, ionizing background spectrum, or reionization history. Thus, in contrast to estimates of P(k) from galaxy clustering, there are no uncertain "bias parameters" in the recovery of the mass power spectrum. We test the full procedure on SPH simulations of 3 cosmological models and show that it recovers their true mass power spectra on comoving scales ~1-10/h Mpc, the upper scale being set by the size of the simulation boxes. The procedure works even for noisy (S/N ~ 10), moderate resolution (~40 km/s pixels) spectra. We present an illustrative application to Q1422+231; the recovered P(k) is consistent with an Ω=1, σ_8=0.5 CDM model. Application to large QSO samples should yield the power spectrum of mass fluctuations on small scales at z ~ 2-4. (Compressed)

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arXiv1997

Quasar Clustering and Spacetime Geometry

The non-Euclidean geometry of spacetime induces an anisotropy in the apparent correlation function of high-redshift quasars. This effect can constrain the cosmological constant Λindependent of any assumptions about evolution of luminosities, sizes, or clustering. We examine the prospects for distinguishing between low-density (Ω_0=0.1-0.4) cosmological models with flat and open space geometry using the quasar samples anticipated from the 2dF and Sloan redshift surveys. We show that even these large quasar surveys are likely to reside in the "sparse sampling" regime, so that measurement errors in the correlation function obey Poisson statistics. As a result: (a) one can devise a simple maximum-likelihood scheme for estimating clustering parameters, (b) one can generate Monte Carlo realizations of correlation function measurements without creating artificial quasar distributions, and (c) for fixed quasar number, a deeper survey over a smaller area has greater statistical power than a shallow, large-area survey. Adopting recent (quite uncertain) estimates of the quasar correlation length, we find that the 2dF and Sloan samples can provide clear discrimination between flat and open geometries for Ω<= 0.2 but only marginal discrimination for Ω= 0.4. Clear discrimination is possible for Ω= 0.4 if the true quasar correlation length is a factor of two larger, and a high-density survey of 30,000 quasars in 200 square degrees would provide clear discrimination even for the lower correlation length.

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arXiv1997

Intergalactic Helium Absorption in Cold Dark Matter Models

Observations from the HUT and the HST have recently detected HeII absorption along the lines of sight to two high redshift quasars. We use cosmological simulations with gas dynamics to investigate HeII absorption in the cold dark matter (CDM) theory of structure formation. We consider two Omega=1 CDM models with different normalizations and one Omega_0=0.4 CDM model, all incorporating the photoionizing UV background spectrum computed by Haardt & Madau (1996). The simulated gas distribution, combined with the H&M spectral shape, accounts for the relative observed values of taubar_HI and taubar_HeII, the effective mean optical depths for HI and HeII absorption. If the background intensity is as high as H&M predict, then matching the absolute values of taubar_HI and taubar_HeII requires a baryon abundance larger (by factors between 1.5 and 3 for the various CDM models) than our assumed value of Omega_b h^2=0.0125. The simulations reproduce the evolution of taubar_heII over the observed redshift range, 2.2 < z < 3.3, if the HeII photoionization rate remains roughly constant. HeII absorption in the CDM simulations is produced by a diffuse, fluctuating, intergalactic medium, which also gives rise to the HI ly-alpha forest. Much of the HeII opacity arises in underdense regions where the HI optical depth is very low. We compute statistical properties of the HeII and HI absorption that can be used to test the CDM models and distinguish them from an alternative scenario in which the HeII absorption is caused by discrete, compact clouds. The CDM scenario predicts that a substantial amount of baryonic material resides in underdense regions at high redshift. HeII absorption is the only sensitive probe of such extremely diffuse, intergalactic gas, so it can provide a vital test of this fundamental prediction.

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arXiv1997

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.

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arXiv1997

Cosmological Constraints from High-Redshift Damped Lyman-Alpha Systems

Any viable cosmological model must produce enough structure at early epochs to explain the amount of gas associated with high-redshift damped Ly$α$ systems. We study the evolution of damped Ly$α$ systems at redshifts $z\ge 2$ in cold dark matter (CDM) and cold+hot dark matter (CDM+HDM) models using both N-body and hydrodynamic simulations. Our approach incorporates the effects of gas dynamics, and we find that all earlier estimates which assumed that all the baryons in dark matter halos would contribute to damped Ly$α$ absorption have overestimated the column density distribution $f(N)$ and the fraction of neutral dense gas $Ω_g$ in damped Ly$α$ systems. The differences are driven by ionization of hydrogen in the outskirts of galactic halos and by gaseous dissipation near the halo centers, and they tend to exacerbate the problem of late galaxy formation in CDM+HDM models. We only include systems up to the highest observed column density $N\sim 10^{21.8}$ cm$^{-2}$ in the estimation of $Ω_g$ for a fair comparison with data. If the observed $f(N)$ and $Ω_g$ inferred from a small number of confirmed and candidate absorbers are robust, the amount of gas in damped Ly$α$ systems at high redshifts in the $Ω_ν=0.2$ CDM+HDM model falls well below the observations.

