arXiv subjects

Renyue Cen

Publications and source records attributed to Renyue Cen.

At least 235 records · Page 13Linked to original sources

arXiv1998

Supernovae, Pulsars and Gamma-Ray Bursts: A Unified Picture

A scenario is proposed that explains both the observed high pulsar velocities and extragalactic gamma-ray bursts (GRBs). The model involves an ultra- relativistic jet from a supernova (SN), that produces a GRB and its afterglow, whose characteristics are similar to an isotropic fireball GRB perhaps with some differences at late times in the afterglow once some significant transverse diffusion has occurred. The time scales and many other properties of GRBs and their afterglows in this model are consistent with observations. GRBs in this model have special intrinsic properties, that can either falsify or prove this model unambiguously by observations. The most direct proof is the detection of a SN about the same time as the luminous GRB event. Most GRBs and SNe are expected occur at moderate redshift ($z\sim 1-3$), if they follow the observed universal star formation history, as implied in this model. Searching for GRB/SN associations is a challenge, because majority of the SNe will be faint. Some additional, dramatic observable consequences are predicted, which can also be utilized to test the model.

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arXiv1998

Testing Cold Dark Matter Models At Moderate to High Redshift

The COBE microwave background temperature fluctuations and the abundance of local rich clusters of galaxies provide the two most powerful constraints on cosmological models. When all variants of the standard cold dark matter (CDM) model are subject to the combined constraint, the power spectrum of any model is fixed to $\sim 10%$ accuracy in both the shape and overall amplitude. These constrained models are not expected to differ dramatically in their local large-scale structure properties. However, their evolutionary histories differ, resulting in dramatic differences towards high redshift. We examine in detail six standardized, COBE and cluster normalized CDM models with respect to a large set of independent observations. The observations include correlation function of rich clusters of galaxies, galaxy power spectrum, evolution of rich cluster abundance, gravitational lensing by moderate -to-high redshift clusters, $\lya$ forest, damped $\lya$ systems, high redshift galaxies, reionization of the universe and future CMB experiments. It seems that each of the independent observations examined is or potentially is capable of distinguishing between at least some of the models. The combined power of several or all of these observations is tremendous. Thus, we appear to be on the verge of being able to make dramatic tests of all models in the near future using a rapidly growing set of observations, mostly at moderate to high redshift. Consistency or inconsistency between different observed phenomena on different scales and/or at different epochs with respect to the models will have profound implications for theory of growth of cosmic structure.

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arXiv1998

Gaussian Peaks and Clusters of Galaxies

(shortened) We develop and test a method to compute mass and auto-correlation functions of rich clusters of galaxies from linear density fluctuations, based on the formalism of Gaussian peaks (Bardeen et al 1986). The essential, new ingredient in the current approach is a simultaneous and unique fixture of the size of the smoothing window for the density field, $r_f$, and the critical height for collapse of a density peak, $δ_c$, for a given cluster mass (enclosed within the sphere of a given radius rather than the virial radius, which is hard to measure observationally). Of these two parameters, $r_f$ depends on both the mass of the cluster in question and $Ω$, whereas $δ_c$ is a function of only $Ω$ and $Λ$. These two parameters have formerly been treated as adjustable and approximate parameters. Thus, for the first time, the Gaussian Peak Method (GPM) becomes unambiguous, and more importantly, accurate, as is shown here. We apply this method to constrain all variants of the Gaussian cold dark matter (CDM) cosmological model using the observed abundance of local rich clusters of galaxies and the microwave background temperature fluctuations observed by COBE. The combined constraint fixes the power spectrum of any model to $\sim 10%$ accuracy in both the shape and overall amplitude. To set the context for analyzing CDM models, we choose six representative models of current interest, including an $Ω_0=1$ tilted cold dark matter model, a mixed hot and cold dark matter model with 20% mass in neutrinos, two lower density open models with $Ω_0=0.25$ and $Ω_0=0.40$, and two lower density flat models with $(Ω_0=0.25, Λ_0=0.75)$ and $(Ω_0=0.40,Λ_0=0.60)$. This suite of CDM models

