Reconstructed Density Field Research Articles

Nonlinear Structure Formation and the Acoustic Scale

We present high signal-to-noise ratio measurements of the acoustic scale in the presence of nonlinear growth and redshift distortions using 320 h−3 Gpc 3 of cosmological particle-mesh simulations. Using simple fitting methods, we obtain robust measurements of the acoustic scale with scatter close to that predicted by the Fisher matrix. We detect and quantify the shift in the acoustic scale by analyzing the power spectrum: we detect at greater than 5 σ a decrease in the acoustic scale in the real-space matter power spectrum of 0.2% at z = 1.5, growing to 0.45% at z = 0.3. In redshift space, the shifts are about 25% larger: we detect a decrease of 0.25% at z = 1.5 and 0.54% at z = 0.3. Despite the nonzero amounts, these shifts are highly predictable numerically, and hence removable within the standard ruler analysis of clustering data. Moreover, we show that a simple density field reconstruction method substantially reduces the scatter and nonlinear shifts of the acoustic scale measurements: the shifts are reduced to less than 0.1% at z = 0.3–1.5, even in the presence of nonnegligible shot noise. Finally, we show that the ratio of the cosmological distance to the sound horizon that would be inferred from these fits is robust to variations in the parameterization of the fitting method and reasonable differences in the template cosmology.

Large-scale structure in the H I Parkes All-Sky Survey: filling the voids with H I galaxies?

We estimate the two-point correlation function in redshift space of the recently compiled HI Parkes All-Sky Survey neutral hydrogen (HI) sources catalogue, which if modelled as a power law, xi(r) = (r(0)/r)(gamma), the best-fitting parameters for the HI selected galaxies are found to be r(0) = 3.3 +/- 0.3 h(-1) Mpc with gamma = 1.38 +/- 0.24. Fixing the slope to its universal value gamma = 1.8, we obtain r(0) = 3.2 +/- 0.2 h(-1) Mpc. Comparing the measured two-point correlation function with the predictions of the concordance cosmological model (Omega(Lambda) = 0.74), we find that at the present epoch the HI selected galaxies are antibiased with respect to the underlying matter fluctuation field with their bias value being b(0) similar or equal to 0.68. Furthermore, dividing the HI galaxies into two richness subsamples we find that the low-mass HI galaxies have a very low present bias factor (b(0) similar or equal to 0.48), while the high-mass HI galaxies trace the underlying matter distribution as the optical galaxies (b(0) similar or equal to 1). Using our derived present-day HI galaxy bias we estimate their redshift-space distortion parameter, and correct accordingly the correlation function for peculiar motions. The resulting real-space correlation length is r(0)(re) = 1.8 +/- 0.2 h(-1) Mpc r(0)(re) = 3.9 +/- 0.6 h(-1) Mpc for the low- and high-mass HI galaxies, respectively. The low-mass HI galaxies appear to have the lowest correlation length among all extragalactic populations studied to date. In order to corroborate these results we have correlated the IRAS-Point Source Catalogue for Redshift (PSCz) reconstructed density field, smoothed over scales of 5 h(-1) Mpc, with the positions of the HI galaxies, to find that indeed the HI galaxies are typically found in negative overdensity regions (delta rho/rho(PSCz) less than or similar to 0), even more so the low-mass HI galaxies. Finally, we also study the redshift evolution of the HI galaxy linear bias factor and find that the HI-galaxy population is antibiased up to z similar to 1.3. While at large redshifts similar to 3, we predict that the HI galaxies are strongly biased. Our bias evolution predictions are consistent with the observational bias results of Ly alpha galaxies.

Annual Rossby waves in the Arabian Sea from TOPEX/POSEIDON altimeter and in situ data

Sea-surface height data acquired by the TOPEX/POSEIDON satellite over the Arabian Sea from October 1992 to October 1998 are analyzed. Strong seasonal fluctuations are found between 61 and 101N, which are mainly associated with westward propagating annual Rossby waves radiated from the western side of the Indian subcontinent and that are continuously forced by the action of the wind-stress curl over the central Arabian Sea. An analysis of hydrographic data acquired during August 1993 and during January 1998 at 81N in the Arabian Sea reveals the existence of first- and second-mode annual Rossby waves. These waves, which can be traced as perturbations in the density fields, have wavelengths of 12 � 10 3 and 4.4 � 10 3 km as well as phase velocities of 0.38 and 0.14 m/s, respectively. The waves are associated with a time-dependent meridional overturning cell that sloshes water northward and southward. Between 581 and 681E in the central Arabian Sea, we found a Rossby-wave induced transport in the upper 500 m of about 10 Sv southward in August 1993 and northward in January 1998. Below 2000 m, there was still a northward transport of 3.2 Sv in August 1993 and a southward transport of 4.8 Sv in January 1998. A comparison of steric height differences between August 1993 and January 1998 calculated from the observed density fields as well as calculated from the reconstructed density fields using first- and second-mode annual Rossby waves agree quite well with the corresponding sea-surface height differences. Implications resulting from the reflection of annual Rossby waves, like fluctuations of the western boundary currents, are discussed. r 2002 Elsevier Science Ltd. All rights reserved.

