arXiv subjects

Ue-Li Pen

Publications and source records attributed to Ue-Li Pen.

At least 235 records · Page 13Linked to original sources

arXiv2005

Mapping dark matter with cosmic magnification

We develop a new tool to generate statistically precise dark matter maps from the cosmic magnification of galaxies with distance estimates. We show how to overcome the intrinsic clustering problem using the slope of the luminosity function, because magnificability changes strongly over the luminosity function, while intrinsic clustering only changes weakly. This may allow precision cosmology beyond most current systematic limitations. SKA is able to reconstruct projected matter density map at smoothing scale $\sim 10^{'}$ with S/N$\geq 1$, at the rate of 200-4000 deg$^2$ per year, depending on the abundance and evolution of 21cm emitting galaxies. This power of mapping dark matter is comparable to, or even better than that of cosmic shear from deep optical surveys or 21cm surveys.

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arXiv2005

Reconstruction of the One-point Distribution of Convergence from Weak Lensing by Large-scale Structure

Weak lensing measurements are starting to provide statistical maps of the distribution of matter in the universe that are increasingly precise and complementary to cosmic microwave background maps. The probability distribution (PDF) provides a powerful tool to test non-Gausian features in the convergence field and to discriminate the different cosmological models. In this letter, we present a new PDF space Wiener filter approach to reconstruct the probability density function of the convergence from the noisy convergence field. We find that for parameters comparable to the CFHT legacy survey, the averaged PDF of the convergence in a 3 degree field can be reconstructed with an uncertainty of about 10%, even though the pointwise PDF is noise dominated.

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arXiv2005

Observational Evidence for Extra Dimensions from Dark Matter

Recent astronomical observations of systems of dark matter, which have been cited as providing possible support for self-interacting cold dark matter, may provide evidence for the extra dimensions predicted by superstring scenarios. We find that the properties of the required dark matter self-interaction are precisely the consequences of a world with 3 large extra dimensions of size \~1nm, where gravity follows the r^{-5} law at scales below ~1nm. From the cross sections measured for various dark matter systems, we also constrain the mass of dark matter particles to be m_x ~ 3*10^{-16} proton mass, consistent with the mass of axions.

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arXiv2005

Rotation in Gravitational Lenses

Gravitational lensing deflects light. A single lens deflector can only shear images, but cannot induce rotations. Multiple lens planes can induce rotations. Such rotations can be observed in quadruply imaged sources, and can be used to distinguish between two proposed solutions of the flux anomaly problem: substructures in lensing galaxies vs large scale structure. We predict the expected amount of rotation due to large scale structure in strong lensing systems, and show how this effect can be measured using ~ mas VLBI astrometry of quadruple lenses with extended source structures. The magnitude of rotation is around one degree. The biggest theoretical uncertainty is the power spectrum of dark matter on very small scales. This procedure can potentially be turned around to measure the dark matter power spectrum on very small scales. We list the predicted RMS rotation angles for several quadruple lenses with known lens and source redshifts.

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arXiv2005

Towards optimal parallel PM N-body codes: PMFAST

We present a new parallel PM N-body code named PMFAST that is freely available to the public. PMFAST is based on a two-level mesh gravity solver where the gravitational forces are separated into long and short range components. The decomposition scheme minimizes communication costs and allows tolerance for slow networks. The code approaches optimality in several dimensions. The force computations are local and exploit highly optimized vendor FFT libraries. It features minimal memory overhead, with the particle positions and velocities being the main cost. The code features support for distributed and shared memory parallelization through the use of MPI and OpenMP, respectively. The current release version uses two grid levels on a slab decomposition, with periodic boundary conditions for cosmological applications. Open boundary conditions could be added with little computational overhead. We present timing information and results from a recent cosmological production run of the code using a 3712^3 mesh with 6.4 x 10^9 particles. PMFAST is cost-effective, memory-efficient, and is publicly available.