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arXiv1997

Large-scale structure in COBE-normalized cold dark matter cosmogonies

We study the clustering of the mass distribution in COBE-normalized open and flat CDM models using large N-body simulations. With an age of the universe of 14 Gyr (12 Gyr) for the flat (open) models and a baryon density fixed by nucleosynthesis constraints, the observed abundance of rich galaxy clusters leads to tight constraints on the density parameter; 0.25 0.2, implies that galaxies are overabundant in clusters relative to the field. The tilted Omega_0=1 model, on the other hand, does require that galaxies be positively biased on all scales. We also compute the topology of isodensity contours in these models, obtaining theoretical predictions that are less sensitive to galaxy bias.

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arXiv1997

Characterization of Lyman Alpha Spectra and Predictions of Structure Formation Models: A Flux Statistics Approach

In gravitational instability models, \lya absorption arises from a continuous fluctuating medium, so that spectra provide a non-linear one-dimensional ``map'' of the underlying density field. We characterise this continuous absorption using statistical measures applied to the distribution of absorbed flux. We describe two simple members of a family of statistics which we apply to simulated spectra in order to show their sensitivity as probes of cosmological parameters (H$_{0}$, $Ω$, the initial power spectrum of matter fluctuations) and the physical state of the IGM. We make use of SPH simulation results to test the flux statistics, as well as presenting a preliminary application to Keck HIRES data.

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arXiv1997

The Topology of Large Scale Structure in the 1.2 Jy IRAS Redshift Survey

We measure the topology (genus) of isodensity contour surfaces in volume limited subsets of the 1.2 Jy IRAS redshift survey, for smoothing scales $λ=4\hmpc$, $7\hmpc$, and $12\hmpc$. At $12\hmpc$, the observed genus curve has a symmetric form similar to that predicted for a Gaussian random field. At the shorter smoothing lengths, the observed genus curve shows a modest shift in the direction of an isolated cluster or ``meatball'' topology. We use mock catalogs drawn from cosmological N-body simulations to investigate the systematic biases that affect topology measurements in samples of this size and to determine the full covariance matrix of the expected random errors. We incorporate the error correlations into our evaluations of theoretical models, obtaining both frequentist assessments of absolute goodness-of-fit and Bayesian assessments of models' relative likelihoods. We compare the observed topology of the 1.2 Jy survey to the predictions of dynamically evolved, unbiased, gravitational instability models that have Gaussian initial conditions. The model with an $n=-1$, power-law initial power spectrum achieves the best overall agreement with the data, though models with a low-density cold dark matter power spectrum and an $n=0$ power-law spectrum are also consistent. The observed topology is inconsistent with an initially Gaussian model that has $n=-2$, and it is strongly inconsistent with a Voronoi foam model, which has a non-Gaussian, bubble topology.

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arXiv1997

Metal Lines Associated with Lyman Alpha Absorbers: A Comparison of Theory and Observations

We study metal line absorption of CIV, CII, SiIV, and NV at redshifts z=3.5 to z=2 within the framework of a cosmological model for the Lyman alpha forest, comparing the results of numerical simulations to recent observations by Songaila & Cowie (1996, SC). In agreement with Rauch, Haehnelt & Steinmetz (1996), we find that the observed mean value of the CIV/HI ratio around z=3 is reproduced if a uniform metallicity of [C/H] = -2.5 is assumed in our model, but that the observed scatter in this ratio is larger than predicted, implying a scatter in the metallicity of the absorbing systems of roughly an order of magnitude. We further argue that absorbers with relatively low column densities (log(N_HI) < 15) likely have a mean metallicity [C/H] less than -2.5, a result which is basically independent of the model considered, and similar to what is predicted in the simulations by Gnedin & Ostriker (1997). Our model predicts no significant evolution in the mean values of metal line column densities between z=3.5 and z=2. Comparison of the predicted and observed numbers of SiIV and NV systems suggests that the photoionizing background radiation field around z=3 is somewhat softer than that proposed by Haardt & Madau (1996). Our model does not account for the increase in the SiIV/CIV ratio at z=3.2 found by SC. While SC suggested that the increase could be explained by a softening of the radiation field due to HeII absorption at z > 3, such a modification does not raise the mean value of SiIV/CIV in our simulation because it shifts numerous weak SiIV systems to just above the detection limit, thus keeping the mean column density of observable SiIV systems low.

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