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arXiv1998

On the Cluster Sunyaev-Zel'dovich Effect and Hubble Constant

This study shows one important effect of preexistent cosmic microwave background temperature fluctuations on the determination of the Hubble constant through Sunyaev-Zel'dovich effect of clusters of galaxies, especially when coupled with the gravitational lensing effect by the same clusters. The effect results in a broad distribution of the apparent Hubble constant. The combination of this effect with other systematic effects such as the Loeb-Refregier Effect seems to provide an explanation for the observationally derived values of the Hubble constant currently available based on the Sunyaev-Zel'dovich effect, if the true value of the Hubble constant is 60-80 km/s/Mpc. It thus becomes possible that the values of the Hubble constant measured by other techniques which generally give a value around 60-80 km/s/Mpc be reconciled with the SZ effect determined values of the Hubble constant, where are systematically lower than others and have a broad distribution.

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arXiv1997

Determining the Amplitude of Mass Fluctuations in the Universe

We present a method for determining the rms mass fluctuations on 8 h^-1 Mpc scale, sigma8. The method utilizes the rate of evolution of the abundance of rich clusters of galaxies. Using the Press-Schechter approximation, we show that the cluster abundance evolution is a strong function of sigma8: d log n/dz ~ -1/sigma8^2; low sigma8 models evolve exponentially faster than high sigma8 models, for a given mass cluster. For example, the number density of Coma-like clusters decreases by a factor of ~10^3$ from z = 0 to z ~ 0.5 for sigma8=0.5 models, while the decrease is only a factor of ~5 for sigma8 ~ 1. The strong exponential dependence on sigma8 arises because clusters represent rarer density peaks in low sigma8 models. We show that the evolution rate at z < 1 is insensitive to the density parameter $Ω$ or to the exact shape of the power spectrum. Cluster evolution therefore provides a powerful constraint on sigma8. Using available cluster data to z ~ 0.8, we find sigma8 = 0.83 +/- 0.15. This amplitude implies a bias parameter b ~ 1/sigma8 = 1.2 +/- 0.2, i.e., a nearly unbiased universe with mass approximately tracing light on large scales.

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arXiv1997

On the Clustering of Lyman Alpha Clouds

We examine the correlational property of $\lya$ clouds in detail and compare it to that of mass and galaxies. We show that the correlation strength of $\lya$ clouds is somewhat weaker than that of the underlying matter, which in turn is weaker than that of galaxies (biased galaxy formation). On the scales probed, $10-300$km/s, for $\lya$ clouds, we find that higher density, higher optical depth, higher column density regions are more strongly clustered than lower density, lower optical depth, lower column density regions. Thus, a consistent picture seems to emerge: the correlation strength for a given set of objects is positively correlated with their characteristic global density and the differences among the correlations of galaxies, $\lya$ clouds and mass reflect the differences in density that each trace. Significant positive correlations with a strength of $0.1-1.0$ are found for $\lya$ clouds in the velocity range $50-300$km/s. Among the correlational measures examined, an optical depth correlation function (Equation 5) proposed here may serve as the best correlational measure. It reasonably faithfully represents the true correlation of the underlying matter, enabling a better indication of both matter correlation and the relationship between galaxies and $\lya$ clouds. Furthermore, it appears to be an alternative to the conventional line-line correlation function with the virtue that it does not require ambiguous post-observation fitting procedures such as those commonly employed in the conventional line-finding methods. Neither does it depend sensitively on the observational resolution (e.g., FWHM), insofaras the clouds are resolved Conveniently, it can be easily measured with the current observational sensitivity without being contaminated significantly by the presence of noise.

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arXiv1997

A Critical Test of Topological Defect Models: Spatial Clustering of Clusters of Galaxies

Gaussian cosmological models, typified by the inflationary cold dark matter models, and non-Gaussian topological defect based cosmological models, such as the texture seeded model, differ in the origin of large-scale cosmic structures. In the former it is believed that peaks at appropriate scales in the initial high density field are the sites onto which matter accretes and collapses to form the present galaxies and clusters of galaxies, whereas in the latter these structures can form around the density perturbation seeds (which are textures in the texture model). Textures initially are randomly distributed on scales larger than their size, in sharp contrast to the initial high density peaks in the Gaussian models which are already strongly clustered before any gravitational evolution has occured. One thus expects that the resultant correlation of large cosmic objects such as clusters of galaxies in the texture model should be significantly weaker than its Gaussian counterpart. We show that an $Ω_0=1$ biased $b=2$ (as required by cluster abundance observations) texture model (or any random seed model) predicts a two-point correlation length of $\le 6.0h^{-1}$Mpc for rich clusters, independent of richness. On the other hand, the observed correlation length for rich clusters is $\ge 10.0h^{-1}$Mpc at an approximately $2σ$ confidence level. It thus appears that the global texture cosmological model or any random seed cosmological models of this sort are ruled out at a very high confidence ($>3σ$).

