arXiv subjects

Richard Matzner

Publications and source records attributed to Richard Matzner.

23 records · Page 2Linked to original sources

arXiv2002

Light Propagation in Inhomogeneous Universes. III. Distributions of Image Separations

Using an analytical model, we compute the distribution of image separations resulting from gravitational lensing of distant sources, for 7 COBE-normalized CDM models with various combinations of Omega_0 and lambda_0. Our model assumes that multiple imaging results from strong lensing by individual galaxies. We model galaxies as nonsingular isothermal spheres, and take into account the finite angular size of the sources. Our model neglects the contribution of the background matter distribution, and assumes that lensing is entirely caused by galaxies. To test the validity of this assumption, we performed a series of ray-tracing experiments to study the effect of the background matter on the distribution of image separations. The analytical model predicts that the distributions of image separations are virtually indistinguishable for flat, cosmological constant models with different values of Omega_0. For models with no cosmological constant, the distributions of image separations do depend upon Omega_0, but this dependence is weak. We conclude that while the number of multiple-imaged sources can put strong constraints on the cosmological parameters, the distribution of image separations does not constrain the cosmological models in any significant way, and mostly provides constraints on the structure of the galaxies responsible for lensing.

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arXiv2001

Light Propagation in Inhomogeneous Universes II. Cosmological Parameter Survey

Using a multiple-lens plane algorithm, we study light propagation in inhomogeneous universes, for 43 different COBE-normalized Cold Dark Matter models, with various values of the density parameter Omega_0, cosmological constant lambda_0, Hubble constant H_0, and rms density fluctuation sigma_8. This is the largest cosmological parameter survey ever done in this field. We performed a total of 3,798 experiments, each experiment consisting of propagating a square beam of angular size 21.9'' x 21.9'' composed of 116,281 light rays from the observer up to redshift z=3. These experiments provide statistics of the magnification, shear, and multiple imaging of distant sources. The results of these experiments can be compared with observations, and eventually help constraining the possible values of the cosmological parameters. Additionally, they provide insight into the gravitational lensing process and its complex relationship with the various cosmological parameters.

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arXiv1999

A Database of COBE-Normalized CDM Simulations (Abbreviated Version)

We have simulated the formation and evolution of large-scale structure in the universe, for 68 different COBE-normalized cosmological models. For each cosmological model, we have performed between 1 and 3 simulations, for a total of 160 simulations. This constitutes the largest database of cosmological simulations ever assembled, and the largest cosmological parameter space ever covered by such simulations. We are making this database available to the astronomical community. We provide instructions for accessing the database and for converting the data from computational units to physical units. The database includes Tilted Cold Dark Matter (TCDM) models, Tilted Open Cold Dark Matter (TOCDM) models, and Tilted Lambda Cold Dark Matter (TLCDM) models. (For several simulations, the primordial exponent n of the power spectrum is near unity, hence these simulations can be considered as "untilted.") The simulations cover a 4-dimensional cosmological parameter phase space, the parameters being the present density parameter Omega_0, cosmological constant lambda_0, and Hubble constant H_0, and the rms density fluctuation sigma_8 at scale 8/h Mpc. All simulations were performed using a P3M algorithm with 64^3 particles on a 128^3 mesh, in a cubic volume of comoving size 128 Mpc. Each simulation starts at a redshift of 24, and is carried up to the present. More simulations will be added to the database in the future.

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arXiv1997

Morphological Evolution of Galaxies

We simulate the growth of large-scale structure in the universe using a N-body code. By combining these simulations with a Monte-Carlo method, we generate galaxy distributions at present that reproduces the observed morphology-density relation, with most ellipticals concentrated in the densest regions. By "tying" each galaxy to its nearest particle, we trace the trajectory of that galaxy back in time. This enables us to reconstruct the distribution of galaxies at high redshift, and the trajectory of each galaxy from its formation epoch to the present. Our goal is to determine whether the morphological type of galaxies is primarily determined by the initial conditions in which these galaxies form, or by evolutionary processes occurring later. We compare the environment in which galaxies are at the epoch of galaxy formation (z=3) and at the present. Making the null hypothesis that morphological types do not evolve, we compare the galaxies that form in low density environments but end up later in high density environments to the ones that form also in low density environment but remain in low density environment. The first group contains a larger proportion of E and S0 galaxies than the second group. We assume that the galaxy formation process cannot distinguish a low density environment that will always remain low density from one that will eventually become high density. Hence, these results force us to discard the null hypothesis of no morphological evolution. Our study suggests that 75% of the E and S0 galaxies observed at present formed as such, while the remaining 25% formed as spirals, and underwent morphological evolution. We conclude that most galaxies did not undergo morphological evolution, but a non-negligible fraction did.

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arXiv1997

Light Propagation in Inhomogeneous Universes I: Methodology and Preliminary Results

We describe a numerical algorithm which simulates the propagation of light in inhomogeneous universes, using the multiple lens-plane method. The deformation and deflection of light beams as they interact with each lens plane are computed using the filled-beam approximation. We use a N-body code to simulate large-scale structure formation in the universe. We extend the length resolution of the simulations to sub-Megaparsec scales by using a Monte-Carlo method for locating galaxies according to the underlying distribution of background matter. The observed galaxy 2-point correlation function and morphology-density relation are reproduced. This algorithm constitutes a major improvement over previous methods. We test this algorithm for three different CDM models: an Einstein-de Sitter model, an open model, and a flat model with a nonzero cosmological constant. In each case, we compute the shear and magnification caused by the presence of inhomogeneities. Our results are the following: (1) The magnification is totally dominated by the convergence; the shear contributes less than one part in 10^4. (2) Most of the cumulative shear and magnification is contributed by matter located at intermediate redshifts z=1-2. (3) The redshift where the largest contribution to shear and magnification occurs is model-dependent. (4) The bulk of the shear and magnification does not originate from direct hits with galaxies, but rather from the tidal influence of nearby and more distant galaxies, and background matter. (5) The average contributions of background matter and nearby galaxies to the shear is comparable for models with small Omega0. For the Einstein-de Sitter model, the contribution of the background matter exceeds the one of nearby galaxies by nearly one order of magnitude.

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