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Ue-Li Pen

Publications and source records attributed to Ue-Li Pen.

At least 271 records · Page 15Linked to original sources
arXiv2000

Spin induced galaxy alignments and their implications for weak lensing measurements

Large scale correlations in the orientations of galaxies can result from alignments in their angular momentum vectors. These alignments arise from the tidal torques exerted on neighboring proto-galaxies by the smoothly varying shear field. We compute the predicted amplitude of such ellipticity correlations using the Zel'dovich approximation for a realistic distribution of galaxy shapes. Weak gravitational lensing can also induce ellipticity correlations since the images of neighboring galaxies will be distorted coherently. On comparing these two effects that induce shape correlations, we find that for current weak lensing surveys with a median redshift of z_m = 1, the intrinsic signal is of order 1 - 10 percent of the measured signal. However, for shallower surveys with z_m < 0.3, the intrinsic correlations dominate over the lensing signal. The distortions induced by lensing are curl-free, whereas those resulting from intrinsic alignments are not. This difference can be used to disentangle these two sources of ellipticity correlations.

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arXiv2000

Strong Lensing Reconstruction

We present a general linear algorithm for measuring the surface mass density 1-κfrom the observable reduced shear g=γ/(1-κ) in the strong lensing regime. We show that in general, the observed polarization field can be decomposed into ``electric'' and ``magnetic'' components, which have independent and redundant solutions, but perfectly orthogonal noise properties. By combining these solutions, one can increase the signal-to-noise ratio by \sqrt{2}. The solutions allow dynamic optimization of signal and noise, both in real and Fourier space (using arbitrary smoothing windows). Boundary conditions have no effect on the reconstructions, apart from its effect on the signal-to-noise. Many existing reconstruction techniques are recovered as special cases of this framework. The magnetic solution has the added benefit of yielding the global and local parity of the reconstruction in a single step.

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arXiv2000

Cosmic Shear from Galaxy Spins

We discuss the origin of galactic angular momentum, and the statistics of the present day spin distribution. It is expected that the galaxy spin axes are correlated with the intermediate principal axis of the gravitational shear tensor. This allows one to reconstruct the shear field and thereby the full gravitational potential from the observed galaxy spin fields. We use the direction of the angular momentum vector without any information of its magnitude, which requires a measurement of the position angle and inclination on the sky of each disk galaxy. We present the maximum likelihood shear inversion procedure, which involves a constrained linear minimization. The theory is tested against numerical simulations. We find the correlation strength of nonlinear structures with the initial shear field, and show that accurate large scale density reconstructions are possible at the expected noise level.

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arXiv2000

Sunyaev-Zeldovich effect from hydrodynamical simulations: maps and low order statistics

We use moving mesh hydrodynamical simulations to make maps of Sunyaev-Zeldovich effect. We present these maps for several cosmological models and explore their lowest moments. We find that the first moment, the mean Compton $y$ parameter, is typically between $1-2\times 10^{-6}$ for cluster abundance normalized models, the lower value corresponding to the high density models, and scales as $σ_8^{3-4}$. Rms fluctuations at 10' scale have an amplitude $ ΔT/T \sim 1-3 \times 10^{-6}$ in the Rayleigh-Jeans regime. The amplitude of the power spectrum strongly depends on the power spectrum normalization and scales as $σ_8^{6-9}$. On smaller scales ($l>1000$) the spectrum is dominated by halos below $10^{13}M_{\sun}$ and so is sensitive to the thermal history of galactic halo gas. On larger scales ($l<1000$) the power spectrum is less sensitive to nongravitational energy injection, but becomes very noisy, with large field to field variations in the power spectrum caused by rare bright sources in the maps, which dominate in the spectrum over the large scale structure correlations. Cross-correlation power spectrum with weak lensing or projected galaxy distribution is significant and the cross-correlation coefficient is around 0.5 over a wide range of scales. Comparison with Press-Schechter predictions gives very good agreement for all the statistics, indicating that there is no significant contribution to SZ from non-virialized structures. The one point distribution function shows clear deviations from gaussianity and can be well-approximated as log-normal on small scales.

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arXiv1999

Application of Wavelets to Filtering of Noisy Data

I discuss approaches to optimally remove noise from images. A generalization of Wiener filtering to Non-Gaussian distributions and wavelets is described, as well as an approach to measure the errors in the reconstructed images. We argue that the wavelet basis is highly advantageous over either Fourier or real space analysis if the data is intermittent in nature, i.e. if the filling factor of objects is small.

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arXiv1999

Broken Defects

Recent indications of a neutrino mass raise the possibility that the dark matter may be hot, which would be a severe challenge for structure formation theory. We argue that generic defect theories fare very well in hot dark matter cosmologies, and also work in the presence of curvature or a cosmological constant. If this model is correct, then the MAP and PLANCK missions will not measure what people expect them to (oscillations); rather, they will measure a broad hump.