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arXiv2005

Ionization fronts and their interaction with density fluctuations: implications for reionization

The propagation of cosmological ionization fronts (I-fronts) during reionization is strongly influenced by small-scale structure. Here we summarize our recent attemps to understand the effect of this small-scale structure. We present high resolution cosmological N-body simulations at high-z (z>6) which resolve a wide range of halo mass, from minihalos to clusters of large, rare halos. We also study how minihalos affect I-fronts, through simulations of minihalo photoevaporation. Furthermore, we modify the I-front propagation equations to account for evolving small-scale structure, and incorporate these results into a semi-analytical reionization model. When intergalactic medium clumping and minihalo clustering around sources are included, small-scale structure affects reionization by slowing it down and extending it in time. This helps to explain observations by the Wilkinson Microwave Anisotropy Probe, which imply an early and extended reionization epoch. We also study how source clustering affects the evolution and size of H II regions, finding, in agreement with simulations, that H II regions usually expand, rarely shrinking. Hence, "relic H II regions'' are an exception, rather than the rule. When the suppression of small-mass sources in already-ionized regions by Jeans-mass filtering is accounted for, H II regions are smaller, delaying overlap. We also present a new numerical method for radiative transfer which is fast, efficient, and easily coupled to hydrodynamics and N-body codes, along with sample tests and applications.

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arXiv2005

Fast n-point correlation functions and three-point lensing application

We present a new algorithm to rapidly compute the two-point (2PCF), three-point (3PCF) and n-point (n-PCF) correlation functions in roughly O(N log N) time for N particles, instead of O(N^n) as required by brute force approaches. The algorithm enables an estimate of the full 3PCF for as many as 10^6 galaxies. This technique exploits node-to-node correlations of a recursive bisectional binary tree. A balanced tree construction minimizes the depth of the tree and the worst case error at each node. The algorithm presented in this paper can be applied to problems with arbitrary geometry. We describe the detailed implementation to compute the two point function and all eight components of the 3PCF for a two-component field, with attention to shear fields generated by gravitational lensing. We also generalize the algorithm to compute the n-point correlation function for a scalar field in k dimensions where n and k are arbitrary positive integers.

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arXiv2005

Searching for Early Ionization with the Primeval Structure Telescope

The Primeval Structure Telescope (PaST), will be used search for and study the era the of the first luminous objects, the epoch of reionization. The first stars ionized the gas around them producing a pattern of ionization that reflects the large scale density structure present at the time. The PaST array will be used in an attempt to sense and study this ionization, by mapping the brightness of 21-cm neutral hydrogen emission at redshifts from 6 to 25. This emission disappears on ionization, allowing the study of large scale structure and of star formation at this very early epoch. The 10,000 antenna PaST array will be used to image ionized structures by creating 1 million pixel images of the sky. The angular scales of the images to be produced span from 5 arc-minutes to 10 degrees. The array is currently under construction and over 2000 antennas have been installed.

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arXiv2004

The intergalactic medium temperature and Compton y parameter

(Abridged) The precision modeling of the thermal Sunyaev Zeldovich (SZ) effect and future high accuracy measurements will provide a powerful way to constrain the thermal history of the universe. In this paper, we combine high resolution adiabatic hydro simulations and analytical models to provide a precision modeling of the gas density weighted temperature $\bar{T}_g$ and the mean SZ Compton $y$ parameter. Simulation artifacts are quantified and corrected and our analytical model passes tests of various simulations and is able to predict $\bar{T}_g$ and $\bar{y}$ to several percent accuracy. For low matter density $Λ$CDM cosmology, the present $\bar{T}_g$ is $0.32 (σ_8/0.84)^{3.05-0.15Ω_m}(Ω_m/0.268)^{1.28-0.2σ_8} {\rm keV}$. The mean $y$ parameter is $2.6\times 10^{-6} (σ_8/0.84)^{4.1-2Ω_m}(Ω_m/0.268)^{1.28-0.2σ_8}$. The current upper limit of $y<1.5\times 10^{-5}$ measured by FIRAS has already ruled out combinations of high $σ_8\ga 1.1$ and high $Ω_m\ga 0.5$.

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arXiv2004

Beating Lensing Cosmic Variance with Galaxy Tomography

I discuss the use of cross correlations between galaxies with distance information and projected weak lensing dark matter maps to obtain a fully three dimensional dark matter map and power spectrum. On large scales l<100 one expects the galaxies to be biased, but not stochastic. I show that this allows a simultaneous solution of the full 3-D evolving galaxy bias and the dark matter power spectrum simultaneously. Within the photo-z information of the CFH lensing legacy survey, this allows a threefold reduction of statistical error, while a cross correlation with CLAR or other deep spectroscopic surveys allows a tenfold improvement in dark matter power accuracy on linear scales. This makes lensing surveys more sensitive to the cosmic equation of state and the neutrino masses.

