arXiv subjects

Y. P. Jing

Publications and source records attributed to Y. P. Jing.

At least 145 records · Page 8Linked to original sources

arXiv2000

The density profile of equilibrium and non-equilibrium dark matter halos

We study the diversity of the density profiles of dark matter halos based on a large set of high-resolution cosmological simulations of 256^3 particles. The cosmological models include four scale-free models and three representative cold dark matter models. The simulations have good force resolution, and there are about 400 massive halos with more than 10^4 particles within the virial radius in each cosmological model. Our unbiased selection of all massive halos enables to quantify how well the bulk of dark matter halos can be described by the Navarro, Frenk & White (NFW) profile which was established for equilibrium halos. We find that about seventy percent of the halos can be fitted by the NFW profile with a fitting residual dvi_{max} less than 30% in Omega_0=1 universes. This percentage is higher in lower density cosmological models. The rest of the halos exhibits larger deviations from the NFW profile for more significant internal substructures. There is a considerable amount of variation in the density profile even for the halos which can be fitted by the NFW profile (i.e. dvi_{max}<0.30). The distribution of the profile parameter, the concentration $c$, can be well described by a lognormal function with the mean value \bar c slightly smaller (15%) than the NFW result and the dispersion σ_c in \ln c about 0.25. The more virialized halos with dvi_{max}<0.15 have the mean value \bar c in good agreement with the NFW result and a slightly smaller dispersion σ_c (about 0.2). Our results can alleviate some of the conflicts found recently between the theoretical NFW profile and observational results. Implications for theoretical and observational studies of galaxy formation are discussed.

astro-ph↗

arXiv2000

Cosmological SPH simulations with four million particles: statistical properties of X-ray clusters in a low-density universe

We present results from a series of cosmological SPH (smoothed particle hydrodynamics) simulations coupled with the P3M (Particle-Particle-Particle-Mesh) solver for the gravitational force. The simulations are designed to predict the statistical properties of X-ray clusters of galaxies as well as to study the formation of galaxies. We have seven simulation runs with different assumptions on the thermal state of the intracluster gas. Following the recent work by Pearce et al., we modify our SPH algorithm so as to phenomenologically incorporate the galaxy formation by decoupling the cooled gas particles from the hot gas particles. All the simulations employ 128^3 particles both for dark matter and for gas components, and thus constitute the largest systematic catalogues of simulated clusters in the SPH method performed so far. These enable us to compare the analytical predictions on statistical properties of X-ray clusters against our direct simulation results in an unbiased manner. We find that the luminosities of the simulated clusters are quite sensitive to the thermal history and also to the numerical resolution of the simulations, and thus are not reliable. On the other hand, the mass-temperature relation for the simulated clusters is fairly insensitive to the assumptions of the thermal state of the intracluster gas, robust against the numerical resolution, and in fact agrees well with the analytic prediction. Therefore the prediction for the X-ray temperature function of clusters on the basis of the Press-Schechter mass function and the virial equilibrium is fairly reliable.

astro-ph↗

arXiv1999

Density Profiles of Dark Matter Halo are not Universal

We perform a series of high-resolution N-body simulations designed to examine the density profiles of dark matter halos. From 12 simulated halos ranging the mass of $2\times10^{12}\sim 5\times10^{14} h^{-1}{\rm M_\odot}$ (represented by $\sim 1$ million particles within the virial radius), we find a clear systematic correlation between the halo mass and the slope of the density profile at 1% of the virial radius, in addition to the variations of the slope among halos of the similar mass. More specifically, the slope is $\sim -1.5$, -1.3, and -1.1 for galaxy, group, and cluster mass halos, respectively. While we confirm the earlier simulation results that the inner slope is steeper than the {\it universal} profile originally proposed by Navarro, Frenk & White, this mass dependence is inconsistent with the several analytical arguments attempting to link the inner slope with the primordial index of the fluctuation spectrum. Thus we conclude that the dark matter density profiles, especially in the inner region, are not universal.

astro-ph↗

arXiv1999

Two-point correlation functions of X-ray selected clusters of galaxies: theoretical predictions for flux-limited surveys