Using Perturbative Least Action to Reconstruct Redshift‐Space Distortions

In this paper, we present a redshift space reconstruction scheme which is analogous to and extends the Perturbative Least Action (PLA) method described by Goldberg & Spergel (2000). We first show that this scheme is effective in reconstructing even nonlinear observations. We then suggest that by varying the cosmology to minimize the quadrupole moment of a reconstructed density field, it may be possible to lower the errorbars on the redshift distortion parameter, $\beta$ as well as to break the degeneracy between the linear bias parameter, $b$, and $\Omega_M$. Finally, we discuss how PLA might be applied to realistic redshift surveys.

The importance of Fourier phases for the morphology of gravitational clustering

The phases of the Fourier modes appearing in a plane-wave expansion of cosmological density fields play a vital role in determining the morphology of gravitationally-developed clustering. We demonstrate this qualitatively and quantitatively using simulations. In particular, we use cross-correlation and rank-correlation techniques to quantify the agreement between a simulated distribution and phase-only reconstructions. The phase-only reconstructions exhibit a high degree of correlation with the original distributions, showing how meaningful spatial reconstruction of cosmological density fields depends more on phase accuracy than on amplitudes.

The Universe behind the Milky Way

Due to the foreground extinction of the Milky Way, galaxies appear increasingly fainter the closer they lie to the Galactic Equator, creating a “zone of avoidance” of about 25% in the distribution of optically visible galaxies. A “whole-sky” map of galaxies is essential, however, for understanding the dynamics in our local Universe, in particular the peculiar velocity of the Local Group with respect to the Cosmic Microwave Background and velocity flow fields such as in the Great Attractor region. Various dynamically important structures behind the Milky Way have only recently been made “visible” through dedicated deep surveys at various wavelengths. The wide range of observational searches (optical, near infrared, far infrared, radio and X-ray) for galaxies in the Zone of Avoidance are reviewed, including a discussion on the limitations and selection effects of these partly complementary approaches. The uncovered and suspected large-scale structures are summarized. Reconstruction methods of the density field in the Zone of Avoidance are described and the resulting predictions compared with observational evidence. The comparison between reconstructed density fields and the observed galaxy distribution allow derivations of the density and biasing parameters $\Omega_0$ and b.

Reconstruction of cosmological initial conditions from galaxy redshift catalogues

We present and test a new method for the reconstruction of cosmological initial conditions from a full-sky galaxy catalogue. This method, called ZTRACE, is based on a self-consistent solution of the growing mode of gravitational instabilities according to the Zel'dovich approximation and higher order in Lagrangian perturbation theory. Given the evolved redshift-space density field, smoothed on some scale, ZTRACE finds via an iterative procedure, an approximation to the initial density field for any given set of cosmological parameters; real-space densities and peculiar velocities are also reconstructed. The method is tested by applying it to N-body simulations of an Einstein-de Sitter and an open cold dark matter universe. It is shown that errors in the estimate of the density contrast dominate the noise of the reconstruction. As a consequence, the reconstruction of real space density and peculiar velocity fields using non-linear algorithms is little improved over those based on linear theory. The use of a mass-preserving adaptive smoothing, equivalent to a smoothing in Lagrangian space, allows an unbiased (although noisy) reconstruction of initial conditions, as long as the (linearly extrapolated) density contrast does not exceed unity. The probability distribution function of the initial conditions is recovered to high precision, even for Gaussian smoothing scales of ~ 5 Mpc/h, except for the tail at delta >~ 1. This result is insensitive to the assumptions of the background cosmology.

The inverse redshift-space operator: reconstructing cosmological density and velocity fields

We present the linear inverse redshift-space operator that maps the galaxy density field derived from redshift surveys from redshift space to real space. Expressions are presented for observers in both the cosmic microwave background radiation and Local Group rest frames. We show how these results can be generalized to flux-limited galaxy redshift surveys. These results allow the straightforward reconstruction of real-space density and velocity fields without resort to iterative or numerically intensive inverse methods. As a corollary to the inversion of the density in the Local Group rest frame we present an expression for estimating the real-space velocity dipole from redshift space, allowing one to estimate the Local Group dipole without full reconstruction of the redshift survey. We test these results on some simple models and find that the reconstruction is very accurate. A new spherical harmonic representation of the redshift distortion and its inverse is developed, which simplifies the reconstruction and allows analytic calculation of the properties of the reconstructed redshift survey. We use this representation to analyse the uncertainties in the reconstruction of the density and velocity fields from redshift space, caused by only a finite volume being available. Both sampling and shot-noise variance terms are derived and we discuss the limits of reconstruction analysis. We compare the reconstructed velocity field with the true velocity field and show that reconstruction in the Local Group rest frame is preferable, since this eliminates the major source of uncertainty from the dipole mode. These results can be used to transform redshift surveys to real space and may be used as part of a full likelihood analysis to extract cosmological parameters.

