arXiv subjects

Weiguang Cui

Publications and source records attributed to Weiguang Cui.

150 records · Page 9Linked to original sources

arXiv2012

The halo mass function in interacting Dark Energy models

We present a detailed investigation of the effects that a direct interaction between Dark Energy (DE) and Cold Dark Matter (CDM) particles imprints on the Halo Mass Function (HMF) of groups and clusters of galaxies. Making use of the public halo catalogs of the {\small CoDECS} simulations, we derive the HMF for several different types of coupled DE scenarios both based on the FoF algorithm and on the SO halo identification for different values of the overdensity threshold $Δ_{c}$. We compare the computed HMFs for coupled DE cosmologies with $Λ$CDM as well as with the predictions of the standard analytic fitting functions. Our results show that the standard fitting functions still reproduce reasonably well both the FoF and the SO HMFs of interacting DE cosmologies at intermediate masses and at low redshifts, once rescaled to the characteristic amplitude of linear density perturbations of each specific model as given by $σ_{8}$. However, we also find that such apparent degeneracy with $σ_{8}$ is broken both by the high-mass tail and by the redshift evolution of our HMFs, with deviations beyond $\sim 10%$ for most of the models under investigation. Furthermore, the discrepancy with respect to the predictions of standard fitting functions rescaled with the characteristic value of $σ_{8}$ shows -- for some models -- a strong dependence on the spherical overdensity threshold $Δ_{c}$ used for the halo identification. We find that such effect is due to a significant increase of halo concentration at low redshifts in these models, that is however absent in the majority of the cosmological scenarios considered in this work. We can therefore conclude that the universality of the HMF is violated by cosmological models that feature a direct interaction between DE and CDM.

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arXiv2012

The effects of baryons on the halo mass function

We present an analysis of the effects of baryon physics on the halo mass function. The analysis is based on simulations of a cosmological volume. Besides a Dark Matter (DM) only simulation, we also carry out two other hydrodynamical simulations. We identified halos using a spherical overdensity algorithm and their masses are computed at three different overdensities (with respect to the critical one), $Δ_c=200$, 500 and 1500. We find the fractional difference between halo masses in the hydrodynamical and in the DM simulations to be almost constant, at least for halos more massive than $\log (M_{Δ_c} / \hMsun)\geq 13.5$. In this range, mass increase in the hydrodynamical simulations is of about 4-5 per cent at $Δ_c=500$ and $\sim 1$ - 2 per cent at $Δ_c=200$. Quite interestingly, these differences are nearly the same for both radiative and non-radiative simulations. Such variations of halo masses induce corresponding variations of the halo mass function (HMF). At $z=0$, the HMFs for GH and CSF simulations are close to the DM one, with differences of $\mincir 3$ per cent at $Δ_c = 200$, and $\simeq 7$ per cent at $Δ_c=500$, with $\sim 10$ - 20 per cent differences reached at $Δ_c = 1500$. At this higher overdensity, the increase of the HMF for the radiative case is larger by about a factor 2 with respect to the non--radiative case. Assuming a constant mass shift to rescale the HMF from the hydrodynamic to the DM simulations, brings the HMF difference with respect to the DM case to be consistent with zero. Our results have interesting implications to bracket uncertainties in the mass function calibration associated to the uncertain baryon physics, in view of cosmological applications of future large surveys of galaxy clusters. (Abridged)

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arXiv2011

Gaussianizing the non-Gaussian lensing convergence field I: the performance of the Gaussianization

Motivated by recent works of Neyrinck et al. 2009 and Scherrer et al. 2010, we proposed a Gaussianization transform to Gaussianize the non-Gaussian lensing convergence field $κ$. It performs a local monotonic transformation $κ\rightarrow y$ pixel by pixel to make the unsmoothed one-point probability distribution function of the new variable $y$ Gaussian. We tested whether the whole $y$ field is Gaussian against N-body simulations. (1) We found that the proposed Gaussianization suppresses the non-Gaussianity by orders of magnitude, in measures of the skewness, the kurtosis, the 5th- and 6th-order cumulants of the $y$ field smoothed over various angular scales relative to that of the corresponding smoothed $κ$ field. The residual non-Gaussianities are often consistent with zero within the statistical errors. (2) The Gaussianization significantly suppresses the bispectrum. Furthermore, the residual scatters around zero, depending on the configuration in the Fourier space. (3) The Gaussianization works with even better performance for the 2D fields of the matter density projected over $\sim 300 \mpch$ distance interval centered at $z\in(0,2)$, which can be reconstructed from the weak lensing tomography. (4) We identified imperfectness and complexities of the proposed Gaussianization. We noticed weak residual non-Gaussianity in the $y$ field. We verified the widely used logarithmic transformation as a good approximation to the Gaussianization transformation. However, we also found noticeable deviations.

