arXiv subjects

Avishai Dekel

Publications and source records attributed to Avishai Dekel.

At least 271 records · Page 15Linked to original sources

arXiv1999

Cosmological Parameters and Power Spectrum from Peculiar Velocities

The power spectrum of mass density fluctuations is evaluated from the Mark III and the SFI catalogs of peculiar velocities by a maximum likelihood analysis, using parametric models for the power spectrum and for the errors. The applications to the two different data sets, using generalized CDM models with and without COBE normalization, give consistent results. The general result is a relatively high amplitude of the power spectrum, with σ_8 Ω_m^{0.6} = 0.8+/-0.2 at 90% confidence. Casting the results in the Ω_m - Ω_Λplane, yields complementary constraints to those of the high-redshift supernovae, together favoring a nearly flat, unbound and accelerating universe with comparable contributions from Ω_m and Ω_Λ. Further implications on the cosmological parameters, arising from a joint analysis of the velocities together with small-scale CMB anisotropies and the high-redshift supernovae, are also briefly described.

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arXiv1999

Evidence for a Positive Cosmological Constant from Flows of Galaxies and Distant Supernovae

Recent observations of high-redshift supernovae seem to suggest that the global geometry of the Universe may be affected by a `cosmological constant', which acts to accelerate the expansion rate with time. But these data by themselves still permit an open universe of low mass density and no cosmological constant. Here we derive an independent constraint on the lower bound to the mass density, based on deviations of galaxy velocities from a smooth universal expansion. This constraint rules out a low-density open universe with a vanishing cosmological constant, and together the two favour a nearly flat universe in which the contributions from mass density and the cosmological constant are comparable. This type of universe, however, seems to require a degree of fine tuning of the initial conditions that is in apparent conflict with `common wisdom'.

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arXiv1999

Young galaxies: what turns them on?

Lyman break galaxies (LBGs) at $z \sim 3$ exhibit number densities and clustering similar to local $L_*$ galaxies; however, their star formation rates (SFRs) are much higher. We explore the scenario in which LBGs are starburst galaxies triggered by collisions, and thus provide an explanation for these key properties. The relative paucity of starburst galaxies at low redshift can be attributed to a much higher collision rate in the past. We use high-resolution cosmological $N$-body simulations and a hierarchical halo finder to estimate the galaxy collision rate as a function of time in the popular $Λ$CDM cosmological model. We find that bright collisional starbursts are frequent enough to account for most of the high-$z$ ($2.5-4.5$) LBGs. Moreover, many of the objects are of relatively small mass, but they cluster about large-mass halos. They therefore exhibit strong clustering, similar to that observed and stronger than that of the relevant massive halos.

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arXiv1999

Large-Scale Power Spectrum and Cosmological Parameters from SFI Peculiar Velocities

We estimate the power spectrum of mass density fluctuations from peculiar velocities of galaxies by applying an improved maximum-likelihood technique to the new all-sky SFI catalog. Parametric models are used for the power spectrum and the errors, and the free parameters are determined by assuming Gaussian velocity fields and errors and maximizing the probability of the data given the model. It has been applied to generalized CDM models with and without COBE normalization. The method has been carefully tested using artificial SFI catalogs. The most likely distance errors are found to be similar to the original error estimates in the SFI data. The general result that is not very sensitive to the prior model used is a relatively high amplitude of the power spectrum. For example, at k=0.1 h/Mpc we find P(k)Ω^{1.2}=(4.4+/-1.7)X10^3 (Mpc/h)^3. An integral over the power spectrum yields σ_8Ω^{0.6}=0.82+/-0.12. Model-dependent constraints on the cosmological parameters are obtained for families of CDM models. For example, for COBE-normalized ΛCDM models (scalar fluctuations only), the maximum-likelihood result can be approximated by Ωn^{2} h_{60}^{1.3} =0.58+/-0.11. The formal random errors quoted correspond to the 90% confidence level. The total uncertainty, including systematic errors associated with nonlinear effects, may be larger by a factor of ~2. These results are in agreement with an application of a similar method to other data (Mark III).

