arXiv subjects

Ue-Li Pen

Publications and source records attributed to Ue-Li Pen.

At least 253 records · Page 14Linked to original sources

arXiv2003

A Free, Fast, Simple and Efficient TVD MHD Code

We describe a numerical method to solve the magnetohydrodynamic (MHD) equations. The fluid variables are updated along each direction using the flux conservative, 2nd order, total variation diminishing (TVD), upwind scheme of Jin and Xin. The magnetic field is updated separately in two-dimensional advection-constraint steps. The electromotive force (EMF) is computed in the advection step using the TVD scheme, and this same EMF is used immediately in the constraint step in order to preserve \grad.B=0 without the need to store intermediate fluxes. Operator splitting is used to extend the code to three dimensions, and Runge-Kutta is used to get second order accuracy in time. The advantages of this code are high resolution per grid cell, second order accuracy in space and time, enforcement of the \grad.B=0 constraint to machine precision, no memory overhead, speed, and simplicity. A 3-D Fortran implementation less than 400 lines long is made freely available. We also implemented a fully scalable message-passing parallel MPI version. We present tests of the code on MHD waves and shocks.

astro-ph↗

arXiv2003

Fast Power Spectrum Estimation

We present a new algorithm to rapidly and optimally compute power spectra. This new algorithm is based on a generalization of iterative multigrid, and has computational cost O(N log N), compared to the standard brute force approach which costs O(N^3). The procedure retains this speed on the full sky and for ill-conditioned matrices. It is applicable to galaxy power spectra, CMB, polarization and weak lensing data. We present a mathematical convergence analysis, and performance results.

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arXiv2003

Detection of dark matter Skewness in the VIRMOS-DESCART survey: Implications for Ω_0

Weak gravitational lensing provides a direct statistical measure of the dark matter distribution. The variance is easiest to measure, which constrains the degenerate product σ_8Ω^0.6. The degeneracy is broken by measuring the skewness arising from the fact that densities must remain positive, which is not possible when the initially symmetric perturbations become non-linear. Skewness measures the non-linear mass scale, which in combination with the variance measures Ωdirectly. We present the first detection of dark matter skewness from the Virmos-Decart survey. We have measured the full three point function, and its projections onto windowed skewness. We separate the lensing mode and the B mode. The lensing skewness is detected for a compensated Gaussian on scales of 5.37 arc minutes to be κ^3=1.06+/-0.06x10^-6. The B-modes are consistent with zero at this scale. The variance for the same window function is κ^2= 5.32+/-0.62+/-0.98x10^-5, resulting in S_3=375^{+342}_{-124}. Comparing to N-body simulations, we find Ω_0<0.5 at 90% confidence. The Canada-France-Hawaii-Telescope legacy survey and newer simulations should be able to improve significantly on the constraint.

astro-ph↗

arXiv2003

Measuring Feedback Using the Intergalactic Medium State and Evolution Inferred from the Soft X-ray Background

We explore the intergalactic medium (IGM) as a potential source of the unresolved soft X-ray background (XRB) and the feasibility to extract the IGM state and evolution from XRB observations. We build two analytical models, the continuum field model and the halo model, to calculate the IGM XRB mean flux, angular auto correlation and cross correlation with galaxies. Our results suggest that the IGM may contribute a significant fraction to the unresolved soft XRB flux and correlations. We calibrated non-Gaussian errors estimated against our $512^3$ moving mesh hydro simulation and estimate that the ROSAT all sky survey plus Sloan galaxy photometric redshift survey would allow a $\sim 10%$ accuracy in the IGM XRB-galaxy cross correlation power spectrum measurement for $800<l<5000$ and a $\sim 20%$ accuracy in the redshift resolved X-ray emissivity-galaxy cross correlation power spectrum measurement for $z\lesssim 0.5$. At small scales, non-gravitational heating, e.g. feedback, dominates over gravity and leaves unique signatures in the IGM XRB, which allows a comparable accuracy in the measurement of the amount of non-gravitational heating and the length scales where non-gravitational energy balances gravity.