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arXiv1998

Heating of the IGM

Using the cosmic virial theorem, Press-Schechter analysis and numerical simulations, we compute the expected X-ray background (XRB) from the diffuse IGM with the clumping factor expected from gravitational shock heating. The predicted fluxes and temperatures are excluded from the observed XRB. The predicted clumping can be reduced by entropy injection. The required energy is computed from the two-point correlation function, as well as from Press-Schechter formalisms. The minimal energy injection of 1 keV/nucleon excludes radiative or gravitational heating as a primary energy source. We argue that the intergalactic medium (IGM) must have been heated through violent processes such as massive supernova bursts. If the heating proceeded through supernova explosions, it likely proceeded in bursts which may be observable in high redshift supernova searches. Within our model we reproduce the observed cluster luminosity-temperature relation with energy injection of 1 keV/nucleon if this injection is assumed to be uncorrelated with the local density. These parameters predict that the diffuse IGM soft XRB has a temperature of ~1 keV with a flux near 10 keV/cm^2 s str keV, which may be detectable in the near future.

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arXiv1998

Topological Defects with Broken Scale Invariance

Defect models have recently been declared dead (Watson 1997), because they predict microwave background and matter fluctuations grossly out of line with what we see. In this talk we apply the fact that many defects are automatically destroyed at the time of radiation-matter transition, thus resurrecting the defects model. Moreover, the resurrected version predicts a cosmological constant, explains the apparent excess of hot clusters and the non-Gaussianity observed in galaxy surveys. If this model is correct, then the MAP and PLANCK missions will not measure what people expect them to (oscillations); rather, they will measure a broad hump.

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arXiv1998

Ω_0 from the Apparent Evolution of Gas Fractions in Rich Clusters of Galaxies

We apply a unique gas fraction estimator to published X-ray cluster properties and we compare the derived gas fractions of observed clusters to simulated ones. The observations are consistent with a universal gas fraction of 0.15+/-0.01h_{50}^{-3/2} for the low redshift clusters that meet our selection criteria. The fair sampling hypothesis states that all clusters should have a universal constant gas fraction for all times. Consequently, any apparent evolution would most likely be explained by an incorrect assumption for the angular-diameter distance relation. We show that the high redshift cluster data is consistent with this hypothesis for Ω_0<0.63 (95% formal confidence, flat $Λ$ model) or Ω_0<0.60 (95% formal confidence, hyperbolic open model). The maximum likelihood occurs at Ω_0=0.2 for a spatially flat cosmological constant model.

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arXiv1997

Gamma Ray Bursts from Baryon Decay in Neutron Stars

The standard unbroken electroweak theory is known to erase baryon number. The baryon number symmetry can be restored in the core of a neutron star as its density diverges via gravitational instability due to a binary merger event. We argue that for certain double Higgs models with discrete symmetries, this process may result in an expanding self-sustained burning front which would convert the entire neutron matter into radiation. This process would release ~10^{54} ergs in electromagnetic radiation over ~10^{-4} sec, with negligible baryonic contamination. The resulting fireball would have all the properties necessary to produce a gamma-ray burst as a result of its interaction with ambient interstellar gas. The subsequent Higgs decay would produce a millisecond burst of ~10^{52} ergs in ~100 GeV neutrinos which might be observable. The above mechanism may have also caused electroweak baryogenesis in the early universe, giving rise to the observed matter-antimatter asymmetry today.

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arXiv1997

Normalizing the Temperature Function of Clusters of Galaxies

We re-examine the constraints which can be robustly obtained from the observed temperature function of X-ray cluster of galaxies. The cluster mass function has been thoroughly studied in simulations and analytically, but a direct simulation of the temperature function is presented here for the first time. Adaptive hydrodynamic simulations using the cosmological Moving Mesh Hydro code of Pen (1997a) are used to calibrate the temperature function for different popular cosmologies. Applying the new normalizations to the present-day cluster abundances, we find $σ_8=0.53\pm 0.05 Ω_0^{-0.45}$ for a hyperbolic universe, and $σ_8=0.53\pm 0.05 Ω_0^{-0.53}$ for a spatially flat universe with a cosmological constant. The simulations followed the gravitational shock heating of the gas and dark matter, and used a crude model for potential energy injection by supernova heating. The error bars are dominated by uncertainties in the heating/cooling models. We present fitting formulae for the mass-temperature conversions and cluster abundances based on these simulations.

