arXiv subjects

G. Boerner

Publications and source records attributed to G. Boerner.

At least 19 recordsLinked to original sources

arXiv2009

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 varies with mass scale dramatically 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 (MAHs), 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. According to two simple but tight correlations we find from the simulation results, we develop new empirical models for both the MAHs 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 the model predictions are highly accurate even when the histories are traced to very high redshift. These models are also simple and easy to implement. A web calculator and a user-friendly code to make the relevant calculations are available from http://www.shao.ac.cn/dhzhao/mandc.html . We explain why Lambda CDM halos on nearly all mass scales show two distinct phases in their evolution histories.

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arXiv2009

Mass and Redshift Dependence of Dark Halo Structure

Using a combination of N-body simulations with different resolutions, we study in detail how the concentrations of cold dark matter (CDM) halos depend on halo mass at different redshifts. We confirm that halo concentrations at the present time depend strongly on halo mass, but our results also show marked differences from the predictions of some early empirical models. Our main result is that the mass dependence of the concentrations becomes weaker at higher redshifts, and at z >~ 3 halos of mass greater than 10^{11} Msun/h all have a similar median concentration, c ~ 3.5. While the median concentrations of low-mass halos grow significantly with time, those of massive halos change only little with redshifts. These results are quantitatively in good agreement with the empirical model proposed by Zhao et al. which shows that halos in the early fast accretion phase all have similar concentrations.

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arXiv2005

Semi-analytical model of galaxy formation with high-resolution N-body simulations

We model the galaxy formation in a series of high-resolution N-body simulations using the semi-analytical approach. Unlike many earlier investigations based on semi-analytical models, we make use of the subhalos resolved in the $N$-body simulations to follow the mergers of galaxies in dark halos, and we show that this is pivotal in modeling correctly the galaxy luminosity function at the bright end and the bimodal nature of galaxy color distribution. Mergers of galaxies based on subhalos also result in many more bright red galaxies at high $z$. The semi-analytical model we adopt is similar to those used in earlier semi-analytical studies, except that we consider the effect of a prolonged cooling in small halos and that we explicitly follow the chemical enrichment in the interstellar medium. We use our model to make predictions for the properties of the galaxy population at low redshift and compare them with various current observations. We find that our model predictions can match the luminosity functions of galaxies in various wavebands redder than the u-band. The shape of the luminosity function at bright end is well reproduced if galaxy mergers are modeled with the merger trees of subhalos and the steep faint-end slope can be moderated if the gas cooling time in low-mass halos is comparable to the age of the universe. The model with subhalos resolved can reproduce the main features in the observed color bimodal distribution, though it still predicts too many bright blue galaxies. The same model can also match the color-magnitude relation for elliptical galaxies in clusters, the metallicity-luminosity relation.

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arXiv2004

The pairwise velocity dispersion of galaxies: luminosity dependence and a new test of galaxy formation models

We present the first determination of the pairwise velocity dispersion (PVD) for galaxies in different luminosity intervals using the final release of the Two-Degree Field Galaxy Redshift Survey (2dFGRS). We have discovered quite surprisingly that the relative velocities of the faint galaxies at small separation are very high, around $700 \kms$, reaching similar values as the brightest galaxies. At intermediate luminosities $M^*-1$ ($M^*$ is the characteristic luminosity of the Schechter function), the relative velocities exhibit a well defined steep minimum near $400 \kms$. This result has been derived using a novel method to determine the real space power spectrum and the PVD from the redshift space power spectrum of the 2dFGRS. Both quantities can be determined quite reliably. We have taken the parameter $β$ equal to 0.45. But we have also checked that the results are changed very little, if we allow a variation of $β$ with luminosity. The result is a challenge to the current halo model of galaxies of Yang et al. that was obtained by matching the clustering and luminosity function of the 2dFGRS, but cannot reproduce the luminosity dependence of the PVD. It may also be an mportant constraint in general on theories of galaxy formation, such as semi-analytical models and hydro/N-body simulations of galaxy formation. Combined with the observed luminosity dependence of clustering, our result implies that a substantial fraction of faint galaxies, as well as the brightest ones, are in massive halos of galaxy cluster size, but most of the $M^*$ galaxies are in galactic halos.

