arXiv subjects

Chung-Pei Ma

Publications and source records attributed to Chung-Pei Ma.

160 records · Page 9Linked to original sources

arXiv1999

Lensed Arcs and Inner Structure of Abell 697

We present new optical observations of the z=0.282 cluster Abell 697 from the Keck II telescope. Images show an unusual disturbed structure in the cD halo and a previously unknown faint gravitational lens arc. A spectrum of the arc did not yield a redshift, but its spectrum and colors suggest it lies at z>1.3. We construct models to reproduce the arc that show the potential is likely to be highly elliptical. We suggest that this cluster may have undergone a recent merger and is in the process of forming its cD galaxy. Analysis of X-ray data from ROSAT and ASCA suggests that the merging process is sufficiently advanced that the gas in the cluster has relaxed, and A697 lies near the L_x-T_x relation for normal clusters.

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arXiv1999

The Mass Power Spectrum in Quintessence Cosmological Models

We present simple analytic approximations for the linear and fully evolved nonlinear mass power spectrum for spatially flat cold dark matter (CDM) cosmological models with quintessence (Q). Quintessence is a time evolving, spatially inhomogeneous energy component with negative pressure and an equation of state w_Q < 0. It clusters gravitationally on large length scales but remains smooth like the cosmological constant on small length scales. We show that the clustering scale is determined by the Compton wavelength of the Q-field and derive a shape parameter, Γ_Q, to characterize the linear mass power spectrum. The growth of linear perturbations as functions of redshift, w_Q, and matter density Ω_m is also quantified. Calibrating to N-body simulations, we construct a simple extension of the formula by Ma (1998) that closely approximates the nonlinear power spectrum for a range of plausible QCDM models.

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arXiv1999

Galaxy Clustering and Large-Scale Structure from z = 0.2 to z = 0.5 in Two Norris Redshift Surveys

(abridged) We present a study of the nature and evolution of large-scale structure based on two independent redshift surveys of faint field galaxies conducted with the 176-fiber Norris Spectrograph on the Palomar 200-inch telescope. The two surveys together sparsely cover ~20 sq. degrees and contain 835 r < 21 mag galaxies with redshifts 0.2 < z < 0.5. Both surveys have a median redshift of z = 0.30. In order to obtain a rough estimate of the cosmic variance, we analyze the two surveys independently. We measure the comoving correlation length to be 3.70 +/- 0.13 h^-1 Mpc at z = 0.30 with a power-law slope gamma = 1.77 +/- 0.05. Dividing the sample into low (0.2 < z < 0.3) and high (0.32 < z < 0.5) redshift intervals, we see no evidence for a change in the comoving correlation length over the redshift range 0.2 < z < 0.5. Similar to the well-established results in the local universe, we find that intrinsically bright galaxies are more strongly clustered than intrinsically faint galaxies and that galaxies with little ongoing star formation, as judged from the rest-frame equivalent width of the [OII]3727, are more strongly clustered than galaxies with significant ongoing star formation. The rest-frame pairwise velocity dispersion of the sample is 326^+67_-52 km s^-1, ~25% lower than typical values measured locally. The appearance of the galaxy distribution, particularly in the more densely sampled Abell 104 field, is quite striking. The pattern of sheets and voids which has been observed locally continues at least to z ~ 0.5. A friends-of-friends analysis of the galaxy distribution supports the visual impression that > 90% of all galaxies at z < 0.5 are part of larger structures with overdensities of > 5.

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arXiv1999

Neutrinos and Dark Matter

In these lectures I highlight some key features of massive neutrinos in the context of cosmology. I first review the thermal history and the free-streaming kinematics of the uniform cosmic background neutrinos. I then describe how fluctuations in the phase space distributions of neutrinos and other particles arise and evolve after neutrino decoupling according to the linear perturbation theory of gravitational instability. The different clustering properties of massive neutrinos (aka hot dark matter) and cold dark matter are contrasted. The last part discusses the nonlinear stage of gravitational clustering and highlights the effects of massive neutrinos on the formation of cosmological structure.

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arXiv1998

Nonlinear Cosmological Structure Formation

I present a general discussion of the evolution and model-dependence of both the linear and nonlinear power spectrum of density fluctuations. The features of the linear power spectrum in cosmological models with cold dark matter (CDM) and cold+hot dark matter (C+HDM) are reviewed, and useful analytical approximations are summarized. Cosmological numerical simulation results are then used to illustrate the process of nonlinear gravitational clustering and to compute the nonlinear power spectrum. An analytical approximation relating the linear and nonlinear power spectrum in C+HDM models is presented.