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arXiv1997

The Galaxy Pairwise Velocity Dispersion as a Function of Local Density

The standard method of measuring the galaxy pairwise velocity dispersion on small scales is heavily weighted by the densest regions in a way that is difficult to calibrate. We propose a new statistic which measures the small-scale velocity dispersion as an explicit function of density. Computing this statistic for a volume-limited subsample of the Optical Redshift Survey, we find that the small-scale velocity dispersion rises from 220 to 760 km/s as density increases. We calculate this statistic for a series of mock catalogs drawn from a hydrodynamic simulation of an Ωh = 0.5 Cold Dark Matter universe (standard CDM), and find that the observed velocity distribution lies ~1 σbelow the simulations in each of eight density bins, formally ruling out this model at the 7.4 σlevel, quantifying the well-known problem that this model produces too high a velocity dispersion. This comparison is insensitive to the normalization of the power spectrum, although it is quite sensitive to the density and velocity bias of galaxies relative to dark matter on small scales. (abridged)

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arXiv1997

Constraining Omega with Cluster Evolution

We show that the evolution of the number density of rich clusters of galaxies breaks the degeneracy between Omega (the mass density ratio of the universe) and sigma_{8} (the normalization of the power spectrum), sigma_{8}Omega^{0.5} \simeq 0.5, that follows from the observed present-day abundance of rich clusters. The evolution of high-mass (Coma-like) clusters is strong in Omega=1, low-sigma_{8} models (such as the standard biased CDM model with sigma_{8} \simeq 0.5), where the number density of clusters decreases by a factor of \sim 10^{3} from z = 0 to z \simeq 0.5; the same clusters show only mild evolution in low-Omega, high-sigma_{8} models, where the decrease is a factor of \sim 10. This diagnostic provides a most powerful constraint on Omega. Using observations of clusters to z \simeq 0.5-1, we find only mild evolution in the observed cluster abundance. We find Omega = 0.3 \pm 0.1 and sigma_{8} = 0.85 \pm 0.15 (for Lambda = 0 models; for Omega + Lambda = 1 models, Omega = 0.34 \pm 0.13). These results imply, if confirmed by future surveys, that we live in a low-den sity, low-bias universe.

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arXiv1997

Towards Understanding Galaxy Clusters and Their Constituents:Projection Effects on Velocity Dispersion, X-Ray Emission, Mass Estimates, Gas Fraction and Substructure

We study the projection effects on various observables of clusters of galaxies at redshift near zero, including cluster richness, velocity dispersion, X-ray luminosity, three total mass estimates (velocity-based, temperature-based and gravitational lensing derived), gas fraction and substructure. Unlike previous studies focusing on the Abell clusters, we conservatively assume that both optical and X-ray observations can determine the source (galaxy or hot X-ray gas) positions along the line of sight as well as in the sky plane accurately; hence we only include sources inside the velocity space defined by the cluster galaxies (filtered through the pessimistic 3 sigma clipping algorithm) as possible contamination sources. Projection effects are found to be important for some quantities but insignificant for others. We show that, on average, the gas to total mass ratio in clusters appears to be 30-40% higher than its corresponding global ratio. Independent of its mean value, the broadness of the observed distribution of gas to total mass ratio is adequately accounted for by projection effects, alleviating the need to invoke (though not preventing) other non gravitational physical processes. While the moderate boost in the ratio narrows the gap, it is still not quite sufficient to reconcile the standard nucleosynthesis value of $Ω_b=0.0125(H_0/100)^{-2}$ and $Ω=1$ with the observed gas to mass ratio value in clusters of galaxies, $0.05(H_0/100)^{-3/2}$, for any plausible value of $H_0$. However, it is worth noting that real observations of X-ray clusters, especially X-ray imaging observations, may be subject to more projection contaminations than we allow for in our analysis. In contrast, the X-ray luminosity of a cluster within a radius $\leq 1.0h^{-1}$Mpc is hardly