We have developed a model to describe two-point correlation functions of clusters of galaxies in X-ray flux-limited surveys. Our model properly takes account of nonlinear gravitational evolution of mass fluctuations, redshift-space distortion due to linear peculiar velocity field and to finger-of-god, cluster abundance and bias evolution on the basis of the Press -- Schechter theory, the light-cone effect, and the selection function due to the X-ray flux, temperature and luminosity limits. Applying this model in representative cosmological models, we have presented quantitative predictions for X-ray selected samples feasible from the future surveys with the X-ray satellites including Astro-E, Chandra, and XMM. The comparison of these predictions and the observed cluster clustering will place important cosmological constraints which are complementary to the cluster abundance and the cosmic microwave background.

astro-ph↗

arXiv1999

Cosmological redshift-space distortion on clustering of high-redshift objects: correction for nonlinear effects in power spectrum and tests with N-body simulations

We examine the cosmological redshift-space distortion effect on the power spectrum of the objects at high-redshifts, which is an unavoidable observational contamination in general relativistic cosmology. In particular, we consider the nonlinear effects of density and velocity evolution using high-resolution N-body simulations in cold dark matter models. We find that the theoretical modeling on the basis of the fitting formulae of nonlinear density and velocity fields accurately describes the numerical results, especially in quasi-nonlinear regimes. These corrections for nonlinear effects are essential in order to use the the cosmological redshift-space distortion as a cosmological test. We perform a feasibility test to derive constraints from the future catalogues of high-redshift quasars using our theoretical modeling and results of N-body simulations. Applying the present methodology to the future data from on-going surveys of high-redshift galaxies and quasars will provide a useful tool to constrain a geometry of the universe.

astro-ph↗

arXiv1999

Deciphering cosmological information from redshift surveys of high-z objects - the cosmological light-cone effect and redshift-space distortion -

The three-dimensional distribution of astronomical objects observed in redshift space significantly differs from the true distribution since the distance to each object cannot be determined by its redshift $z$ only; for $z \ll 1$ the peculiar velocity field contaminates the true recession velocity of the Hubble flow, while the true distance for objects at $z < 1$ sensitively depends on the (unknown and thus assumed) cosmological parameters. This hampers the effort to understand the true distribution of large-scale structure of the universe. In addition, all cosmological observations are carried out on a light-cone, the null hypersurface of an observer at $z=0$. This implies that their intrinsic properties and clustering statistics should change even within the survey volume. Therefore a proper comparison taking account of the light-cone effect is important to extract any cosmological information from redshift catalogues, especially for $z < 1$. We present recent theoretical development on the two effects -- the cosmological light-cone effect and the cosmological redshift-space distortion -- which should play key roles in observational cosmology in the 21st century.

astro-ph↗

arXiv1999

Accurate determination of the Lagrangian bias for the dark matter halos

We use a new method, the cross power spectrum between the linear density field and the halo number density field, to measure the Lagrangian bias for dark matter halos. The method has several important advantages over the conventional correlation function analysis. By applying this method to a set of high-resolution simulations of 256^3 particles, we have accurately determined the Lagrangian bias, over 4 magnitudes in halo mass, for four scale-free models with the index n=-0.5, -1.0, -1.5 and -2.0 and three typical CDM models. Our result for massive halos with $M \ge M_*$ ($M_*$ is a characteristic non-linear mass) is in very good agreement with the analytical formula of Mo & White for the Lagrangian bias, but the analytical formula significantly underestimates the Lagrangian clustering for the less massive halos $M < M_*. Our simulation result however can be satisfactorily described, with an accuracy better than 15%, by the fitting formula of Jing for Eulerian bias under the assumption that the Lagrangian clustering and the Eulerian clustering are related with a linear mapping. It implies that it is the failure of the Press-Schechter theories for describing the formation of small halos that leads to the inaccuracy of the Mo & White formula for the Eulerian bias. The non-linear mapping between the Lagrangian clustering and the Eulerian clustering, which was speculated as another possible cause for the inaccuracy of the Mo & White formula, must at most have a second-order effect. Our result indicates that the halo formation model adopted by the Press-Schechter theories must be improved.