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arXiv1998

Analytical Approximation to the Nonlinear Power Spectrum of Gravitational Clustering

This paper presents an analytical formula that closely approximates the fully nonlinear power spectrum of matter fluctuations for redshift $z\approx 5$ to 0 over a wide range of cosmologically interesting flat models with varying matter density $Ω_m$ and neutrino fraction $Ω_ν$. The functional form is motivated by analytical solutions in asymptotic regimes, but in order to obtain accurate approximations, the coefficients are calculated from fits to the nonlinear power spectrum computed from numerical simulations of four cosmological models. The transformation from the linear to nonlinear power spectrum depends on $Ω_m, Ω_ν$, and time. A simple scaling rule is introduced, which greatly simplifies the construction of the functional form and allows the formula to depend directly on the rms linear mass fluctuation $σ_8$ instead of on an effective spectral index as in earlier work.

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arXiv1998

Redshift Evolution of the Nonlinear Two-Point Correlation Function

This paper presents a detailed theoretical study of the two-point correlation function $ξ$ for both dark matter halos and the matter density field in five cosmological models with varying matter density $Ω_m$ and neutrino fraction $Ω_ν$. The objectives of this systematic study are to evaluate the nonlinear gravitational effects on $ξ$, to contrast the behavior of $ξ$ for halos vs. matter, and to quantify the redshift evolution of $ξ$ and its dependence on cosmological parameters. Overall, $ξ$ for halos exhibits markedly slower evolution than $ξ$ for matter, and its redshift dependence is much more intricate than the single power-law parameterization used in the literature. Of particular interest is that the redshift evolution of the halo-halo correlation length $r_0$ depends strongly on $Ω_m$ and $Ω_ν$, being slower in models with lower $Ω_m$ or higher $Ω_ν$. Measurements of $ξ$ to higher redshifts can therefore be a potential discriminator of cosmological parameters. The evolution rate of $r_0$ for halos within a given model increases with time, passing the phase of fixed comoving clustering at $z\sim 1$ to 3 toward the regime of stable clustering at $z\sim 0$. The shape of the halo-halo $ξ$, on the other hand, is well approximated by a power law with slope -1.8 in all models and is not a sensitive model discriminator.

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arXiv1998

Weighing Superclusters

We present a summary report on a detailed study of the mass and dynamical state of the Corona Borealis supercluster using the largest redshift survey of this object to date. Extensive $N$-body experiments are performed on simulated superclusters in both critically-flat and low-density cosmological models to test the robustness and errors of the mass estimators used on the data. From the redshifts and dynamics of 528 galaxies in the supercluster, the mass of Corona Borealis is estimated to be at least $3\times 10^{16} h^{-1} M_\odot$, and the mass-to-light ratio is $560 h (M/L)_\odot$, yielding a matter density parameter of $Ω_m\approx 0.4$. We also discuss the prospects for mapping the mass distribution on supercluster scales with gravitational lensing.

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arXiv1997

The Norris Survey of the Corona Borealis Supercluster: III. Structure and Mass of the Supercluster

(Abridged) We present a study of the structure and dynamics of the Corona Borealis Supercluster (z ~ 0.07) based on the redshifts for 528 galaxies in the supercluster. The galaxy distribution within Corona Borealis is clumpy and appears overall to be far from relaxed. A background supercluster at z ~ 0.11 makes a substantial contribution to the projected surface density of galaxies in the Corona Borealis field. In order to estimate the mass of the supercluster, we have assumed that the mass of the supercluster is proportional to v^2r, where v and r are suitable scale velocity and radius, respectively, and we have used N-body simulations of both critical- and low-density universes to determine the applicability of standard mass estimators based on this assumption. Although superclusters are obviously not in equilibrium, our simulations demonstrate that the virial mass estimator yields mass estimates with an insignificant bias and a dispersion of only ~25% for objects with overdensities >~ 5. Non-uniform spatial sampling can, however, cause systematic underestimates by as much as 30%. All of our simulated superclusters turn out to be bound, and based on the overdensity of the Corona Borealis supercluster, we believe it is also very likely to be bound and may well have started to collapse. The mass of Corona Borealis is at least 3 x 10^16 solar masses, which yields a B(AB)-band mass-to-light ratio of 564h (M/L)_solar on scales of ~20 h^-1 Mpc. The background supercluster has a mass-to-light ratio of 726h (M/L)_solar. By comparing the supercluster mass-to-light ratios with the critical mass-to-light ratio required to close the universe, we determine that Omega_0 >~ 0.4 on supercluster scales.