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arXiv1997

Sizes, Shapes, and Correlations of Lyman Alpha Clouds and Their Evolution in the CDM$+Λ$ Universe

This study analyzes the sizes, shapes and correlations of $\lya$ clouds produced by a hydrodynamic simulation of a spatially flat CDM universe with a non-zero cosmological constant ($Ω_0=0.4$, $Λ_0=0.6$, $σ_8 =0.79$), over the redshift range $2\le z \le 4$. The $\lya$ clouds range in size from several kiloparsecs to about a hundred kiloparsecs in proper units, and they range in shape from roundish, high column density regions with $\nhi\ge 10^{15} cm^{-2}$ to low column density sheet-like structures with $\nhi \le 10^{13} cm^{-2}$ at z=3. The most common shape found in the simulation resembles that of a flattened cigar. The physical size of a typical cloud grows with time roughly as $(1+z)^{-3/2}$ while its shape hardly evolves (except for the most dense regions $ρ_{cut}>30$). Our result indicates that any simple model with a population of spheres (or other shapes) of a uniform size is oversimplified; if such a model agrees with observational evidence, it is probably only by coincidence. We also illustrate why the use of double quasar sightlines to set lower limits on cloud sizes is useful only when the perpendicular sightline separation is small ($Δr \le 50h^{-1}$ kpc). Finally, we conjecture that high column density $\lya$ clouds ($\nhi\ge 10^{15} cm^{-2}$) may be the progenitors of the lower redshift faint blue galaxies. This seems plausible because their correlation length, number density (extrapolated to lower redshift) and their masses are in fair agreement with those observed.

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arXiv1997

Testing Cosmological Models With A $\lya$ Forest Statistic: The High End Of The Optical Depth Distribution

We pay particular attention to the high end of the $\lya$ optical depth distribution of a quasar spectrum. Based on the flux distribution (Miralda-Escudé et al 1996), a simple yet seemingly cosmological model -differentiating statistic, $Δ_{τ_0}$ -- the cumulative probability of a quasar spectrum with $\lya$ optical depth greater than a high value $τ_0$ -- is emphasized. It is shown that two different models -- the cold dark matter model with a cosmological constant and the mixed hot and cold dark matter model, both normalized to COBE and local galaxy cluster abundance -- yield quite different values of $Δ_{τ_0}$: 0.13 of the former versus 0.058 of the latter for $τ_0=3.0$ at $z=3$. Moreover, it is argued that $Δ_{τ_0}$ may be fairly robust to compute theoretically because it does not seem to depend sensitively on small variations of simulations parameters such as radiation field, cooling, feedback process, radiative transfer, resolution and simulation volume within the plausible ranges of the concerned quantities. Furthermore, it is illustrated that $Δ_{τ_0}$ can be obtained sufficiently accurately from currently available observed quasar spectra for $τ_0\sim 3.0-4.0$, when observational noise is properly taken into account. We anticipate that analyses of observations of quasar $\lya$ absorption spectra over a range of redshift may be able to constrain the redshift evolution of the amplitude of the density fluctuations on small-to-intermediate scales, therefore providing an independent constraint on $Ω_0$, $Ω_{0,HDM}$ and $Λ_0$.

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arXiv1996

Testing Cosmological Models by Gravitational Lensing: I. Method and First Applications

Gravitational lensing directly measures mass density fluctuations along the lines of sight to very distant objects. No assumptions need to be made concerning bias, the ratio of fluctuations in galaxy density to mass density. Hence, lensing is a very useful tool to study the universe at low to moderate redshifts. We describe in detail a new method to trace light rays from redshift zero through a three dimensional mass distribution to high redshift. As an example, this method is applied here to a standard cold dark matter universe. We obtain a variety of results, some of them statistical in nature, others from rather detailed case studies of individual ``lines of sight". Among the former are the frequency of multiply imaged quasars, the distribution of separation of the multiple quasars, and the redshift distribution of lenses: all that as a function of quasar redshift. We find effects from very weak lensing to highly magnified multiple images of high redshift objects, which, for extended background sources, (i.e. galaxies), range from slight deformations of the shapes through tangentially aligned arclets up to giant luminous arcs. Different cosmological models differ, increasingly with redshift, in their predictions for the mass (thus gravitational potential) distributions. Our ultimate goal is to apply this method to a number of cosmogonic models and to eliminate some models whose gravitational lensing properties are inconsistent with those observed.

