10,178 research outputs found
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.Comment: 72 pages including 33 figures, submitted to Ap
Converted wave imaging and velocity analysis using elastic reverse-time migration
Master's thesis in petroleum geosciences engineeringAlong the continuous evolution of exploration seismology, the main objective has been producing better subsurface seismic images that lead to lower risk exploration and enhanced production. The unique characteristics of converted (P-S) waves enable retrieving more accurate subsurface information, which made it play a complementary role in hydrocarbon seismic exploration, where the primary method of conventional compressional wave (P-P) data has limited capabilities. Conventional processing techniques of P-S data are based on approximations that do not respect the elastic nature of the subsurface and the vector nature of the recorded wave-fields, which urge the need for accurate modeling of subsurface velocity fields, and elastic imaging algorithm that can overcome the shortcomings following the conventional approximations. In this thesis we presented a novel workflow for accurate depth imaging and velocity analysis for multicomponent data. The workflow is based on elastic reverse-time migration as a robust migration algorithm, and automatic wave equation migration velocity analysis techniques. We practically tested novel imaging conditions for elastic reverse-time migration in order to overcome the polarity reversal problem and investigated the cross-talking between wave-modes. For velocity analysis we applied stack-power maximization to produce improved velocity fields that enhance the image coherency, then we applied co-depthing technique based on novel Born modeling/demigration method and target image fitting procedure in order to produce the shear-wave velocity model that result in depth consistent P-S and P-P images. We successfully implemented the workflow on synthetic and field datasets. The results obtained show the robustness and practicality of the workflow to produce enhanced velocity models and accurate subsurface elastic images
Observational biases in Lagrangian reconstructions of cosmic velocity fields
Lagrangian reconstruction of large-scale peculiar velocity fields can be
strongly affected by observational biases. We develop a thorough analysis of
these systematic effects by relying on specially selected mock catalogues. For
the purpose of this paper, we use the MAK reconstruction method, although any
other Lagrangian reconstruction method should be sensitive to the same
problems. We extensively study the uncertainty in the mass-to-light assignment
due to luminosity incompleteness, and the poorly-determined relation between
mass and luminosity. The impact of redshift distortion corrections is analyzed
in the context of MAK and we check the importance of edge and finite-volume
effects on the reconstructed velocities. Using three mock catalogues with
different average densities, we also study the effect of cosmic variance. In
particular, one of them presents the same global features as found in
observational catalogues that extend to 80 Mpc/h scales. We give recipes,
checked using the aforementioned mock catalogues, to handle these particular
observational effects, after having introduced them into the mock catalogues so
as to quantitatively mimic the most densely sampled currently available galaxy
catalogue of the nearby universe. Once biases have been taken care of, the
typical resulting error in reconstructed velocities is typically about a
quarter of the overall velocity dispersion, and without significant bias. We
finally model our reconstruction errors to propose an improved Bayesian
approach to measure Omega_m in an unbiased way by comparing the reconstructed
velocities to the measured ones in distance space, even though they may be
plagued by large errors. We show that, in the context of observational data, a
nearly unbiased estimator of Omega_m may be built using MAK reconstruction.Comment: 29 pages, 21 figures, 6 tables, Accepted by MNRAS on 2007 October 2.
Received 2007 September 30; in original form 2007 July 2
Measurability of kinetic temperature from metal absorption-line spectra formed in chaotic media
We present a new method for recovering the kinetic temperature of the
intervening diffuse gas to an accuracy of 10%. The method is based on the
comparison of unsaturated absorption-line profiles of two species with
different atomic weights. The species are assumed to have the same temperature
and bulk motion within the absorbing region. The computational technique
involves the Fourier transform of the absorption profiles and the consequent
Entropy-Regularized chi^2-Minimization [ERM] to estimate the model parameters.