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arXiv1997

Cosmological Constraints from High-Redshift Damped Lyman-Alpha Systems

Any viable cosmological model must produce enough structure at early epochs to explain the amount of gas associated with high-redshift damped Ly$α$ systems. We study the evolution of damped Ly$α$ systems at redshifts $z\ge 2$ in cold dark matter (CDM) and cold+hot dark matter (CDM+HDM) models using both N-body and hydrodynamic simulations. Our approach incorporates the effects of gas dynamics, and we find that all earlier estimates which assumed that all the baryons in dark matter halos would contribute to damped Ly$α$ absorption have overestimated the column density distribution $f(N)$ and the fraction of neutral dense gas $Ω_g$ in damped Ly$α$ systems. The differences are driven by ionization of hydrogen in the outskirts of galactic halos and by gaseous dissipation near the halo centers, and they tend to exacerbate the problem of late galaxy formation in CDM+HDM models. We only include systems up to the highest observed column density $N\sim 10^{21.8}$ cm$^{-2}$ in the estimation of $Ω_g$ for a fair comparison with data. If the observed $f(N)$ and $Ω_g$ inferred from a small number of confirmed and candidate absorbers are robust, the amount of gas in damped Ly$α$ systems at high redshifts in the $Ω_ν=0.2$ CDM+HDM model falls well below the observations.

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arXiv1996

Linear Power Spectra in Cold+Hot Dark Matter Models: Analytical Approximations and Applications

This paper presents simple analytic approximations to the linear power spectra, linear growth rates, and rms mass fluctuations for both components in a family of cold+hot dark matter (CDM+HDM) models that are of current cosmological interest. The formulas are valid for a wide range of wavenumber, neutrino fraction, redshift, and Hubble constant: $k\lo 10\,h$ Mpc$^{-1}$, $0.05\lo \onu\lo 0.3$, $0\le z\lo 15$, and $0.5\lo h \lo 0.8$. A new, redshift-dependent shape parameter $Γ_ν=a^{1/2}\onu h^2$ is introduced to simplify the multi-dimensional parameter space and to characterize the effect of massive neutrinos on the power spectrum. The physical origin of $Γ_ν$ lies in the neutrino free-streaming process, and the analytic approximations can be simplified to depend only on this variable and $\onu$. Linear calculations with these power spectra as input are performed to compare the predictions of $\onu\lo 0.3$ models with observational constraints from the reconstructed linear power spectrum and cluster abundance. The usual assumption of an exact scale-invariant primordial power spectrum is relaxed to allow a spectral index of $0.8\lo n\le 1$. It is found that a slight tilt of $n=0.9$ (no tensor mode) or $n=0.95$ (with tensor mode) in $\onu\sim 0.1-0.2$ CDM+HDM models gives a power spectrum similar to that of an open CDM model with a shape parameter $Γ=0.25$, providing good agreement with the power spectrum reconstructed by Peacock and Dodds (1994) and the observed cluster abundance.

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arXiv1994

Massive Neutrinos and Galaxy Formation

We report the most recent results from high-resolution numerical simulations of structure formation in two flat cold+hot dark matter models with neutrino mass densities $\onu=0.2$ and 0.3. We find that structure forms too late in all CDM+HDM models with $\onu>0.2$ to account for the amount of dense neutral gas in high-redshift damped Lyman-$α$ systems. The $\onu=0.2$ model at $z\approx0$ provides a better match to observations than the pure CDM model.

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arXiv1994

Do Galactic Systems Form Too Late in Cold+Hot Dark Matter Models?