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arXiv2004

Gravitational Lensing of Epoch-of-reionization Gas

I present a weak lensing sensitivity estimate for upcoming high redshift (epoch of reionization and beyond) 21cm surveys. Instruments such as PAST, LOFAR and SKA should be able to measure the weak lensing power spectrum to precisions far exceeding conceivable optical surveys. Three types of sources are detectable, which include the re-ionization stromgren spheres, large scale structure, and minihalos. Tomography allows the measurement of the time evolution of the dark matter power spectrum. Raw sensitivities allow measurement of many cosmological parameters, including dark energy, neutrino mass and cosmic equations of state, to percent accuracy. It also has the potential for inflationary gravity wave measurements. Ultimate limits may be achievable through radio observations of 10^18 minihalos. Inflationary Hubble parameters H_I down to 10^-9 M_Planck can be detected through this effect. Second order effects may also be observable, allowing tests for backreaction and the quantum mechanical origin of perturbations.

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arXiv2003

Precision era of the kinetic Sunyaev-Zeldovich effect: simulations, analytical models and observations and the power to constrain reionization

The kinetic SZ effect, which is the dominant CMB source at arc-minute scales and $ν\sim 217$ Ghz, probes the ionized gas peculiar momentum up to the epoch of reionization and is a sensitive measure of the reionization history. We ran high resolution self-similar and $Λ$CDM hydro simulations and built an analytical model to study this effect. Our model reproduces the $Λ$CDM simulation results to several percent accuracy, passes various tests against self-similar simulations, and shows a wider range of applicability than previous analytical models. Our model in its continuous version is free of simulation limitations such as finite simulation box and finite resolution and allows an accurate prediction of the kinetic SZ power spectrum $C_l$. For the WMAP cosmology, we find $l^2C_l/(2π)\simeq 0.91 \times 10^{-12} [(1+z_{\rm reion})/10]^{0.34}(l/5000)^{0.23-0.015(z_{\rm reion}-9)}$ for the reionization redshift $6<z_{\rm reion}<20$ and $3000<l<9000$. The corresponding temperature fluctuation is several $μ$K at these ranges. The dependence of $C_l$ on the reionization history allows an accurate measurement of the reionization epoch. For the Atacama cosmology telescope experiment, $C_l$ can be measured with $\sim 1%$ accuracy. $C_l$ scales as $(Ω_b h)^2 σ_8^{4\sim 6}$. Given cosmological parameters, ACT would be able to constrain $z_{\rm reion}$ with several percent accuracy. Some multi-reionization scenarios degenerate in the primary CMB temperature and TE measurement can be distinguished with $\sim 10 σ$ confidence.

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arXiv2003

A Moving Frame Algorithm for High Mach Number Hydrodynamics

We present a new approach to Eulerian computational fluid dynamics that is designed to work at high Mach numbers encountered in astrophysical hydrodynamic simulations. The Eulerian fluid conservation equations are solved in an adaptive frame moving with the fluid where Mach numbers are minimized. The moving frame approach uses a velocity decomposition technique to define local kinetic variables while storing the bulk kinetic components in a smoothed background velocity field that is associated with the grid velocity. Gravitationally induced accelerations are added to the grid, thereby minimizing the spurious heating problem encountered in cold gas flows. Separately tracking local and bulk flow components allows thermodynamic variables to be accurately calculated in both subsonic and supersonic regions. A main feature of the algorithm, that is not possible in previous Eulerian implementations, is the ability to resolve shocks and prevent spurious heating where both the preshock and postshock Mach numbers are high. The hybrid algorithm combines the high resolution shock capturing ability of the second-order accurate Eulerian TVD scheme with a low-diffusion Lagrangian advection scheme. We have implemented a cosmological code where the hydrodynamic evolution of the baryons is captured using the moving frame algorithm while the gravitational evolution of the collisionless dark matter is tracked using a particle-mesh N-body algorithm. The MACH code is highly suited for simulating the evolution of the IGM where accurate thermodynamic evolution is needed for studies of the Lyman alpha forest, the Sunyaev-Zeldovich effect, and the X-ray background. Hydrodynamic and cosmological tests are described and results presented. The current code is fast, memory-friendly, and parallelized for shared-memory machines.

