Y. P. Jing

Triaxial Modeling of Halo Density Profiles with High-Resolution N-Body Simulations

We present a detailed nonspherical modeling of dark matter halos on the basis of a combined analysis of high-resolution halo simulations (12 halos with N ~ 106 particles within their virial radius) and large cosmological simulations (five realizations with N = 5123 particles in a 100 h-1 Mpc box size). The density profiles of those simulated halos are well approximated by a sequence of the concentric triaxial distribution with their axis directions being fairly aligned. We characterize the triaxial model quantitatively by generalizing the universal density profile, that has previously been discussed only in the framework of the spherical model. We obtain a series of practically useful fitting formulae in applying the triaxial model: the mass and redshift dependence of the axis ratio, the mean of the concentration parameter, and the probability distribution functions of the axis ratio and the concentration parameter. These accurate fitting formulae form a complete description of the triaxial density profiles of halos in cold dark matter models. Our current description of the dark halos will be particularly useful in predicting a variety of nonsphericity effects, to a reasonably reliable degree, including the weak and strong lens statistics, the orbital evolution of galactic satellites and triaxiality of galactic halos, and the nonlinear clustering of dark matter. In addition, this provides a useful framework for the nonspherical modeling of the intracluster gas, which is crucial in discussing the gas and temperature profiles of X-ray clusters and the Hubble constant estimated via the Sunyaev-Zeldovich effect.

ACCURATE UNIVERSAL MODELS FOR THE MASS ACCRETION HISTORIES AND CONCENTRATIONS OF DARK MATTER HALOS

A large amount of observations have constrained cosmological parameters and the initial density fluctuation spectrum to a very high accuracy. However, cosmological parameters change with time and the power index of the power spectrum dramatically varies with mass scale in the so-called concordance Lambda CDM cosmology. Thus, any successful model for its structural evolution should work well simultaneously for various cosmological models and different power spectra. We use a large set of high-resolution N-body simulations of a variety of structure formation models (scale-free, standard CDM, open CDM, and Lambda CDM) to study the mass accretion histories, the mass and redshift dependence of concentrations, and the concentration evolution histories of dark matter halos. We find that there is significant disagreement between the much-used empirical models in the literature and our simulations. Based on our simulation results, we find that the mass accretion rate of a halo is tightly correlated with a simple function of its mass, the redshift, parameters of the cosmology, and of the initial density fluctuation spectrum, which correctly disentangles the effects of all these factors and halo environments. We also find that the concentration of a halo is strongly correlated with the universe age when its progenitor on the mass accretion history first reaches 4% of its current mass. According to these correlations, we develop new empirical models for both the mass accretion histories and the concentration evolution histories of dark matter halos, and the latter can also be used to predict the mass and redshift dependence of halo concentrations. These models are accurate and universal: the same set of model parameters works well for different cosmological models and for halos of different masses at different redshifts, and in the Lambda CDM case the model predictions match the simulation results very well even though halo mass is traced to about 0.0005 times the final mass, when cosmological parameters and the power index of the initial density fluctuation spectrum have changed dramatically. Our model predictions also match the PINOCCHIO mass accretion histories very well, which are much independent of our numerical simulations and our definitions of halo merger trees. These models are also simple and easy to implement, making them very useful in modeling the growth and structure of dark matter halos. We provide appendices describing the step-by-step implementation of our models. A calculator which allows one to interactively generate data for any given cosmological model is provided on the Web, together with a user-friendly code to make the relevant calculations and some tables listing the expected concentration as a function of halo mass and redshift in several popular cosmological models. We explain why Lambda CDM and open CDM halos on nearly all mass scales show two distinct phases in their mass growth histories. We discuss implications of the universal relations we find in connection to the formation of dark matter halos in the cosmic density field.

The Density Profiles of the 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 in mass from 2x1012 to 5x1014 h-1 M middle dot in circle (represented by approximately 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 similar mass. More specifically, the slope is approximately -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 universal profile originally proposed by Navarro, Frenk, & White, this mass dependence is inconsistent with 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.

Spatial Correlation Function and Pairwise Velocity Dispersion of Galaxies: Cold Dark Matter Models versus the Las Campanas Survey

We show, with the help of large N-body simulations, that both the real-space two-point correlation function and pairwise velocity dispersion of galaxies can be measured reliably from the Las Campanas Redshift Survey. The real-space correlation function is well fitted by the power law ξ(r) = (r0/r)γ with r0 = (5.06 ± 0.12) h-1 Mpc and γ = 1.862 ± 0.034, and the pairwise velocity dispersion at 1 h-1 Mpc is 570 ± 80 km s-1. A detailed comparison between these observational results and the predictions of current cold dark matter (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 1 h-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 σ8 Ω0.60~0.4-0.5 and Γ ~ 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 Γ ~ 0.2 and 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.