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arXiv2004

Velocity of galaxies with different luminosity

We present the first determination of the pairwise velocity dispersion of galaxies at different luminosity with the final release of the Two-Degree Field Galaxy Redshift Survey (2dFGRS). Our result surprisingly shows that the random velocities of the faint galaxies are very high, around $ 700 \kms$, reaching similar values as the brightest galaxies. At intermediate luminosities slightly brighter than the characteristic luminosity $M_\star$, the velocities exhibit a well defined steep minimum near $ 400 \kms$. The result challenges the current halo model of galaxies of Yang et al. that was obtained by matching the clustering and luminosity function of 2dFGRS, and can be an important constraint in general on theories of galaxy formation, e.g., the semi-analytical model. Combining the observed luminosity dependence of clustering, our result implies that quite a fraction of faint galaxies are in massive halos of galaxy clusters as the brightest ones, but most of the $M_\star$ galaxies are in galactic halos.

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arXiv2004

The Three-point Correlation Function of Galaxies Determined from the 2dF Galaxy Redshift Survey

In a detailed analysis of the three point correlation function (3PCF) for the 2dF Galaxy Redshift Survey we have accurately measured the 3PCF for galaxies of different luminosity. The 3PCF amplitudes [$\Qsu$ or $\Qrpu$] of the galaxies generally decrease with increasing triangle size and increase with the shape parameter $v$, in qualitative agreement with the predictions for the clustering of dark matter in popular hierarchical CDM models. The 2dFGRS results agree well with the results of Jing & Börner for the Las Camapanas Redshift Survey (LCRS), though the measurement accuracy is greatly improved in the present study. The dependence of the 3PCF on luminosity is not significant, but there seems to be a trend for the brightest galaxy sample to have a lower amplitude than the fainter ones. Comparing the measured 3PCF amplitudes [$\Qsu$ or $\Qrpu$] to the prediction of a WMAP concordance model, we find that the measured values are consistently lower than the predicted ones for dark matter. This is most pronounced for the brightest galaxies, for which about one-half of the predicted $Q$ value provides a good description of $\Qrpu$ for the 2dFGRS data. For a less luminous sample, the $Q$ values are also smaller than in the dark matter model on small scales, but on scales larger than $s=8 \mpc$ and $r_p=3.25 \mpc$ they reach the model values. We discuss implications for current theories of galaxy formation.

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arXiv2002

Testing Theoretical Models for the Higher-Order Moments of Dark Halo Distribution

Using high--resolution N--body simulations, we test two theoretical models, based either on spherical or on ellipsoidal collapse model, for the higher--order moments of the dark matter halo distribution in CDM models. We find that a theoretical model based on spherical collapse describes accurately the simulated counts--in--cells moments for haloes of several mass ranges. It appears that the model using ellipsoidal collapse instead of spherical collapse in defining dark haloes is unable to improve the models for the higher--order moments of halo distribution, for haloes much smaller than $M^*$ (the mass scale on which the fluctuation of the density field has a rms about 1). Both models are particularly accurate for the descendants of haloes selected at high redshift, and so are quite useful in interpreting the high--order moments of galaxies. As an application we use the theoretical model to predict the higher--order moments of the Lyman break galaxies observed at $z\approx 3$ and their descendants at lower redshifts.

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arXiv2002

Pairwise velocity dispersion of galaxies at high redshift: theoretical predictions

We investigate the feasibility of determining the pairwise velocity dispersion (PVD) for Lyman Break Galaxies (LBGs), and of using this quantity as a discriminator among theoretical models. We find that when the Two Point Correlation Function over the range of observable separations is fixed, different schemes of galaxy formation will lead to significant changes in PVD, while influence of different cosmogonies is invisible. Our test with mock samples shows furthermore that one can discriminate among such models already with currently available observational samples (if the measurement error of the redshift is negligible) which have a typical error of $80\kms$. The error will be reduced by a factor of 2 if the samples are increased four times. We also show that an erroneous assumption about the geometry of the universe and different infall models only slightly change the results. Therefore the PVD will become another promising statistic to test galaxy formation models with redshift samples of LBGs.