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arXiv1998

Cosmological Implications of Lyman-Break Galaxy Clustering

We review our analysis of the clustering properties of ``Lyman-break'' galaxies (LBGs) at redshift z~3, previously discussed in Wechsler et al (1998). We examine the likelihood of spikes found by Steidel et al (1998) in the redshift distribution of LBGs, within a suite of models for the evolution of structure in the Universe. Using high-resolution dissipationless N-body simulations, we analyze deep pencil-beam surveys from these models in the same way that they are actually observed, identifying LBGs with the most massive dark matter halos. We find that all the models (with SCDM as a marginal exception) have a substantial probability of producing spikes similar to those observed, because the massive halos are much more clumped than the underlying matter -- i.e., they are biased. Therefore, the likelihood of such a spike is not a good discriminator among these models. The LBG correlation functions are less steep than galaxies today (gamma~1.4), but show similar or slightly longer correlation lengths. We have extened this analysis and include a preliminary comparison to the new data presented in Adelberger et al (1998). We also discuss work in progress, in which we use semi-analytic models to identify Lyman-break galaxies within dark-matter halos.

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arXiv1998

Stochastic Nonlinear Galaxy Biasing

We propose a general formalism for galaxy biasing and apply it to methods for measuring cosmological parameters, such as regression of light versus mass, the analysis of redshift distortions, measures involving skewness and the cosmic virial theorem. The common linear and deterministic relation g=b*d between the density fluctuation fields of galaxies g and mass d is replaced by the conditional distribution P(g|d) of these random fields, smoothed at a given scale at a given time. The nonlinearity is characterized by the conditional mean =b(d)*d, while the local scatter is represented by the conditional variance s_b^2(d) and higher moments. The scatter arises from hidden factors affecting galaxy formation and from shot noise unless it has been properly removed. For applications involving second-order local moments, the biasing is defined by three natural parameters: the slope b_h of the regression of g on d, a nonlinearity b_t, and a scatter s_b. The ratio of variances b_v^2 and the correlation coefficient r mix these parameters. The nonlinearity and the scatter lead to underestimates of order b_t^2/b_h^2 and s_b^2/b_h^2 in the different estimators of beta (=Omega^0.6/b_h). The nonlinear effects are typically smaller. Local stochasticity affects the redshift-distortion analysis only by limiting the useful range of scales, especially for power spectra. In this range, for linear stochastic biasing, the analysis reduces to Kaiser's formula for b_h (not b_v), independent of the scatter. The distortion analysis is affected by nonlinear properties of biasing but in a weak way. Estimates of the nontrivial features of the biasing scheme are made based on simulations and toy models, and strategies for measuring them are discussed. They may partly explain the range of estimates for beta.

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arXiv1998

Wiener Reconstruction of Large-Scale Structure from Peculiar Velocities

We present an alternative, Bayesian method for large-scale reconstruction from observed peculiar velocity data. The method stresses a rigorous treatment of the random errors and it allows extrapolation into poorly sampled regions in real space or in k-space. A likelihood analysis is used to determine the fluctuation power spectrum, followed by a Wiener Filter (WF) analysis to obtain the minimum-variance mean fields of velocity and mass density. Constrained Realizations (CR) are then used to sample the statistical scatter about the WF mean field. The WF/CR method is applied as a demonstration to the Mark III data with 1200 km/s, 900 km/s, and 500 km/s resolutions. The main reconstructed structures are consistent with those extracted by the POTENT method. A comparison with the structures in the distribution of IRAS 1.2Jy galaxies yields a general agreement. The reconstructed velocity field is decomposed into its divergent and tidal components relative to a cube of +/-8000 km/s centered on the Local Group. The divergent component is very similar to the velocity field predicted from the distribution of IRAS galaxies. The tidal component is dominated by a bulk flow of 194 +/- 32 km/s towards the general direction of the Shapley concentration, and it also indicates a significant quadrupole.