astro-ph↗

arXiv2002

A Primer on Eulerian Computational Fluid Dynamics for Astrophysics

We present a pedagogical review of some of the methods employed in Eulerian computational fluid dynamics (CFD). Fluid mechanics is governed by the Euler equations, which are conservation laws for mass, momentum, and energy. The standard approach to Eulerian CFD is to divide space into finite volumes or cells and store the cell-averaged values of conserved hydro quantities. The integral Euler equations are then solved by computing the flux of the mass, momentum, and energy across cell boundaries. We review both first-order and second-order flux assignment schemes. All linear schemes are either dispersive or diffusive. The nonlinear, second-order accurate total variation diminishing (TVD) approach provides high resolution capturing of shocks and prevents unphysical oscillations. We review the relaxing TVD scheme, a simple and robust method to solve systems of conservation laws like the Euler equations. A 3-D relaxing TVD code is applied to the Sedov-Taylor blast wave test. The propagation of the blast wave is accurately captured and the shock front is sharply resolved. We apply a 3-D self-gravitating hydro code to simulating the formation of blue straggler stars through stellar mergers and present some numerical results. A sample 3-D relaxing TVD code is provided in the appendix.

astro-ph↗

arXiv2002

Hydrodynamical Simulations of the IGM at High Mach Numbers

We present a new approach to doing Eulerian computational fluid dynamics that is designed to work at high Mach numbers encountered in hydrodynamical simulations of the IGM. In conventional Eulerian CFD, the thermal energy is poorly tracked in supersonic bulk flows where local fluid variables cannot be accurately separated from the much larger bulk flow components. We described a method in which local fluid quantities can be directly tracked and the Eulerian fluid equations solved in a local frame moving with the flow. The new algorithm has been used to run large hydrodynamical simulations on a 1024^3 grid to study the kinetic SZ effect. The KSZ power spectrum is broadly peaked at l~10^4 with temperature fluctuations on micro Kelvin levels.

astro-ph↗

arXiv2002

The Sunyaev Zel'dovich effect: simulation and observation

The Sunyaev Zel'dovich effect (SZ effect) is a complete probe of ionized baryons, the majority of which are likely hiding in the intergalactic medium. We ran a $512^3$ $Λ$CDM simulation using a moving mesh hydro code to compute the statistics of the thermal and kinetic SZ effect such as the power spectra and measures of non-Gaussianity. The thermal SZ power spectrum has a very broad peak at multipole $l\sim 2000-10^4$ with temperature fluctuations $ΔT \sim 15μ$K. The power spectrum is consistent with available observations and suggests a high $σ_8\simeq 1.0$ and a possible role of non-gravitational heating. The non-Gaussianity is significant and increases the cosmic variance of the power spectrum by a factor of $\sim 5$ for $l<6000$. We explore optimal driftscan survey strategies for the AMIBA CMB interferometer and their dependence on cosmology. For SZ power spectrum estimation, we find that the optimal sky coverage for a 1000 hours of integration time is several hundred square degrees. One achieves an accuracy better than 40% in the SZ measurement of power spectrum and an accuracy better than 20% in the cross correlation with Sloan galaxies for $2000<l<5000$. For cluster searches, the optimal scan rate is around 280 hours per square degree with a cluster detection rate 1 every 7 hours, allowing for a false positive rate of 20% and better than 30% accuracy in the cluster SZ distribution function measurement.

astro-ph↗

arXiv2002

The Distance to Clusters: Correcting for Asphericity

X-ray and Sunyaev-Zeldovich effect observations can be combined to measure the distance to clusters of galaxies. If the intracluster gas distribution is not spherical, but elongated by a factor of Z along the line of sight, the inferred distance is increased by Z, and H0 is decreased by the same factor. Averaging the inferred value of H0 over a sufficiently large sample of clusters can reduce any systematic bias due to cluster shapes, provided the clusters are selected without any preferred orientation. Even so, elongation contributes significantly to the variance in the measured distances and in the inferred value of H0. With the addition of gravitational lensing observations, it is possible to infer the three-dimensional shape of clusters, provided the gas is in hydrostatic equilibrium. We demonstrate a specific method for finding the shape and correcting the measured distances to individual clusters. To test this method, we apply it to artificial observations of simple model clusters. We recover the true distances to each of our clusters without detectable bias, and with statistical errors due to measurement uncertainties of 4 to 6%.