Overview of Uncovered and Suspected Large-scale Structures behind the Milky Way

AbstractVarious dynamically important extragalactic large-scale structures in the local Universe lie behind the Milky Way. Most of these structures (predicted and unexpected) have only recently been made ‘visible’ through dedicated deep surveys at various wavelengths. The wide range of observational searches (optical, near infrared, far infrared, radio and X-ray) for galaxies in the Zone of Avoidance (ZOA) will be reviewed and the uncovered and suspected large-scale structures summarised. Particular emphasis is given to the Great Attractor region where the existence of yet another cluster is suspected (Woudt 1998). Predictions from reconstructions of the density field in the ZOA are discussed and compared with observational evidence. Although no major structures are predicted out to about v ≲ 10,000 km s−1 for which no observational evidence exists, the comparison between reconstructed density fields and the observed galaxy distribution remain important as they allow derivations of the density and biasing parameters.

C504 Tomographic Reconstruction of Density Fields Using Modified Algebraic Reconstruction Technique for Laser Speckle Photography Projections

The reconstructed images from a tomographic reconstruction technique such as an algebraic reconstruction technique (ART) are examined for laser speckle photography. Because of non-algebraic characteristics of line-of-sight projections of speckle photography, the ART is modified to solve this problem. The modified ART converts non-algebraic speckle photographic projection data into algebraic interferometric projections so that the linear ART can proceed. The modified ART of the speckle photography shows good reconstruction results for both of computer synthesized phantom fields and asymmetric helium jets in experiments although it requires an additional step of integration.

Reconstruction of cosmological density and velocity fields in the Lagrangian Zel'dovich approximation

We present a method for reconstructing cosmological densityn for and velocity fields using the Lagrangian Zel'dovich formalism. . The method involves finding the least action solution for straight line particle paths in an evolving density field. Our starting point is the final, evolved density , so that we are in effect carrying out the standard Zel'dovich Approximation based process in reverse. Using a simple numerical algorithm we are able to minimise the action for the trajectories of several million particles. We apply our method to the evolved density taken from N-body simulations of different cold dark matter dominated universes, testing both the prediction for the present day velocity field and for the initial density field. The method is easy to apply, reproduces the accuracy of the forward Zel'dovich Approximation, and also works directly in redshift space with minimal modification.

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.

Wiener reconstruction of density, velocity and potential fields from all-sky galaxy redshift surveys

We present a new method for recovering the cosmological density, velocity, and potential fields from all-sky redshift catalogues. The method is based on an expansion of the fields in orthogonal radial (Bessel) and angular (spherical harmonic) functions. In this coordinate system, peculiar velocities introduce a coupling of the radial harmonics describing the density field in redshift space but leave the angular modes unaffected. In the harmonic transform space, this radial coupling is described by a distortion matrix which can be computed analytically within the context of linear theory; the redshift space harmonics can then be converted to their real space values by inversion of this matrix. Statistical noise is mitigated by regularizing the matrix inversion with a Wiener filter. The method yields a minimum variance estimate of the density field in real space. In this coordinate system, the minimum variance harmonics of the peculiar velocity and potential fields are related to those of the density field by simple linear transformations. Tests of the method with simulations of a CDM universe and comparison with previously proposed methods demonstrate it to be a very promising new reconstruction method for the local density and velocity field. A first application to the 1.2 Jy IRAS redshift survey is presented.

The choice of a dynamico-stochastic model for ocean monitoring — A compromise between the accuracy and cost of computations

In this paper we compare, two dynamico-stochastic models (DSMs) with different hydrodynamic parts, designed to study a large-scale North Atlantic region. As a performance criterion, RMS errors (over the horizon) in density and velocity field reconstruction are used. It has been shown that in some cases it seems rational to use DSMs with simplified hydrodynamic parts. This helps to expedite the computations and to reduce the related costs without much detriment to the quality of hydrophysical field simulation.

Holographic interferometry of transparent media using light scattered by embedded test objects

Fringe formation and localization in holographic interferometry of transparent media are discussed for configurations in which light enters the medium and is scattered back through it by an embedded diffuse object. Fringe order numbers are doubled, and the fringe localization region is translated and compressed by a factor of 2. The results are applicable to tomographic reconstruction of aerodynamic density fields around opaque test objects.