The growth and structure of dark matter haloes

In this paper, we analyse in detail the mass-accretion histories and structural properties of dark haloes in high-resolution N-body simulations. We model the density distribution in individual haloes using the Navarro-Frenk-White (NFW) profile. For a given halo, there is a tight correlation between its inner-scale radius r(s) and the mass within it, M-s, for all its main progenitors. Using this correlation, one can predict quite well the structural properties of a dark halo at any time in its history from its mass-accretion history, implying that the structure properties and the mass-accretion history are closely correlated. The predicted growing rate of concentration c with time tends to increase with decreasing mass-accretion rate. The build-up of dark haloes in cold dark matter (CDM) models generally consists of an early phase of fast accretion (where the halo mass M-h increases with time much faster than the expansion rate of the Universe) and a late phase of slow accretion (where M-h increases with time approximately as the expansion rate). These two phases are separated at a time when c similar to 4 and the typical binding energy of the halo is approximately equal to that of a singular isothermal sphere with the same circular velocity. Haloes in the two accretion phases show systematically different properties, for example, the circular velocity v(h) increases rapidly with time in the fast accretion phase but remains almost constant in the slow accretion phase, the inner properties of a halo, such as r(s) and M-s increase rapidly with time in the fast accretion phase but change only slowly in the slow accretion phase, the inner circular velocity v(s) is approximately equal to v(h) in the fast accretion phase but is larger in the slow accretion phase. The potential well associated with a halo is built up mainly in the fast accretion phase, while a large amount of mass can be accreted in the slow accretion phase without changing the potential well significantly. We discuss our results in connection with the formation of dark haloes and galaxies in hierarchical models.

A halo-based galaxy group finder: calibration and application to the 2dFGRS

We use the halo occupation model to calibrate galaxy group finders in magnitude limited redshift surveys. Because, according to the current scenario of structure formation, galaxy groups are associated with cold dark matter (CDM) haloes, we make use of the properties of the halo population in the design of our group finder. The method starts with an assumed mass-to-light ratio to assign a tentative mass to each group. This mass is used to estimate the size and velocity dispersion of the underlying halo that hosts the group, which in turn is used to determine group membership (in redshift space). This procedure is repeated until no further changes occur in group memberships. We find that the final groups selected this way are insensitive to the mass-to-light ratio assumed. We use mock catalogues, constructed using the conditional luminosity function (CLF), to test the performance of our group finder in terms of completeness of true members and contamination by interlopers. Our group finder is more successful than the conventional friends-of-friends (FOF) group finder in assigning galaxies in common dark matter haloes to a single group. We apply our group finder to the 2-degree Field Galaxy Redshift Survey (2dFGRS) and compare the resulting group properties with model predictions based on the CLF. For the CDM concordance cosmology, we find a clear discrepancy between the model and data in the sense that the model predicts too many rich groups. In order to match the observational results, we have to either increase the mass-to-light ratios of rich clusters to a level significantly higher than current observational estimates, or to assume sigma(8) similar or equal to 0.7, compared with the concordance value of 0.9.

The dependence of clustering on galaxy properties

We use a sample of similar to 200 000 galaxies drawn from the Sloan Digital Sky Survey (SDSS) with 0.01 5 h(-1) Mpc). This large-scale clustering dependence is not seen for the parameters C or mu*. On small scales ( 1.5. In contrast, the dependence of the amplitude of wp(r(p)) on concentration on scales less than 1 h(-1) Mpc is strongest for disc-dominated galaxies with C < 2.6. This demonstrates that different processes are required to explain environmental trends in the structure and in the star formation history of galaxies.

Accurate Fitting Formula for the Two-Point Correlation Function of Dark Matter Halos

An accurate fitting formula is reported for the two-point correlation function ? -->hh(r;M) of dark matter halos in hierarchical clustering models. It is valid for the linearly clustering regime, and its accuracy is about 10% in ? -->hh(r;M) for the halos with mass M > (10 -->?2-10 -->?3)M -->*, where M -->* is the characteristic nonlinear mass. The result is found on the basis of a careful analysis for a large set of scale-free simulations with 2563 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 neff

Semianalytical model of galaxy formation with high-resolution n-body simulations

{r eff}

High-order correlations of peaks and haloes: a step towards understanding galaxy biasing

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