astro-ph↗

arXiv2002

Detection of Galaxy Spin Alignments in the PSCz Shear Field

We report the first direct observational evidence for alignment of galaxy spin axes with the local tidal shear field. We measure quantitatively the strength of this directional correlation of disk galaxies from the Tully catalog with the local shear field reconstructed from PSCz galaxies. We demonstrate that the null hypothesis of random galaxy alignments relative to the shear frame can be ruled out more than 99.98 % confidence. The observed intrinsic correlation averaged over the censored samples that have detected non-zero signals is measured in terms of the alignment parameter 'a=0.17 +/- 0.04', which includes only statistical errors of the censored data, but not the cosmic variance error. The reconstruction procedure is likely to underestimates 'a' systematically. Our result is consistent with the linear tidal torque model, and supports the idea that the present galaxy spins may be used as a probe of primordial tidal shear and mass density fields. The intrinsic alignments of galaxy spins dominate over weak gravitational lensing for shallow surveys such like SDSS, while it should be negligible for deeper surveys at z ~ 1.

astro-ph↗

arXiv2001

Gravity and Non-gravity Modes in the VIRMOS-DESCART Weak Lensing Survey

We analyze the weak lensing data of the VIRMOS imaging survey using projections (called E and B-modes) of the two independents observed correlation functions. The E-mode contains all the lensing signal, while noise and systematics contribute equally to the E and B modes provided that intrinsic alignment is negligible. The mode separation allows a measurement of the signal with a \sqrt{2} smaller error bars, and a separate channel to test for systematic errors. We apply various transformations, including a spherical harmonic space power spectrum C^E_l and C^B_l, which provides a direct measurement of the projected dark matter distribution for 500<l<10^4.

astro-ph↗

arXiv2001

Friedmann Equation and Stability of Inflationary Higher Derivative Gravity

Stability analysis on the De Sitter universe in pure gravity theory is known to be useful in many aspects. We first show how to complete the proof of an earlier argument based on a redundant field equation. It is shown further that the stability condition applies to $k \ne 0$ Friedmann-Robertson-Walker spaces based on the non-redundant Friedmann equation derived from a simple effective Lagrangian. We show how to derive this expression for the Friedmann equation of pure gravity theory. This expression is also generalized to include scalar field interactions.

gr-qc↗

arXiv2001

Galaxy Spin Statistics and Spin-Density Correlation

We present a theoretical study of galaxy spin correlation statistics, with detailed technical derivations. We also find an expression for the spin-density cross-correlation, and apply that to the Tully galaxy catalog. The observational results appear qualitatively consistent with the theoretical predictions, yet the error bars are still large. However, we expect that currently ongoing large surveys such as the Sloan Digital Sky survey (SDSS) will enable us to make a precision measurement of these correlation statistics in the near future. These intrinsic galaxy alignments are expected to dominate over the weak lensing signal in SDSS, and we present the detailed algorithms for the density reconstruction for this case. These observables are tracers of the galaxy-gravity interaction, which may provide us deeper insights into the galaxy formation and large scale matter distribution as well.

astro-ph↗

arXiv2001

Do angular momentum induced ellipticity correlations contaminate weak lensing measurements?

Alignments in the angular momentum vectors of galaxies can induce large scale correlations in their projected orientations. Such alignments arise from the tidal torques exerted on neighboring proto-galaxies by the smoothly varying shear field. Weak gravitational lensing can also induce ellipticity correlations since the images of neighboring galaxies will be distorted coherently by the intervening mass distribution. Comparing these two sources of shape correlations, it is found that for current weak lensing surveys with a median redshift of 1, the intrinsic signal is a contaminant on the order of 1-10% of the measured signal. However, for shallower surveys, 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. When the distortions are dominated by lensing, as occurs at high redshifts, the decomposition provides a valuable tool for understanding properties of the noise and systematic errors.

astro-ph↗

arXiv2000

Discriminating weak lensing from intrinsic spin correlations using the curl-gradient decomposition