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arXiv2002

On the Distribution of Haloes, Galaxies and Mass

The stochasticity in the distribution of dark haloes in the cosmic density field is reflected in the distribution function $P_V(N_h|δ_m)$ which gives the probability of finding $N_h$ haloes in a volume $V$ with mass density contrast $δ_m$. We study the properties of this function using high-resolution $N$-body simulations, and find that $P_V(N_n|δ_m)$ is significantly non-Poisson. The ratio between the variance and the mean goes from $\sim 1$ (Poisson) at $1+δ_m\ll 1$ to $<1$ (sub-Poisson) at $1+δ_m\sim 1$ to $>1$ (super-Poisson) at $1+δ_m\gg 1$. The mean bias relation is found to be well described by halo bias models based on the Press-Schechter formalism. The sub-Poisson variance can be explained as a result of halo-exclusion while the super-Poisson variance at high $δ_m$ may be explained as a result of halo clustering. A simple phenomenological model is proposed to describe the behavior of the variance as a function of $δ_m$. Galaxy distribution in the cosmic density field predicted by semi-analytic models of galaxy formation shows similar stochastic behavior. We discuss the implications of the stochasticity in halo bias to the modelling of higher-order moments of dark haloes and of galaxies.

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arXiv2002

Slow relative motion of IRAS galaxies at small separations: implications for galaxy formation models

We report on the measurement of the two-point correlation function and the pairwise peculiar velocity of galaxies in the IRAS PSCz survey. The real space two-point correlation function can be fitted to a power law $ξ(r) = (r_0/r)^γ$ with $γ=1.69$ and $r_0=3.70 \mpc$. The pairwise peculiar velocity dispersion $σ_{12}(r_p)$ is close to $400 \kms$ at $r_p=3\mpc$ and decreases to about $150 \kms$ at $r_p \approx 0.2 \mpc$. These values are significantly lower than those obtained from the Las Campanas Redshift Survey, but agree very well with the results of blue galaxies reported by the SDSS team later on. We have constructed mock samples from N-body simulations with a cluster-weighted bias and from the theoretically constructed GIF catalog. We find that the two-point correlation function of the mock galaxies can be brought into agreemnt with the observed result, but the model does not reduce the velocity dispersions of galaxies to the level measured in the PSCz data. Thus we conclude that the peculiar velocity dispersions of the PSCz galaxies require a biasing model which substantially reduces the peculiar velocity dispersion on small scales relative to their spatial clustering. The results imply that either the cosmogony model needs to be revised or the velocity bias is important for the velocity dispersion of the IRAS galaxies.

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arXiv2001

Spatial correlation functions and the pairwise peculiar velocity dispersion of galaxies in the PSCz survey: implications for the galaxy biasing in cold dark matter models

We report on the measurement of the two-point correlation function, and the pairwise peculiar velocity of galaxies in the IRAS PSCz survey. We compute these statistics first in redshift space, and then obtain the projected functions which have simple relations to the real-space correlation functions on the basis of the method developed earlier in analyzing the Las Campanas Redshift Survey (LCRS) by Jing, Mo, & Börner (1998). We find that the real space two-point correlation function can be fitted to a power law $ξ(r) = (r_0/r)^γ$ with $γ=1.69$ and $r_0=3.70 \mpc$. The pairwise peculiar velocity dispersion $σ_{12}(r_p)$ is close to $400 \kms$ at $r_p=3\mpc$ and decreases to about $150 \kms$ at $r_p \approx 0.2 \mpc$. These values are significantly lower than those obtained from the LCRS. In order to understand the implications of those measurements on the galaxy biasing, we construct mock samples for a low density spatially-flat cold dark matter model ($Ω_0 = 0.3$, $λ_0=0.7$, $Γ=0.2$, $σ_8=1$) using a set of high-resolution N-body simulations. Applying a stronger cluster-underweight biasing ($\propto M^{-0.25}$) than for the LCRS ($\propto M^{-0.08}$), we are able to reproduce these observational data, except for the strong decrease of the pairwise peculiar velocity at small separations. This is qualitatively ascribed to the different morphological mixture of galaxies in the two catalogues. Disk-dominated galaxy samples drawn from the theoretically constructed GIF catalog yield results rather similar to our mock samples with the simple cluster-underweight biasing.