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arXiv1997

Reconstructing Nonlinear Stochastic Bias from Velocity Space Distortions

We propose a strategy to measure the dark matter power spectrum using minimal assumptions about the galaxy distribution and the galaxy-dark matter cross-correlations. We argue that on large scales the central limit theorem generically assures Gaussianity of each smoothed density field, but not coherence. Asymptotically, the only surviving parameters on a given scale are galaxy variance $σ$, bias $b=Ω^{.6}/β$ and the galaxy-dark matter correlation coefficient $r$. These can all be determined by measuring the quadrupole and octupole velocity distortions in the power spectrum. Measuring them simultaneously may restore consistency between all $β$ determinations independent of galaxy type. The leading deviations from Gaussianity are conveniently parameterized by an Edgeworth expansion. In the mildly non-linear regime, two additional parameters describe the full picture: the skewness parameter $s$ and non-linear bias $b_2$. They can both be determined from the measured skewness combined with second order perturbation theory or from an N-body simulation. By measuring the redshift distortion of the skewness, one can measure the density parameter $Ω$ with minimal assumptions about the galaxy formation process. This formalism also provides a convenient parametrization to quantify statistical galaxy formation properties.

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arXiv1997

Generating Cosmological Gaussian Random Fields

We present a generic algorithm for generating Gaussian random initial conditions for cosmological simulations on periodic rectangular lattices. We show that imposing periodic boundary conditions on the real-space correlator and choosing initial conditions by convolving a white noise random field results in a significantly smaller error than the traditional procedure of using the power spectrum. This convolution picture produces exact correlation functions out to separations of L/2, where L is the box size, which is the maximum theoretically allowed. This method also produces tophat sphere fluctuations which are exact at radii $ R \le L/4 $. It is equivalent to windowing the power spectrum with the simulation volume before discretizing, thus bypassing sparse sampling problems. The mean density perturbation in the volume is no longer constrained to be zero, allowing one to assemble a large simulation using a series of smaller ones. This is especially important for simulations of Lyman-$α$ systems where small boxes with steep power spectra are routinely used. We also present an extension of this procedure which generates exact initial conditions for hierarchical grids at negligible cost.

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arXiv1997

The Scalar, Vector and Tensor Contributions to CMB anisotropies from Cosmic Defects

Recent work has emphasised the importance of vector and tensor contributions to the large scale microwave anisotropy fluctuations produced by cosmic defects. In this paper we provide a general discussion of these contributions, and how their magnitude is constrained by the fundamental assumptions of causality, scaling, and statistical isotropy. We discuss an analytic model which illustrates and explains how the ratios of isotropic and anisotropic scalar, vector and tensor microwave anisotropies are determined. This provides a check of the results from large scale numerical simulations, confirming the numerical finding that vector and tensor modes provide substantial contributions to the large angle anisotropies. This leads to a suppression of the scalar normalisation and consequently of the Doppler peaks.

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arXiv1997

A High-resolution Adaptive Moving Mesh Hydrodynamic Algorithm

An algorithm for simulating self-gravitating cosmological astrophysical fluids is presented. The advantages include a large dynamic range, parallelizability, high resolution per grid element and fast execution speed. The code is based on a finite volume flux conservative Total-Variation-Diminishing (TVD) scheme for the shock capturing hydro, and an iterative multigrid solver for the gravity. The grid is a time dependent field, whose motion is described by a generalized potential flow. Approximately constant mass per cell can be obtained, providing all the advantages of a Lagrangian scheme. The grid deformation combined with appropriate limiting and smoothing schemes guarantees a regular and well behaved grid geometry, where nearest neighbor relationships remain constant. The full hydrodynamic fluid equations are implemented in the curvilinear moving grid, allowing for arbitrary fluid flow relative to the grid geometry. This combination retains all the advantages of the grid based schemes including high speed per fluid element and a rapid gravity solver. The current implementation is described, and empirical simulation results are presented. Accurate execution speed calculations are given in terms of floating point operations per time step per grid cell. This code is freely available to the community.

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arXiv1997

Polarization of the Microwave Background in Defect Models

We compute the polarization power spectra for global strings, monopoles, textures and nontopological textures, and compare them to inflationary models. We find that topological defect models predict a significant (1 microK) contribution to magnetic type polarization on degree angular scales, which is produced by the large vector component of the defect source. We also investigate the effect of decoherence on polarization. It leads to a smoothing of acoustic oscillations both in temperature and polarization power spectra and strongly suppresses the cross-correlation between temperature and polarization relative to inflationary models. Presence or absence of magnetic polarization or cross-correlation would be a strong discriminator between the two theories of structure formation and will be testable with the next generation of CMB satellites.

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arXiv1997

Power Spectra in Global Defect Theories of Cosmic Structure Formation

An efficient technique for computing perturbation power spectra in field ordering theories of cosmic structure formation is introduced, enabling computations to be carried out with unprecedented precision. Large scale simulations are used to measure unequal time correlators of the source stress energy, taking advantage of scaling during matter and radiation domination, and causality, to make optimal use of the available dynamic range. The correlators are then re-expressed in terms of a sum of eigenvector products, a representation which we argue is optimal, enabling the computation of the final power spectra to be performed at high accuracy. Microwave anisotropy and matter perturbation power spectra for global strings, monopoles, textures and non-topological textures are presented and compared with recent observations.

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