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arXiv1998

Galaxy Biasing: Nonlinear, Stochastic and Measurable

I describe a general formalism for galaxy biasing (Dekel & Lahav 1998) and its application to measurements of beta (=Omega^0.6/b), e.g. via direct comparisons of light and mass and via redshift distortions. The linear and deterministic relation g=b*d between the density fluctuation fields of galaxies g and mass d is replaced by the conditional distribution P(g|d) of these as random fields, smoothed at a given scale and at a given time. The mean biasing and its nonlinearity are characterized by the conditional mean =b(d)*d and the local scatter by the conditional variance s_b^2(d). This scatter arises from hidden effects on galaxy formation and from shot noise. For applications involving second-order local moments, the biasing is defined by three natural parameters: the slope b_h of the regression of g on d (replacing b), a nonlinearity parameter b_t, and a scatter parameter s_b. The ratio of variances b_v^2 and the correlation coefficient r mix these parameters. The nonlinearity and scatter lead to underestimates of order b_t^2/b_h^2 and s_b^2/b_h^2 in the different estimators of beta, which may partly explain the range of estimates. Local stochasticity affects the redshift-distortion analysis only by limiting the useful range of scales. In this range, for linear stochastic biasing, the analysis reduces to Kaiser's formula for b_h (not b_v) independent of the scatter. The distortion analysis is affected by nonlinearity but in a weak way. Estimates of the nontrivial features of the biasing scheme are made based on simulations and toy models, and a new method for measuring them via distribution functions is proposed.

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arXiv1998

Evaluating Semi-Analytic Halo Merging Histories

We evaluate the accuracy of semi-analytic merger-trees by comparing them with the merging histories of dark-matter halos in N-body simulations, focusing on the joint distribution of the number of progenitors and their masses. We first confirm that the halo mass function as predicted directly by the Press-Schechter (PS) model deviates from the simulations by up to 50% depending on the mass scale and redshift, while the means of the projected distributions of progenitor number and mass for a halo of a given mass are more accurately predicted by the Extended PS model. We then use the full merger trees to study the joint distribution as a function of redshift and parent-halo mass. We find that while the deviation of the mean quantities due to the inaccuracy of the Extended PS model partly propagates into the higher moments of the distribution, the merger-tree procedure does not introduce a significant additional source of error. In particular, certain properties of the merging history such as the mass ratio of the progenitors and the total accretion rate are reproduced quite accurately for galaxy sized halos ($\sim 10^{12}\msun$), and less so for larger masses. We conclude that although there could be $\sim 50%$ deviations in the absolute numbers and masses of progenitors and in the higher order moment of these distributions, the relative properties of progenitors for a given halo are reproduced fairly well by the merger trees. They can thus provide a useful framework for modelling galaxy formation once the above-mentioned limitations are taken into account.

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arXiv1998

Implications of Spikes in the Redshift Distribution of $z\sim3$ Galaxies

We address the high peaks found by Steidel et al (1997) in the redshift distribution of ``Lyman-break'' objects (LBOs) at redshift z~3. The highest spike represents a relative overdensity of 2.6 in the distribution of LBOs in pixels of comoving size ~10Mpc/h. We examine the likelihood of such a spike in the redshift distribution within a suite of models for the evolution of structure in the Universe, including models with Omega=1 (SCDM and CHDM) and with Omega=0.4-0.5 (LCDM and OCDM). Using high-resolution dissipationless N-body simulations, we analyze deep pencil-beam surveys from these models in the same way that they are actually observed, identifying LBOs with the most massive dark matter halos. We find that all the models (with SCDM as a marginal exception) have a substantial probability of producing spikes similar to those observed, because the massive halos are much more clumped than the underlying matter -- i.e., they are biased. Therefore, the likelihood of such a spike is not a good discriminator among these models. We find in these models that the mean biasing parameter b of LBOs with respect to dark matter varies within a range b ~2-5 on a scale of ~10Mpc/h. We also compute the two-body correlation functions of LBOs predicted in these models. The LBO correlation functions are less steep than galaxies today (gamma ~1.4), but show similar or slightly longer correlation lengths.