The abundance of galactic systems at high redshifts can impose a strong constraint on the cold+hot dark matter (CDM+HDM) models. The hot component reduces the excessive small-scale power in the COBE-normalized CDM model but also delays the epoch of galaxy formation. We present results from the first numerical simulations that have enough dynamic range to address accurately the issue of high-redshift halo abundances in CDM+HDM models. Equivalent high-resolution particle-particle/particle-mesh $N$-body simulations are performed for spatially flat models with $Ω_ν=0.3$ and 0.2 (with $H_0=50$ km s$^{-1}$ Mpc$^{-1}$ and $Ω_b=0.05$). We study the constraints placed on the models by the high-redshift quasar space density and by the mass fraction in neutral dense gas associated with damped Ly$α$ systems. We find that even with optimistic assumptions, the much-studied $Ω_ν=0.3$ model does not produce enough massive halos to account for the observed abundance of quasars at $z>4$. The model passes this test if $Ω_ν$ is decreased to 0.2. Both models do not produce enough high column-density halos to account for the amount of gas in damped Ly$α$ systems at $z\go 3$: the $Ω_ν=0.3$ model falls short by a factor $\sim$80; the $Ω_ν=0.2$ model by a factor $\sim$3. We conclude that only CDM+HDM models with $Ω_ν\lo 0.2$ can match observations at high redshift, implying an upper bound of 4.7 eV on the most massive light neutrino (presumably the $τ$).

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arXiv1994

Cosmological Perturbation Theory in the Synchronous vs. Conformal Newtonian Gauge

We present a systematic treatment of the linear theory of scalar gravitational perturbations in the synchronous gauge and the conformal Newtonian (or longitudinal) gauge. We first derive the transformation law relating the two gauges. We then write down in parallel in both gauges the coupled, linearized Boltzmann, Einstein and fluid equations that govern the evolution of the metric perturbations and the density fluctuations of the particle species. The particle species considered include cold dark matter (CDM), baryons, photons, massless neutrinos, and massive neutrinos (a hot dark matter or HDM candidate), where the CDM and baryon components are treated as fluids while a detailed phase-space description is given to the photons and neutrinos. The linear evolution equations presented are applicable to any $Ω=1$ model with CDM or a mixture of CDM and HDM. Isentropic initial conditions on super-horizon scales are derived. The equations are solved numerically in both gauges for a CDM+HDM model with $Ω_{\rm cold}=0.65,$ $Ω_{\rm hot}=0.3$, and $Ω_{\rm baryon}=0.05$. We discuss the evolution of the metric and the density perturbations and compare their different behaviors outside the horizon in the two gauges. In a companion paper we integrate the geodesic equations for the neutrino particles in the perturbed conformal Newtonian background metric computed here. The purpose is to obtain an accurate sampling of the neutrino phase space for the HDM initial conditions in $N$-body simulations of the CDM+HDM models.

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arXiv1993

A Calculation of the Full Neutrino Phase Space in Cold+Hot Dark Matter Models

This paper presents a general-relativistic N-body technique for evolving the phase space distribution of massive neutrinos in linear perturbation theory. The method provides a much more accurate sampling of the neutrino phase space for the HDM initial conditions of N-body simulations in a cold+hot dark matter universe than previous work. Instead of directly sampling the phase space at the end of the linear era, we first compute the evolution of the metric perturbations by numerically integrating the coupled, linearized Einstein, Boltzmann, and fluid equations for all particle species. We then sample the phase space shortly after neutrino decoupling at redshift z=10^9 when the distribution is Fermi-Dirac. To follow the trajectory of each neutrino, we subsequently integrate the geodesic equations for each neutrino in the perturbed background spacetime from z=10^9 to z=13.55, using the linearized metric found in the previous calculation to eliminate discreteness noise. The positions and momenta resulting from this integration represent a fair sample of the full neutrino phase space and can be used as HDM initial conditions for N-body simulations of nonlinear structure evolution in this model. A total of 21 million neutrino particles are used in a 100 Mpc box, with Omega_cdm=0.65, Omega_hdm=0.30, Omega_baryon=0.05, and Hubble constant H_0=50. We find that correlations develop in the neutrino densities and momenta which are absent when only the zeroth-order Fermi-Dirac distribution is considered.

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arXiv1992

SO(10) Cosmic Strings and Baryon Number Violation

SO(10) cosmic strings formed during the phase transition Spin(10) $\rightarrow$ SU(5) $\times{\cal Z}_2$ are studied. Two types of strings --- one effectively Abelian and one non-Abelian --- are constructed and the string solutions are calculated numerically. The non-Abelian string can catalyze baryon number violation via the ``twisting'' of the scalar field which causes mixing of leptons and quarks in the fermion multiplet. The non-Abelian string is also found to have the lower energy possibly for the entire range of the parameters in the theory. Scattering of fermions in the fields of the strings is analyzed, and the baryon number violation cross section is calculated. The role of the self-adjoint parameters is discussed and the values are computed.

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