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arXiv2003

The three dimensional power spectrum of dark and luminous matter from the VIRMOS-DESCART cosmic shear survey

We present the first optimal power spectrum estimation and three dimensional deprojections for the dark and luminous matter and their cross correlations. The results are obtained using a new optimal fast estimator (Pen 2003) deprojected using minimum variance and SVD techniques. We show the resulting 3-D power spectra for dark matter and galaxies, and their covariance for the VIRMOS-DESCART weak lensing shear and galaxy data. The survey is most sensitive to nonlinear scales k_NL ~ 1 h Mpc^-1. On these scales, our 3-D power spectrum of dark matter is in good agreement with the RCS 3-D power spectrum found by (Hoekstra et al 2002). Our galaxy power is similar to that found by the 2MASS survey, and larger than that of SDSS, APM and RCS, consistent with the expected difference in galaxy population. We find an average bias b=1.24+/-0.18 for the I selected galaxies, and a cross correlation coefficient r=0.75+/-0.23. Together with the power spectra, these results optimally encode the entire two point information about dark matter and galaxies, including galaxy-galaxy lensing. We address some of the implications regarding galaxy halos and mass-to-light ratios. The best fit ``halo'' parameter h=r/b=0.57+/-0.16, suggesting that dynamical masses estimated using galaxies systematically underestimate total mass. Ongoing surveys, such as the Canada-France-Hawaii-Telescope-Legacy survey will significantly improve on the dynamic range, and future photometric redshift catalogs will allow tomography along the same principles.

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arXiv2003

Optimal Weak Lensing Skewness Measurements

Weak lensing measurements are entering a precision era to statistically map the distribution of matter in the universe. The most common measurement has been of the variance of the projected surface density of matter, which corresponds to the induced correlation in alignments of background galaxies. This measurement of the fluctuations is insensitive to the total mass content, like using waves on the ocean to measure its depths. But when the depth is shallow as happens near a beach, waves become skewed. Similarly, a measurement of skewness in the projected matter distribution directly measures the total matter content of the universe. While skewness has already been convincingly detected, its constraint on cosmology is still weak. We address optimal analyses for the CFHT Legacy Survey in the presence of noise. We show that a compensated Gaussian filter with a width of 2.5 arc minutes optimizes the cosmological constraint, yielding $ΔΩ_m/Ω_m\sim 10%$. This is significantly better than other filters which have been considered in the literature. This can be further improved with tomography and other sophisticated analyses.

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arXiv2003

The naked singularity in the global structure of critical collapse spacetimes

We examine the global structure of scalar field critical collapse spacetimes using a characteristic double-null code. It can integrate past the horizon without any coordinate problems, due to the careful choice of constraint equations used in the evolution. The limiting sequence of sub- and supercritical spacetimes presents an apparent paradox in the expected Penrose diagrams, which we address in this paper. We argue that the limiting spacetime converges pointwise to a unique limit for all r>0, but not uniformly. The r=0 line is different in the two limits. We interpret that the two different Penrose diagrams differ by a discontinuous gauge transformation. We conclude that the limiting spacetime possesses a singular event, with a future removable naked singularity.

gr-qc↗

arXiv2003

The Fate of Non-Radiative Magnetized Accretion Flows: Magnetically-Frustrated Convection

We present a scenario for non-radiative accretion onto the supermassive black hole at the galactic center. Conducting MHD simulations with 1400^3 grid zones that break the axial and reflection symmetries of earlier investigations and extend inward from the Bondi radius, we find a quasi-hydrostatic radial density profile rho propto r^{-0.72} with superadiabatic gradient corresponding to an n~0.72 polytrope. Buoyancy generated by magnetic dissipation is resisted by the same fields so effectively that energy is advected inward: a state of magnetically-frustrated convection. This scenario is consistent with observational constraints on energetics andouter boundary conditions.

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arXiv2003

High Performance Commodity Networking in a 512-CPU Teraflop Beowulf Cluster for Computational Astrophysics

We describe a new 512-CPU Beowulf cluster with Teraflop performance dedicated to problems in computational astrophysics. The cluster incorporates a cubic network topology based on inexpensive commodity 24-port gigabit switches and point to point connections through the second gigabit port on each Linux server. This configuration has network performance competitive with more expensive cluster configurations and is scaleable to much larger systems using other network topologies. Networking represents only about 9% of our total system cost of USD$561K. The standard Top 500 HPL Linpack benchmark rating is 1.202 Teraflops on 512 CPUs so computing costs by this measure are $0.47/Megaflop. We also describe 4 different astrophysical applications using complex parallel algorithms for studying large-scale structure formation, galaxy dynamics, magnetohydrodynamic flows onto blackholes and planet formation currently running on the cluster and achieving high parallel performance. The MHD code achieved a sustained speed of 2.2 teraflops in single precision or 44% of the theoretical peak.

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