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arXiv1998

A Local Hubble Bubble from SNe Ia?

We analyze the monopole in the peculiar velocities of 44 Type Ia supernovae (SNe Ia) to test for a local void. The sample extends from 20 to 300 Mpc/h, with distances, deduced from light-curve shapes, accurate to ~6%. Assuming Omega_m=1 and Omega_lambda=0, the most significant deviation we find from the Hubble law is an outwards flow of (6.6+/-2.2)% inside a sphere of radius 70 Mpc/h as would be produced by a void of ~20% underdensity surrounded by a dense shell. This shell roughly coincides with the local Great Walls. Monte Carlo analyses, using Gaussian errors or bootstrap resampling, show the probability for chance occurrence of this result out of a pure Hubble flow to be ~2%. The monopole could be contaminated by higher moments of the velocity field, especially a quadrupole, which are not properly probed by the current limited sky coverage. The void would be less significant if Omega_m is low and Omega_lambda is high. It would be more significant if one outlier is removed from the sample, or if the size of the void is constrained a-priori. This putative void is not in significant conflict with any of the standard cosmological scenarios. It suggests that the Hubble constant as determined within 70 Mpc/h could be overestimated by ~6% and the local value of Omega may be underestimated by ~20%. While the present evidence for a local void is marginal in this data set, the analysis shows that the accumulation of SNe Ia distances will soon provide useful constraints on elusive and important aspects of regional cosmic dynamics.

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arXiv1997

Cosmological Implications of Large-Scale Flows

Cosmological implications of the observed large-scale peculiar velocities are reviewed, alone or combined with redshift surveys and CMB data. The latest version of the POTENT method for reconstructing the underlying three-dimensional velocity and mass-density fields is described. The initial fluctuations and the nature of the dark matter are addressed via statistics such as bulk flow and mass power spectrum. The focus is on constraining the mass density parameter $Ω$, directly or via the parameter $β$ which involves the unknown relation between galaxies and mass. The acceptable range for $Ω$ is found to be $0.4-1.0$. The range of $β$ estimates is likely to reflect non-trivial features in the galaxy biasing scheme, such as scale dependence. Similar constraints on $Ω$ and $Λ$ from global measures are summarized.

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arXiv1996

Maximum-Likelihood Comparisons of Tully-Fisher and Redshift Data: Constraints on Omega and Biasing

We compare Tully-Fisher (TF) data for 838 galaxies within cz=3000 km/sec from the Mark III catalog to the peculiar velocity and density fields predicted from the 1.2 Jy IRAS redshift survey. Our goal is to test the relation between the galaxy density and velocity fields predicted by gravitational instability theory and linear biasing, and thereby to estimate $β_I = Ω^{0.6}/b_I,$ where $b_I$ is the linear bias parameter for IRAS galaxies. Adopting the IRAS velocity and density fields as a prior model, we maximize the likelihood of the raw TF observables, taking into account the full range of selection effects and properly treating triple-valued zones in the redshift-distance relation. Extensive tests with realistic simulated galaxy catalogs demonstrate that the method produces unbiased estimates of $β_I$ and its error. When we apply the method to the real data, we model the presence of a small but significant velocity quadrupole residual (~3.3% of Hubble flow), which we argue is due to density fluctuations incompletely sampled by IRAS. The method then yields a maximum likelihood estimate $β_I=0.49\pm 0.07$ (1-sigma error). We discuss the constraints on $Ω$ and biasing that follow if we assume a COBE-normalized CDM power spectrum. Our model also yields the 1-D noise noise in the velocity field, including IRAS prediction errors, which we find to be be 125 +/- 20 km/sec.