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arXiv2011

Properties of fossil groups in cosmological simulations and galaxy formation models

It has been a long-standing question whether fossil groups are just sampling the tail of the distribution of ordinary groups, or whether they are a physically distinct class of objects, characterized by an unusual and special formation history. To study this question, we here investigate fossil groups identified in the hydrodynamical simulations of the GIMIC project, which consists of resimulations of five regions in the Millennium Simulation (MS) that are characterized by different large-scale densities, ranging from a deep void to a proto-cluster region. For comparison, we also consider semi-analytic models built on top of the MS, as well as a conditional luminosity function approach. We identify galaxies in the GIMIC simulations as groups of stars and use a spectral synthesis code to derive their optical properties. The X-ray luminosity of the groups is estimated in terms of the thermal bremsstrahlung emission of the gas in the host halos, neglecting metallicity effects. We focus on comparing the properties of fossil groups in the theoretical models and observational results, highlighting the differences between them, and trying to identify possible dependencies on environment for which our approach is particularly well set-up. We find that the optical fossil fraction in all of our theoretical models declines with increasing halo mass, and there is no clear environmental dependence. Combining the optical and X-ray selection criteria for fossil groups, the halo mass dependence of the fossil groups seen in optical vanishes. Over the GIMIC halo mass range we resolve best, 9.0\times1012 \sim 4.0\times1013 h-1 M, the central galaxies in the fossil groups show similar properties as those in ordinary groups, in terms of age, metallicity, color, concentration, and mass-to-light ratio. [abridged]

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arXiv2010

Nonlinearities in modified gravity cosmology I: signatures of modified gravity in the nonlinear matter power spectrum

A large fraction of cosmological information on dark energy and gravity is encoded in the nonlinear regime. Precision cosmology thus requires precision modeling of nonlinearities in general dark energy and modified gravity models. We modify the Gadget-2 code and run a series of N-body simulations on modified gravity cosmology to study the nonlinearities. The modified gravity model that we investigate in the present paper is characterized by a single parameter ζ, which determines the enhancement of particle acceleration with respect to general relativity (GR), given the identical mass distribution (ζ= 1 in GR). The first nonlinear statistics we investigate is the nonlinear matter power spectrum at k < 3h/Mpc, which is the relevant range for robust weak lensing power spectrum modeling at l < 2000. In this study, we focus on the relative difference in the nonlinear power spectra at corresponding redshifts where different gravity models have the same linear power spectra. This particular statistics highlights the imprint of modified gravity in the nonlinear regime and the importance to include the nonlinear regime in testing GR. By design, it is less susceptible to the sample variance and numerical artifacts. We adopt a mass assignment method based on wavelet to improve the power spectrum measurement. We run a series of tests to determine the suitable simulation specifications (particle number, box size and initial redshift). We find that, the nonlinear power spectra can differ by ~30% for 10% deviation from GR (|ζ-1| = 0.1) where the rms density fluctuations reach 10. This large difference, on one hand, shows the richness of information on gravity in the corresponding scales, and on the other hand, invalidates simple extrapolations of some existing fitting formulae to modified gravity cosmology.

astro-ph.CO↗

arXiv2008

An ideal mass assignment scheme for measuring the Power Spectrum with FFTs

In measuring the power spectrum of the distribution of large numbers of dark matter particles in simulations, or galaxies in observations, one has to use Fast Fourier Transforms (FFT) for calculational efficiency. However, because of the required mass assignment onto grid points in this method, the measured power spectrum $\la |δ^f(k)|^2\ra$ obtained with an FFT is not the true power spectrum $P(k)$ but instead one that is convolved with a window function $|W(\vec k)|^2$ in Fourier space. In a recent paper, Jing (2005) proposed an elegant algorithm to deconvolve the sampling effects of the window function and to extract the true power spectrum, and tests using N-body simulations show that this algorithm works very well for the three most commonly used mass assignment functions, i.e., the Nearest Grid Point (NGP), the Cloud In Cell (CIC) and the Triangular Shaped Cloud (TSC) methods. In this paper, rather than trying to deconvolve the sampling effects of the window function, we propose to select a particular function in performing the mass assignment that can minimize these effects. An ideal window function should fulfill the following criteria: (i) compact top-hat like support in Fourier space to minimize the sampling effects; (ii) compact support in real space to allow a fast and computationally feasible mass assignment onto grids. We find that the scale functions of Daubechies wavelet transformations are good candidates for such a purpose. Our tests using data from the Millennium Simulation show that the true power spectrum of dark matter can be accurately measured at a level better than 2% up to $k=0.7k_N$, without applying any deconvolution processes. The new scheme is especially valuable for measurements of higher order statistics, e.g. the bi-spectrum,........

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