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arXiv1996

The Protogalactic Origin for Cosmic Magnetic Fields

It is demonstrated that strong magnetic fields are produced from a zero initial magnetic field during the pregalactic era, when galaxies are first forming. Their development proceeds in three phases. In the first phase, weak magnetic fields are created by the Biermann battery mechanism, acting in shocked parts of the intergalactic medium where caustics form and intersect. In the second phase, these weak magnetic fields are amplified to strong magnetic fields by the Kolmogoroff turbulence endemic to gravitational structure formation of galaxies. During this second phase, the magnetic fields reach saturation with the turbulent power, but they are coherent only on the scale of the smallest eddy. In the third phase, the magnetic field strength increases to equipartition with the turbulent energy, and the coherence length of the magnetic fields increases to the scale of the largest turbulent eddy, comparable to the scale of the entire galaxy. The resulting magnetic field represents a galactic magnetic field of primordial origin. No further dynamo action is necessary, after the galaxy forms, to explain the origin of magnetic fields. However, the magnetic field may be altered by dynamo action once the galaxy and the galactic disk have formed. It is first shown by direct numerical simulations, that thermoelectric currentsassociated with the Biermann battery, build the field up from zero to $10^{-21}$ G in the regions about to collapse into galaxies, by $z\sim3$. For weak fields, in the absence of dissipation, the cyclotron frequency ${\bf ω_{cyc}}=e{\bf B } /m_H c $ and $ {\bf ω}/(1+ χ)$, where ${\bf ω= \nabla \times v }$ is the vorticity and $χ$ is the degree of ionization, satisfy the same equations, and initial conditions ${\bf ω_{cyc}=ω}=0$, so that, ${\bf ω_{cyc}}({\bf

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arXiv1996

Effects of Weak Gravitational Lensing from Large-Scale Structure on the Determination of $q_0$

Weak gravitational lensing by large-scale structure affects the determination of the cosmological deceleration parameter $q_0$. We find that the lensing induced dispersions on truly standard candles are $0.04$ and $0.02$ mag at redshift $z=1$ and $z=0.5$, respectively, in a COBE-normalized cold dark matter universe with $Ω_0=0.40$, $Λ_0=0.6$, $H=65$km/s/Mpc and $σ_8=0.79$. It is shown that one would observe $q_0=-0.44^{+0.17}_{-0.05}$ and $q_0=-0.45^{+0.10}_{-0.03}$ (the errorbars are $2σ$ limits) with standard candles with zero intrinsic dispersion at redshift $z=1$ and $z=0.5$, respectively, compared to the truth of $q_0=-0.40$ in this case, i.e., a 10\% error in $q_0$ will be made. A standard COBE normalized $Ω_0=1$ CDM model would produce three times as much variance and a mixed (hot and cold) dark matter model would lead to an intermediate result. One unique signature of this dispersion effect is its non Gaussianity. Although the lensing induced dispersion at lower redshift is still significantly smaller than the currently best observed (total) dispersion of $0.12$ mag in a sample of type Ia supernovae, selected with the multicolor light curve shape method, it becomes significant at higher redshift. We show that there is an optimal redshift, in the range $z\sim 0.5-2.0$ depending on the amplitude of the intrinsic dispersion of the standard candles, at which $q_0$ can be most accurately determined.