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arXiv1996

Measuring Omega

We were asked to debate the value of the cosmological mass-density parameter Omega. Is Omega_m=1 in accordance with the simplest model? Is Omega_m much smaller as indicated by some observations? There is conflicting evidence. We lay out the various methods for measuring Omega_m, mention new developments and current estimates, and focus on prospects versus the associated difficulties. We try to shed light on the uncertainties that are responsible for the span of estimates for Omega_m. We divide the methods into the following four classes: a. Global measures of the properties of space-time that constrain combinations of Omega_m and the other cosmological parameters (Lambda, Ho, to). b. Virialized Systems. Nonlinear dynamics within galaxies and clusters on comoving scales 1-10 Mpc/h. c. Large-scale structure. Mildly-nonlinear gravitational dynamics of fluctuations on scales 10-100 Mpc/h, in particular cosmic flows. d. Growth rate of fluctuations from the last scattering of the CMB or from high redshift galaxies to the present. Methods (b) typically yield low values of Omega_m=0.2-0.3. Methods (a) and (c) typically indicate higher values of Omega_m=0.4-1.

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arXiv1996

Large-Scale Power-Spectrum from Peculiar Velocities

The power spectrum (PS) of mass density fluctuations, in the range $0.05 \leq k \leq 0.2 \ihmpc$, is derived from the Mark III catalog of peculiar velocities of galaxies, independent of ``biasing". It is computed from the density field as recovered by POTENT with Gaussian smoothing of $12\hmpc$, within a sphere of radius $\sim 60\hmpc$ about the Local Group. The density is weighted inversely by the square of the errors. The PS is corrected for the effects of smoothing, random errors, sparse sampling and finite volume using mock catalogs that mimic in detail the Mark III catalog and the dynamics of our cosmological neighborhood. The mock catalogs are also used for error analysis. The PS at $k = 0.1 \ihmpc$ (for example) is $(4.6 \pm 1.4) \times 10^3 Ω^{-1.2} \3hmpc$, with a local logarithmic slope of $-1.45 \pm 0.5$. An itegration yields $σ_8 Ω^{0.6} \simeq 0.7-0.8$, depending on where the PS peak is. Direct comparisons of the mass PS with the galaxy PS derived from sky and redshift surveys show a similarity in shape and yield for $β\equiv Ω^{0.6}/b$ values in the range $0.77-1.21$, with a typical error of $\pm 0.1$ per galaxy sample. A comparison of the mass PS at $\sim 100\hmpc$ with the large-angle CMB fluctuations by COBE provides constraints on cosmological parameters and on the slope of the initial PS ($n$). The ``standard" CDM model is marginally rejected at the $\sim 2σ$ level, while each of the following modifications leads to a good fit: $n \lsim 1$, $Ω_ν\sim 0.3$, or $Ω\lsim 1$. Values of $Ω$ as low as $\sim 0.2$ are ruled out with high confidence (independent of $Λ$).

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arXiv1996

Homogeneous Velocity-Distance Data for Peculiar Velocity Analysis. III. The Mark III Catalog of Galaxy Peculiar Velocities

This is the third in a series of papers in which we assemble and analyze a homogeneous catalog of peculiar velocity data. In Papers I and II, we described the Tully-Fisher (TF) redshift-distance samples that constitute the bulk of the catalog, and our methodology for obtaining mutually consistent TF calibrations for these samples. In this paper, we supply further technical details of the treatment of the data, and present a subset of the catalog in tabular form. The full catalog, known as the Mark III Catalog of Galaxy Peculiar Velocities, is available in accessible on-line databases, as described herein. The electronic catalog incorporates not only the TF samples discussed in Papers I and II, but also elliptical galaxy Dn-sigma samples originally presented elsewhere. The relative zeropointing of the elliptical and spiral data sets is discussed here. The basic elements of the Mark III Catalog are the observables for each object (redshift, magnitude, velocity width, etc.) and inferred distances derived from the TF or Dn-sigma relations. Malmquist bias-corrected distances are computed for each catalog object using density fields obtained from the IRAS 1.2 Jy redshift survey. Distances for both individual objects and groups are provided. A variety of auxiliary data, including distances and local densities predicted from the IRAS redshift survey reconstruction method, are tabulated as well. We study the distributions of TF residuals for three of our samples and conclude that they are well-approximated as Gaussian. For the Mathewson et al. sample we demonstrate a significant decrease in TF scatter with increasing velocity width. We test for, but find no evidence of, a correlation between TF residuals and galaxy morphology.