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arXiv2001

The Scaling of the Redshift Power Spectrum: Observations from the Las Campanas Redshift Survey

In a recent paper we have studied the redshift power spectrum $P^S(k,μ)$ in three CDM models with the help of high resolution simulations. Here we apply the method to the largest available redshift survey, the Las Campanas Redshift Survey (LCRS). The basic model is to express $P^S(k,μ)$ as a product of three factors P^S(k,μ)=P^R(k)(1+βμ^2)^2 D(k,μ). Here $μ$ is the cosine of the angle between the wave vector and the line of sight. The damping function $D$ for the range of scales accessible to an accurate analysis of the LCRS is well approximated by the Lorentz factor D=[1+{1\over 2}(kμσ_{12})^2]^{-1}. We have investigated different values for $β$ ($β=0.4$, 0.5, 0.6), and measured $P^R(k)$ and $σ_{12}(k)$ from $P^S(k,μ)$ for different values of $μ$. The velocity dispersion $σ_{12}(k)$ is nearly a constant from $k=0.5$ to 3 $\mpci$. The average value for this range is $510\pm 70 \kms$. The power spectrum $P^R(k)$ decreases with $k$ approximately with $k^{-1.7}$ for $k$ between 0.1 and 4 $\mpci$. The statistical significance of the results, and the error bars, are found with the help of mock samples constructed from a large set of high resolution simulations. A flat, low-density ($Ω_0=0.2$) CDM model can give a good fit to the data, if a scale-dependent special bias scheme is used which we have called the cluster-under-weighted bias (Jing et al.).

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arXiv2000

Scaling properties of the redshift power spectrum: theoretical models

We report the results of an analysis of the redshift power spectrum $P^S(k,μ)$ in three typical Cold Dark Matter (CDM) cosmological models, where $μ$ is the cosine of the angle between the wave vector and the line-of-sight. Two distinct biased tracers derived from the primordial density peaks of Bardeen et al. and the cluster-underweight model of Jing, Mo, & Börner are considered in addition to the pure dark matter models. Based on a large set of high resolution simulations, we have measured the redshift power spectrum for the three tracers from the linear to the nonlinear regime. We investigate the validity of the relation - guessed from linear theory - in the nonlinear regime $$ P^S(k,μ)=P^R(k)[1+βμ^2]^2D(k,μ,σ_{12}(k)), $$ where $P^R(k)$ is the real space power spectrum, and $β$ equals $Ω_0^{0.6}/b_l$. The damping function $D$ which should generally depend on $k$, $μ$, and $σ_{12}(k)$, is found to be a function of only one variable $kμσ_{12}(k)$. This scaling behavior extends into the nonlinear regime, while $D$ can be accurately expressed as a Lorentz function - well known from linear theory - for values $D > 0.1$. The difference between $σ_{12}(k)$ and the pairwise velocity dispersion defined by the 3-D peculiar velocity of the simulations (taking $r=1/k$) is about 15%. Therefore $σ_{12}(k)$ is a good indicator of the pairwise velocity dispersion. The exact functional form of $D$ depends on the cosmological model and on the bias scheme. We have given an accurate fitting formula for the functional form of $D$ for the models studied.

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arXiv2000

Low Redshift QSO Lyman alpha Absorption Line Systems Associated with Galaxies

In this paper we present Monte-Carlo simulations of Lyman alpha absorption systems which originate in galactic haloes, galaxy discs and dark matter (DM) satellites around big central haloes. It is found that for strong Lyman alpha absorption lines galactic haloes and satellites can explain ~20% and 40% of the line number density of QSO absorption line key project respectively. If big galaxies indeed possess such large numbers of DM satellites and they possess gas, these satellites may play an important role for strong Lyman alpha lines. However the predicted number density of Lyman-limit systems by satellites is \~0.1 (per unit redshift), which is four times smaller than that by halo clouds. Including galactic haloes, satellites and HI discs of spirals, the predicted number density of strong lines can be as much as 60% of the HST result. The models can also predict all of the observed Lyman-limit systems. The average covering factor within 250 kpc/h is estimated to be ~0.36. And the effective absorption radius of a galaxy is estimated to be ~150 kpc/h. The models predict W_r propto rho^{-0.5} L_B^{0.15} (1+z)^{-0.5}. We study the selection effects of selection criteria similar to the imaging and spectroscopic surveys. We simulate mock observations through known QSO lines-of-sight and find that selection effects can statistically tighten the dependence of line width on projected distance. (abridged)

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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.

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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.

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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.

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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

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