astro-ph↗

arXiv1998

Density profiles and clustering of dark matter halos

The density profile and the clustering of dark matter (DM) halos are two very important ingredients for understanding many observations of galaxies. In this talk, I present our new results for these two quantities from a large set of high quality N-body simulations. We show that in an Einstein de Sitter Universe about 35 percent of the halos could not be fitted by the Navarro, Frenk & White (NFW) profile because of too significant substructures, another 50 percent with less substructures can be reasonably described by the NFW profile, and the rest with the least substructures can be fitted by the NFW profile very nicely. For the halos which can be reasonably or better fitted by the NFW profile, the probability distribution function of the concentration parameter $c$ could be well described by a lognormal function with a dispersion $σ=0.27$. The mean value of $c$ is quite close to that predicted by NFW, though it decreases with the increase of the substructures. Our clustering analysis shows a significantly stronger clustering for low mass halos than Mo & White predict. A formula is therefore presented for the two-point correlation function of halos which can fit our simulation results (both CDM models and scale-free clustering models) very accurately.

astro-ph↗

arXiv1998

Accurate fitting formula for the two-point correlation function of the dark matter halos

An accurate fitting formula is reported for the two-point correlation function of the dark matter halos in hierarchical clustering models, It is valid for the the linearly clustering regime, and its accuracy is about 10% in the correlation amplitude for the halos with mass M greater than 1/100 -- 1/1000 of the characteristic non-linear mass M_c. The result is found on the basis of a careful analysis for a large set of scale-free simulations with 17 million particles. The fitting formula has a weak explicit dependence on the index n of the initial power spectrum, but can be equally well applied to the cold dark matter (CDM) cosmological models if the effective index n_{eff} of the CDM power spectrum at the scale of the halo mass replaces the index n. The formula agrees with the analytical formula of Mo & White (MW96) for massive halos with M>M_c, but the MW96 formula significantly underpredicts the correlation for the less massive halos. The difference between the fitting and the analytical formulae amounts to a factor => 2 in the correlation amplitude for M=0.01 M_c. One of the most interesting applications of this fitting formula would be the clustering of galaxies since the majority of halos hosting galaxies satisfies M<< M_c.

astro-ph↗

arXiv1998

A Possible Bias Model for Quasars

We propose that the majority of quasars at redshift $z\sim 1 - 5$ formed in the environment of new born collapsed halos with 1-D velocity dispersion $σ_v^{1d} \sim 400 \kms$. The harboring coefficient $f$ of quasars per halo and the lifetime of quasars depend only on local process, not modulated by the density inhomogeneities on scales larger than the size of the halos. Thus, the bias of quasars on scale larger than the size of these halos is mainly determined by the parameter $σ_v$ used for quasar environment identification. With this model, the popular structure formation models, like SCDM and LCDM, can be fairly well reconciled with the data of quasars, including a. observed feature of the environment for quasars; b. redshift evolution of quasar abundance; c. the two-point correlation functions of quasars. This bias model predicts that the correlation function of quasars doesn't significantly evolve, or only slightly increases with redshift.

astro-ph↗

arXiv1998

The Pairwise Peculiar Velocity Dispersion of Galaxies: Effects of the Infall

We study the reliability of the reconstruction method which uses a modelling of the redshift distortions of the two-point correlation function to estimate the pairwise peculiar velocity dispersion of galaxies. In particular, the dependence of this quantity on different models for the infall velocity is examined for the Las Campanas Redshift Survey. We make extensive use of numerical simulations and of mock catalogs derived from them to discuss the effect of a self-similar infall model, of zero infall, and of the real infall taken from the simulation. The implications for two recent discrepant determinations of the pairwise velocity dispersion for this survey are discussed.

astro-ph↗

arXiv1998

The Three-point Correlation Function of Galaxies Determined from the Las Campanas Redshift Survey

