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  • 1
    Publication Date: 2011-08-18
    Description: Quite soon after the first collapse of structure, almost half the matter in a neutrino-dominated universe is expected to reside in clusters. The masses and binding energies of these neutrino clusters are too large for them to be identified with observed galaxy clusters. Even if such objects were able to suppress all galaxy formation, their X-ray emission would, however, make them highly visible if more than 2.5 percent of their mass was in ordinary matter. Such a low baryon density leads to insuffient cooling for galaxies to form in pancakes. A neutrino-dominated universe appears to conflict with observation irrespective of the details of the processes which govern galaxy formation.
    Keywords: ASTROPHYSICS
    Type: Royal Astronomical Society, Monthly Notices (ISSN 0035-8711); 209; 27P-31P
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  • 2
    Publication Date: 2011-08-17
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 226
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  • 3
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    Publication Date: 2011-08-17
    Description: A model for clustering in an expanding universe is developed based on an application of the coagulation equation to the collision and aggregation of bound condensations. While the growth rate of clustering is determined by the rate at which density fluctuations reach the nonlinear regime and therefore depends on the initial fluctuation spectrum, the mass spectrum rapidly approaches a self-similar limiting form. This form is determined by the tidal processes which lead to the merging of condensations, and is not dependent on initial conditions.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 223
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  • 4
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    Publication Date: 2011-08-19
    Description: The impulse approximation and full N-body experiments are used to investigate the changes in mass and binding energy which result from gravitational encounters between spherical galaxies. Models with King (1966) and de Vaucouleurs (1948) surface density profiles, and radial, tangential, and isotropic velocity distributions. Results based on the impulse approximation depend on two collision parameters (the impact parameter and the ratio of the perturbing mass to the relative velocity at infinity); they agree well with N-body results. The asymptotic limit, valid for distant encounters, is found to approximate the correct results only for impact parameters larger than 10 effective radii. It is therefore of little use in most situations of interest. No simple 'linear' or 'quadratic' dependence of energy and mass loss on collision strength is found, but rather, a smooth intermediate behavior. Cross sections depend very little on the internal velocity structure of the galaxy, but quite strongly on the surface density profile in its outer parts. The reasons why these results disagree with the conclusions of previous workers are discussed.
    Keywords: ASTRONOMY
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 295; 374-387
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  • 5
    Publication Date: 2011-08-19
    Description: The results of numerical simulations of nonlinear gravitational clustering in universes dominated by weakly interacting, 'cold' dark matter are presented. The numerical methods used and the way in which initial conditions were generated are described, and the simulations performed are catalogued. The evolution of the fundamental statistical properties of the models is described and their comparability with observation is discussed. Graphical comparisons of these open models with the observed galaxy distribution in a large redshift survey are made. It is concluded that a model with a cosmological density parameter omega equal to one is quite unacceptable if galaxies trace the mass distribution, and that models with omega of roughly two, while better, still do not provide a fully acceptable match with observation. Finally, a situation in which galaxy formation is suppressed except in sufficiently dense regions is modelled which leads to models which can agree with observation quite well even for omega equal to one.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 292; 371-394
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  • 6
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    Publication Date: 2011-08-19
    Description: An analysis by Doroshkevich (1970) which shows that the angular momentum of galaxies grew to first order (in proportion to t) during the linear phases of protogalactic evolution is expanded. This result is confirmed in N-body simulations of the formation of structure. The well-known study of Peebles (1969) found growth at second order only (in proportion to t to the 5/3-power) because its analysis was restricted to spherical regions. In such regions growth occurs purely as a result of convective effects on the bounding surface; the material initially within a spherical volume gains no angular momentum in second-order perturbation theory. These considerations do not affect estimates of the total angular momentum acquired by a galaxy in the gravitational instability picture.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 286; 38-41
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  • 7
    Publication Date: 2011-08-19
    Description: Techniques for carrying out large N-body simulations of the gravitational evolution of clustering in the fundamental cube of an infinite periodic universe are described and compared. The accuracy of the forces derived from several commonly used particle mesh schemes is examined, showing how submesh resolution can be achieved by including short-range forces between particles by direct summation techniques. The time integration of the equations of motion is discussed, and the accuracy of the codes for various choices of 'time' variable and time step is tested by considering energy conservation as well as by direct analysis of particle trajectories. Methods for generating initial particle positions and velocities corresponding to a growing mode representation of a specified power spectrum of linear density fluctuations are described. The effects of force resolution are studied and different simulation schemes are compared. An algorithm is implemented for generating initial conditions by varying the number of particles, the initial amplitude of density fluctuations, and the initial peculiar velocity field.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series (ISSN 0067-0049); 57; 241-260
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  • 8
    Publication Date: 2011-08-18
    Description: High-resolution X-ray observations of the rich cluster 0016+16 at a redshift of 0.541 are presented. The emitting gas in this cluster is hot and extremely luminous, and its structure resembles that seen in the brightest nearby cluster sources. In most of its properties, 0016+16 resembles a richer version of the Coma cluster, and it offers little support to the hypothesis that clusters at z greater than 0.5 differ fundamentally from nearer objects.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 251
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  • 9
    Publication Date: 2011-08-18
    Description: The X-ray observations of the galaxy clusters Abell 2319 and 576 show a reduced temperature of the microwave background due to the Suniaev-Zeldovich effect. The X-ray maps indicate smooth emission in these clusters concentrated toward the optical center; the surface brightness in the 1-3 keV band falls with the inverse square of radius outside a small core, and fits the Hubble's law of optical surface brightness of elliptical galaxies. The low temperature indicates that in A576 (1) the core radius of the gravitational potential is two to three times larger than inferred from galaxy counts, (2) the temperature rises with radius over most of the cluster and falls to zero at large radii, and (3) the gas does not obey the equation of state.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 241
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  • 10
    Publication Date: 2011-08-17
    Description: The growth and subsequent collapse of homogeneous ellipsoidal perturbations in a uniform expanding background is considered as a simple model for the formation of large-scale aspherical structures in the observed universe. Numerical calculations of the evolution of such perturbations turn out to be well described by an approximate analytic solution of the equations of motion, and simple relationships are found between the initial shape of a perturbation and its shape and kinematic properties at the time of collapse. Perturbations do not change their shape significantly until they reach a density contrast of order unity. As a result, structures with the kinematic properties of the Local Supercluster should form much more commonly in a low-density universe than in a flat universe. The homogeneity of the local Hubble flow, the motion of the Milky Way with respect to the microwave background, and the flattening of the Local Supercluster can be successfully accounted for by these models, provided that the initial perturbation is sufficiently flattened. Viable models are obtained only if the ratio of the lengths of the two smaller axes of the initial perturbation is at least 3:1 in an Einstein-de Sitter universe or at least 1.8:1 in a universe for which the density parameter (Omega) is of order 0.1, when the protocluster pancakes.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 231
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