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arXiv1995

Simulating Our Cosmological Neighborhood: Mock Catalogs for Velocity Analysis

We describe the construction of an N-body simulation that mimics the true velocity and mass-density fields in a box of side 256\hmpc about the Local Group , and the production of mock catalogs that mimic in detail current catalogs of redshifts and peculiar velocities. Our main purpose is to provide a tool for developing and testing reconstruction methods, but the different components of the method can be used on their own in other applications. The initial conditions are based on the IRAS 1.2Jy redshift survey, assuming that galaxies trace mass and Ω=1. A density field smoothed is recovered from the redshift survey, using quasi-linear theory and a power-preserving filter. The corresponding potential field is traced back to the linear regime using the Zel'dovich-Bernoulli equation. Small-scale power is added by means of constrained realization to mimic fluctuations on galactic scales. The gravitating system is evolved forward in time with a PM code of 2\hmpc resolution. The result reproduces the real dynamical structures on large scales and the statistical properties of the structure down to galactic scales. "Galaxies" are identified via a linear biasing scheme (b=1.35) and they are divided into "spirals" and "ellipticals" to obey Dressler's morphology-density relation. The galaxies are assigned internal-velocity parameters and absolute magnitudes scattered about an assumed mean Tully-Fisher relation. They are then "observed" as magnitude-limited samples, trying to mimic the selection criteria of the data sets constituting the Mark III catalog of peculiar velocities. Artificial IRAS 1.2Jy redshift surveys are also compiled. The simulations and mock catalogs will be made available electronically as bench marks for testing reconstruction methods.

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arXiv1994

Homogeneous Velocity-Distance Data for Peculiar Velocity Analysis. I. Calibration of Cluster Samples

We have combined five Tully-Fisher (TF) redshift-distance samples for peculiar velocity analysis: the cluster data of Han, Mould and coworkers (1991-93, HM) and Willick (1991, W91CL), and the field data of Aaronson et al. (1992), Willick (1991), Courteau & Faber (1992), and Mathewson et al. (1992), totaling over 3000 spiral galaxies. We treat the cluster data in this paper, which is the first of a series; in Paper II we treat the field TF samples. These data are to be combined with elliptical data (e.g., Faber et al. 1989) to form the MARK III CATALOG OF GALAXY PECULIAR VELOCITIES, which we will present in Paper III. The catalog will be used as input for POTENT reconstruction of velocity and density fields, described in later papers, as well as for alternative velocity analyses. Our main goal in Papers I & II is to place the TF data onto a self-consistent system by (i) applying a uniform set of corrections to the raw observables, (ii) determining the TF slopes and scatters separately for each sample, and (iii) adjusting the TF zeropoints to ensure mutually consistent distances. The global zeropoint is set by the HM sample, chosen because of its depth and uniformity on the sky and its substantial overlap with each of the other samples. In this paper, we calibrate the ``forward'' and ``inverse'' TF relations for HM and W91CL. We study the selection criteria for these samples and correct for the resultant statistical biases. The bias corrections are validated by comparing forward and inverse cluster distances. We find that many sample clusters are better modeled as ``expanding'' than relaxed, which significantly affects the TF calibrations. Proper corrections for internal extinction are derived self-consistently from the data.

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