We report the measurement of the three-point correlation function (3PCF) of galaxies for the Las Campanas Redshift Survey (LCRS). We have not only measured the 3PCF in redshift space but also developed a method to measure the projected 3PCF which has simple relations to the real space 3PCF. Both quantities have been measured as a function of triangle size and shape with only a fractional uncertainty in each individual bin. Various tests derived from mock catalogs have been carried out to assure that the measurement is stable and that the errors are estimated reliably. Our results indicate that the 3PCFs both in redshift space and in real space have small but significant deviations from the well-known hierarchical form. The 3PCF in redshift space can be fitted by $\Qsu=0.5\cdot 10^{[0.2+0.1({s\over s+1})^2]v^2}$ for $0.8 1\mpc$ is noted. The real-space $\Qru$ for $0.2 \ls r_{12} \ls 3\mpc$ and $r_{31}\ls 6\mpc$ can be well described by half the mean 3PCF predicted by a CDM model with $Ω_0 h=0.2$. Implications for the CDM cosmogonic models are discussed in detail with the help of mock simulation samples.

astro-ph↗

arXiv1998

Confronting cold dark matter cosmologies with strong clustering of Lyman break galaxies at $z\sim3$

We perform a detailed analysis of the statistical significance of a concentration of Lyman break galaxies at $z \sim 3$ recently discovered by Steidel et al. (1997), using a series of N-body simulations with $N=256^3$ particles in a $(100\himpc)^3$ comoving box. While the observed number density of Lyman break galaxies at $z\sim3$ implies that they correspond to systems with dark matter halos of $\simlt 10^{12}M_\odot$, the resulting clustering of such objects on average is not strong enough to be reconciled with the concentration if it is fairly common; we predict one similar concentration approximately per ($6\sim 10$) fields in three representative cold dark matter models. Considering the current observational uncertainty of the frequency of such clustering at $z\sim3$, it would be premature to rule out the models, but the future spectroscopic surveys in a dozen fields could definitely challenge all the existing cosmological models a posteriori fitted to the $z=0$ universe.

astro-ph↗

arXiv1997

Spatial correlation function and pairwise velocity dispersion of galaxies: CDM models versus the Las Campanas Survey

We show, with the help of large N-body simulations, that the real-space two-point correlation function and pairwise velocity dispersion of galaxies can both be measured reliably from the Las Campanas Redshift Survey. The real-space correlation function is well fitted by the power law $ξ(r)=(r_0/r)^γ$ with $r_0=(5.06\pm0.12)\mpc$ and $γ=1.862\pm 0.034$, and the pairwise velocity dispersion at $1\mpc$ is $(570\pm 80)\kms$. A detailed comparison between these observational results and the predictions of current CDM cosmogonies is carried out. We construct 60 mock samples for each theoretical model from a large set of high resolution N-body simulations, which allows us to include various observational selection effects in the analyses and to use exactly the same methods for both real and theoretical samples. We demonstrate that such a procedure is essential in the comparison between models and observations. The observed two-point correlation function is significantly flatter than the mass correlation function in current CDM models on scales $\la 1\mpc$. The observed pairwise velocity dispersion is also lower than that of dark matter particles in these models. We propose a simple antibias model to explain these discrepancies. This model assumes that the number of galaxies per unit dark matter mass, $N/M$, decreases with the mass of dark haloes. The predictions of CDM models with $σ_8Ω_0^{0.6}\sim 0.4$-0.5 and $Ω_0 h\sim 0.2$ are in agreement with the observational results, if the trend of $N/M$ with $M$ is at the level already observed for rich clusters of galaxies. Thus CDM models with cluster-abundance normalization are consistent with the observed correlation function and pairwise velocity dispersion of galaxies. A high level of velocity bias is not required in these models.

astro-ph↗

arXiv1996

Analytical Approximations to the Low-Order Statistics of Dark Matter Distribution