The distortion field defined by the ellipticities of galaxy shapes projected on the sky can be uniquely decomposed into a gradient and a curl component. If the observed ellipticities are induced by weak gravitational lensing, then the distortion field is curl free. Here we show that, in contrast, the distortion field resulting from intrinsic spin alignments is not curl free. This provides a powerful discriminant between lensing and intrinsic contributions to observed ellipticity correlations. We also show how these contributions can be disentangled statistically from the ellipticity correlations or computed locally from circular integrals of the ellipticity field. This allows for an unambiguous detection of intrinsic galaxy alignments in the data. When the distortions are dominated by lensing, as occurs at high redshifts, the decomposition provides a valuable tool for understanding properties of the noise and systematic errors. These techniques can be applied equally well to the polarization of the microwave background, where it can be used to separate curl-free scalar perturbations from those produced by gravity waves or defects.

astro-ph↗

arXiv2000

CMB anisotropy from spatial correlations of clusters of galaxies

The SZ effect from clusters of galaxies is a dominant source of secondary CMB anisotropy in the low-redshift universe. We present analytic predictions for the CMB power spectrum from massive halos arising from the SZ effect. Since halos are discrete, the power spectrum consists of a Poisson and a correlation term. The latter is always smaller than the former, which is dominated by nearby bright rich clusters. In practice however, those bright clusters are easy to indentify and can thus be subtracted from the map. After this subtraction, the correlation term dominates degree-scale fluctuations over the Poisson term, as the main contribution to the correlation term comes from distant clusters. We find that the correlation term is detectable by Planck experiment. Since the degree scale spectrum is quite insensitive to the highly uncertain core structures of halos, our predictions are robust on these scales. Measuring the correlation term on degree scales thus cleanly probes the clustering of distant halos. This has not been measured yet, mainly because optical and X-ray surveys are not sufficiently sensitive to include such distant clusters and groups. Our analytic predictions are also compared to adiabatic hydrodynamic simulations. The agreement is remarkably good, down to ten arcminutes scales, indicating that our predictions are robust for the Planck experiment. Below ten arcminute scales, where the details of the core structure dominates the power spectrum, our analytic and simulated predictions might fail. In the near future, interferometer and bolometer array experiments will measure the SZ power spectrum down to arcminutes scales, and yield new insight into the physics of the intrahalo medium.

astro-ph↗

arXiv2000

Deprojecting Sunyaev-Zeldovich statistics

We apply the hierarchical clustering model and non-linear perturbation theory to the cosmological density and temperature fields. This allows us to calculate the intergalactic gas pressure power spectrum, SZ anisotropy power spectrum, skewness and related statistics. Then we show the effect of the non-gravitational heating. Our model confirms recent simulations yielding mass weighted gas temperature \bar{T}_g \sim 0.35 keV and reproduces the power spectra found in these simulations. While the SZ effect contains only angular information, we show that it is possible to extract the full time resolved gas pressure power spectrum when combined with galaxy photometric redshift surveys by using a variation on the cross-correlation. This method further allows the disentanglement of the gravitational and the non-gravitational heating.

astro-ph↗

arXiv2000

Tentative Detection of Galaxy Spin Correlations in the Tully Catalogue

We report a tentative detection of spin correlations in the Tully catalogue of nearby galaxies. We define a simple but nontrivial spin correlation function, and find an analytic estimate of it in the frame of the linear perturbation theory. Then, we present the observed spin correlation signal from the Tully galaxies with error bars. The three dimensional spin correlation turns out to be significant at the 97% confidence level, detected out to a few $h^{-1}$ Mpc. This observed correlation is consistent with the theoretical prediction based on the gravitational instability picture of galaxy formation. An analysis of systematic errors is also presented. The observed strength of correlation may be sufficient to significantly affect blank field of weak lensing searches.

astro-ph↗

arXiv2000

Gravitational Lensing by Galaxy Groups in the Hubble Deep Field

We investigate strong lensing of galaxies in the Hubble Deep Field by foreground groups and clusters of galaxies with masses from $10^{13}$ to $10^{15} \MSun$. Over this mass range, groups with the profile of Navarro, Frenk, & White (1995) are less efficient than singular isothermal spheres at producing multiple images of galaxies, by factors of $5 \times 10^{- 2}$ to $10^{- 3}$. This difference highlights the sensitivity of the lensing cross section to the central density profile. Nonetheless, with either profile we find that the expected number of galaxies lensed by groups in the Hubble Deep Field is at most $\lesssim 1$, consistent with the lack of clearly identified group lens systems.

astro-ph↗