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arXiv1996

Pattern in Nonlinear Gravatational Clustering: A Numerical Investigation

The nonlinear clustering of dark matter particles in an expanding universe is usually studied by N-body simulations. One can gain some insight into this complex problem if simple relations between physical quantities in the linear and nonlinear regimes can be extracted from the results of N-body simulations. Hamilton et al. (1991) and Nityananda and Padmanabhan (1994) have made an attempt in this direction by relating the mean relative pair velocities to the mean correlation function in a useful manner. We investigate this relation and other closely related issues in detail for the case of six different power spectra: power laws with spectral indexes $n=-2,-1$, cold dark matter (CDM), and hot dark matter models with density parameter $Ω=1$; CDM including a cosmological constant ($Λ$) with $Ω_{CDM}=0.4$, $Ω_Λ=0.6$; and $n=-1$ model with $Ω=0.1$. We find that: (i) Power law spectra lead to self-similar evolution in an $Ω=1$ universe. (ii) Stable clustering does not hold in an $Ω=1$ universe to the extent our simulations can ascertain. (iii) Stable clustering is a better approximation in the case of $Ω<1$ universe in which structure formation freezes out at some low redshift. (iv) The relation between dimensionless pair velocity and the mean correlation function, $\barξ$, is only approximately independent of the shape of the power spectrum. At the nonlinear end, the asymptotic value of the dimensionless pair velocity decreases with increasing small scale power, because the stable clustering assumption is not universally true. (v) The relation between the evolved $\barξ$ and the linear regime $\barξ$ is also not universal but shows a weak spectrum dependence. We present simple theoretical arguments for these conclusions.

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arXiv1996

Hydrodynamic Simulations of the Growth of Cosmological Structure: Summary and Comparisons Among Scenarios

We compute, including a current state-of-the-art treatment of hydrodynamical processes, heating and cooling, a variety of cosmological models into the extreme nonlinear phase to enable comparisons with observations. First, we note the common, model independent results. All have a mean ($z=0$) temperature of $10^{4.5}-10^{5.5}$\Kel, set essentially by photoheating processes. Most gas is in one of two components: either at the photoheating floor $10^{4.5}$\Kel and primarily in low density regions or else shock heated to $10^5-10^6$\Kel and in regions of moderate overdensity (in caustics and near groups and clusters). It presents a major observational challenge to observationally detect this second, abundant component as it is neither an efficient radiator nor absorber. About 2% to 10% of the baryons cool and collapse into galaxies forming on caustics and migrating to clusters. About 1%-2% of baryons are in the very hot X-ray emitting gas near cluster cores, in good agreement with observations. These correspondances between the simulations and the real world imply that there is some significant truth to the underlying standard scenarios for the growth of structure. The differences among model predictions may help us find the path to the correct model. For COBE normalized models the most relevant differences concern epoch of structure formation. In the open variants having $Ω=0.3$, with or without a cosmological constant, structure formation on galactic scales is well advanced at redshift z=5, and reionization occurs early. But if observations require models for which most galaxy formation occurs more recently than $z=2$, then the flat $Ω=1$ models are to be preferred. The velocity dispersion on the $1h^{-1}$Mpc scale also provides a strong discriminant with, as expected, the $Ω=1$ models giving a much higher

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arXiv1995

The Lyman Alpha Forest from Gravitational Collapse in the CDM+Lambda Model

We model the $\lya$ forest in a spatially flat, CDM model with $Ω=0.4$, with an Eulerian hydrodynamic simulation, and find that the intergalactic, photoionized gas collapses into sheet-like and filamentary structures which produce absorption lines like those observed in the $\lya$ forest. A typical filament is $\sim 1 h^{-1} \mpc$ long with thickness $\sim 50-100 h^{-1} \kpc$ (in proper units), and baryonic mass $\sim 10^{10} h^{-1} \msun$. The gas temperature increases with time as structures with larger velocities collapse gravitationally. The predicted distributions of column densities and b-parameters of the absorption lines agree with the observed ones, and their evolution is consistent with our model if the ionizing background has an approximately constant intensity between $z=2$ and $z=4$. We also predict the distribution of transmitted flux and its correlation along a spectrum and on parallel spectra, and the \heii flux decrement as a function of redshift. We predict a correlation length of $\sim 80 h^{-1}\kpc$ perpendicular to the line of sight for features in the Lyman alpha forest. In order to reproduce the observed number of lines and average flux transmission, the baryon content of the clouds may need to be significantly higher than in previous models because of the low densities we predict. If the background intensity $\jhi$ is at least that predicted from the observed quasars, $Ω_b$ needs to be $\sim 0.025 h^{-2}$, higher than expected by light element nucleosynthesis; the model also predicts that most of the baryons at $z>2$ are in $\lya$ clouds, and that the rate at which the baryons move to more overdense regions is slow.

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