We show that in a hierarchical clustering model the low-order statistics of the density and the peculiar velocity fields can all be modelled semianalytically for a given cosmology and an initial density perturbation power spectrum $P(k)$. We present such models for the two-point correlation function $ξ(r)$, the amplitude $Q$ of the three-point correlation function, the mean pairwise peculiar velocity $< v_{12}(r)> $, the pairwise peculiar velocity dispersion $< v_{12}^2(r)>$, and the one-point peculiar velocity dispersion $< v_1^2 >$. We test our models against results derived from N-body simulations. These models allow us to understand in detail how these statistics depend on $P(k)$ and cosmological parameters. They can also help to interpret, and maybe correct for, sampling effects when these statistics are estimated from observations. The dependence of the small-scale pairwise peculiar velocity dispersion on rich clusters in the sample, for instance, can be studied quantitatively. There are also significant implications for the reconstruction of the cosmic density field from measurements in redshift space.

astro-ph↗

arXiv1996

Three-point correlation function in the quasilinear regime

Using the second-order Eulerian perturbation theory (SEPT), we study the three-point correlation function $ζ$ in the quasilinear regime for the SCDM, LCDM and MDM models, with the interesting result that these three models have distinctive three-point correlation functions. We test this SEPT prediction using a large set of high-resolution N-body simulations. The N-body results show that the SEPT prediction for $ζ$ is not accurate even in the quasilinear regime ($r\la 10 \mpc$), in contrast to previous N-body tests on the skewness. However, similar to the perturbation theory, our N-body results still predict a strong dependence of the three-point correlation on the triangle shape which is observable in the distribution of galaxies if the galaxies trace the distribution of the underlying mass

astro-ph↗

arXiv1996

High-Order Correlations of Peaks and Halos: a Step toward Understanding Galaxy Biasing

We develop an analytic model for the hierarchical correlation amplitudes S_j(R)= \bxi_j(R)/\bxi_2^{j-1}(R) of density peaks and dark matter halos in the quasi-linear regime. The statistical distribution of density peaks and dark matter halos within the initial density field are determined by the peak formalism and by an extension of the Press-Schechter formalism, respectively. Modifications of these distributions caused by gravitationally induced motions are treated using a spherical collapse model. We test our model against results from a variety of N-body simulations. The model works well for peaks and for halos that are identified earlier than the time when the moments are calculated. Because halos are spatially exclusive at the time of their identification, our model is only qualitatively correct for halos identified at the same time as the moments are calculated. The S_j depend only weakly on the bias parameter b for large b but increase rapidly with decreasing b at b\sim 1. Thus if galaxies are associated with peaks in the initial density field, or with dark halos formed at high redshifts, a measurement of S_j in the quasilinear regime should determine whether galaxies are significantly biased relative to the mass. We use our model to interpret the observed high order correlation functions of galaxies and clusters. We find that if the values of S_j for galaxies are as high as those given by the APM survey, then APM galaxies should not be significantly biased.

astro-ph↗

arXiv1996

The Correlation Function of Clusters of Galaxies and the Amplitude of Mass Fluctuations in the Universe

We show that if a sample of galaxy clusters is complete above some mass threshold, then hierarchical clustering theories for structure formation predict its autocorrelation function to be determined purely by the cluster abundance and by the spectrum of linear density fluctuations. Thus if the shape of the initial fluctuation spectrum is known, its amplitude $σ_8$ can be estimated directly from the correlation length of a cluster sample in a way which is independent of the value of $Ω_0$. If the cluster mass corresponding to the sample threshold is also known, it provides an independent estimate of the quantity $σ_8Ω_0^{0.6}$. Thus cluster data should allow both $σ_8$ and $Ω_0$ to be determined observationally. We explore these questions using N-body simulations together with a simple but accurate analytical model based on extensions of Press-Schechter theory. Applying our results to currently available data we find that if the linear fluctuation spectrum has a shape similar to that suggested by the APM galaxy survey, then a correlation length $r_0$ in excess of $20\mpch$ for Abell clusters would require $σ_8>1$, while $r_0<15\mpch$ would require $σ_8<0.5$. With conventional estimates of the relevant mass threshold these imply $Ω_0\la 0.3$ and $Ω_0\ga 1$ respectively.

astro-ph↗