The procedure is tested using synthetic spectra of CII, SiII and FeII ions. The
comparison with the standard Voigt fitting analysis is performed and it is
shown that the Voigt deconvolution of the complex absorption-line profiles may
result in estimated temperatures which are not physical. We also successfully
analyze Keck telescope spectra of CII1334 and SiII1260 lines observed at the
redshift z = 3.572 toward the quasar Q1937--1009 by Tytler {\it et al.}.Comment: 25 pages, 6 Postscript figures, aaspp4.sty file, submit. Ap
MECI: A Method for Eclipsing Component Identification
We describe an automated method for assigning the most probable physical
parameters to the components of an eclipsing binary, using only its photometric
light curve and combined colors. With traditional methods, one attempts to
optimize a multi-parameter model over many iterations, so as to minimize the
chi-squared value. We suggest an alternative method, where one selects pairs of
coeval stars from a set of theoretical stellar models, and compares their
simulated light curves and combined colors with the observations. This approach
greatly reduces the parameter space over which one needs to search, and allows
one to estimate the components' masses, radii and absolute magnitudes, without
spectroscopic data. We have implemented this method in an automated program
using published theoretical isochrones and limb-darkening coefficients. Since
it is easy to automate, this method lends itself to systematic analyses of
datasets consisting of photometric time series of large numbers of stars, such
as those produced by OGLE, MACHO, TrES, HAT, and many others surveys.Comment: 25 pages, 7 figures, accepted for publication in Ap
The Frontier Fields Lens Modeling Comparison Project
Gravitational lensing by clusters of galaxies offers a powerful probe of
their structure and mass distribution. Deriving a lens magnification map for a
galaxy cluster is a classic inversion problem and many methods have been
developed over the past two decades to solve it. Several research groups have
developed techniques independently to map the predominantly dark matter
distribution in cluster lenses. While these methods have all provided
remarkably high precision mass maps, particularly with exquisite imaging data
from the Hubble Space Telescope (HST), the reconstructions themselves have
never been directly compared. In this paper, we report the results of comparing
various independent lens modeling techniques employed by individual research
groups in the community. Here we present for the first time a detailed and
robust comparison of methodologies for fidelity, accuracy and precision. For
this collaborative exercise, the lens modeling community was provided simulated
cluster images -- of two clusters Ares and Hera -- that mimic the depth and
resolution of the ongoing HST Frontier Fields. The results of the submitted
reconstructions with the un-blinded true mass profile of these two clusters are
presented here. Parametric, free-form and hybrid techniques have been deployed
by the participating groups and we detail the strengths and trade-offs in
accuracy and systematics that arise for each methodology. We note in conclusion
that lensing reconstruction methods produce reliable mass distributions that
enable the use of clusters as extremely valuable astrophysical laboratories and
cosmological probes.Comment: 38 pages, 25 figures, submitted to MNRAS, version with full
resolution images can be found at
http://pico.bo.astro.it/~massimo/papers/FFsims.pd
Measuring the galaxy power spectrum with multiresolution decomposition -- II. diagonal and off-diagonal power spectra of the LCRS galaxies
The power spectrum estimator based on the discrete wavelet transform (DWT)
for 3-dimensional samples has been studied. The DWT estimator for
multi-dimensional samples provides two types of spectra with respect to
diagonal and off-diagonal modes, which are very flexible to deal with
configuration-related problems in the power spectrum detection. With simulation
samples and mock catalogues of the Las Campanas redshift survey (LCRS), we show
(1) the slice-like geometry of the LCRS doesn't affect the off-diagonal power
spectrum with ``slice-like'' mode; (2) the Poisson sampling with the LCRS
selection function doesn't cause more than 1- error in the DWT power
spectrum; and (3) the powers of peculiar velocity fluctuations, which cause the
redshift distortion, are approximately scale-independent. These results insure
that the uncertainties of the power spectrum measurement are under control. The
scatter of the DWT power spectra of the six strips of the LCRS survey is found
to be rather small. It is less than 1- of the cosmic variance of mock
samples in the wavenumber range h Mpc. To fit the detected
LCRS diagonal DWT power spectrum with CDM models, we find that the best-fitting
redshift distortion parameter is about the same as that obtained from
the Fourier power spectrum. The velocity dispersions for SCDM and
CDM models are also consistent with other detections with
the LCRS. A systematic difference between the best-fitting parameters of
diagonal and off-diagonal power spectra has been significantly measured. This
indicates that the off-diagonal power spectra are capable of providing
information about the power spectrum of galaxy velocity field.Comment: AAS LaTeX file, 41 pages, 10 figures included, accepted for
publication in Ap
A Neural Model of How the Brain Computes Heading from Optic Flow in Realistic Scenes
Animals avoid obstacles and approach goals in novel cluttered environments using visual information, notably optic flow, to compute heading, or direction of travel, with respect to objects in the environment. We present a neural model of how heading is computed that describes interactions among neurons in several visual areas of the primate magnocellular pathway, from retina through V1, MT+, and MSTd. The model produces outputs which are qualitatively and quantitatively similar to human heading estimation data in response to complex natural scenes. The model estimates heading to within 1.5° in random dot or photo-realistically rendered scenes and within 3° in video streams from driving in real-world environments. Simulated rotations of less than 1 degree per second do not affect model performance, but faster simulated rotation rates deteriorate performance, as in humans. The model is part of a larger navigational system that identifies and tracks objects while navigating in cluttered environments.National Science Foundation (SBE-0354378, BCS-0235398); Office of Naval Research (N00014-01-1-0624); National-Geospatial Intelligence Agency (NMA201-01-1-2016
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