Showing papers on "Structure formation published in 1988"

Cosmological structure formation from soft topological defects

Abstract: Some models have extremely low-mass pseudo-Goldstone bosons that can lead to vacuum phase transitions at late times, after the decoupling of the microwave background.. This can generate structure formation at redshifts z greater than or approx 10 on mass scales as large as M approx 10 to the 18th solar masses. Such low energy transitions can lead to large but phenomenologically acceptable density inhomogeneities in soft topological defects (e.g., domain walls) with minimal variations in the microwave anisotropy, as small as delta Y/T less than or approx 10 to the minus 6 power. This mechanism is independent of the existence of hot, cold, or baryonic dark matter. It is a novel alternative to both cosmic string and to inflationary quantum fluctuations as the origin of structure in the Universe.

On the Dynamical Role of Cosmic Rays in Cooling Flows in Clusters of Galaxies

Abstract: Cosmic rays are generated in galaxies, presumably by supernovae and active galactic nuclei, and are released into the surrounding medium. Theoretical estimates and implications from radio observations suggest that the nuclear component of the cosmic rays may be dynamically important in the intergalactic medium of clusters of galaxies, in particular in compact clusters with cooling flows. The propagation and dynamical role of the cosmic rays in the intracluster gas were studied by means of steady spherically symmetric models. With expected values for the cosmic-ray production rate and the transport coefficients, it is found that the cosmic-ray pressure may be comparable to the thermal gas pressure in the halo region around central galaxies in clusters with cooling flows. This should result in Rayleigh-Taylor instabilities inducing an inhomogeneous structure in the cooling flow region. These inhomogeneities may be an important trigger for subsequent thermal condensation leading to the formation of filaments and star birth regions. 48 references.

The angular distribution of the cosmic background radiation

Abstract: The cosmic background radiation is thermal radiation with a temperature of 2.75 K still present throughout the Universe, a relic of its hot, big bang, initial phase. Detailed studies of this radiation, and particularly its angular variations, can provide information obtainable in no other way about the global geometry and expansion of the Universe, about the distribution of mass near our Galaxy, and about the process of galaxy formation. The present review treats searches for such angular variations and the consequences of the resulting measurements of, or upper limits on, variations in the cosmic background radiation.

Hot gas in evolving galaxy clusters and groups - Implications for the microwave and X-ray backgrounds

Abstract: A phenomenological model for the luminosity function of galaxy groups and clusters allows its evolution to be described in terms of the two-point galaxy correlation function. The observable properties of hot gas in virialized groups and clusters at high redshift have been computed. These include the spectral distortion by Comptonization of the cosmic blackbody spectrum, fluctuations in the cosmic microwave background arising by this same process, and the diffuse X-ray background and fluctuations therein arising from thermal emission by the intracluster gas.

Distortions and Anisotropies of the Cosmic Background Radiation

Abstract: The general theory of spectral distortions and angular anisotropies in the cosmic background radiation is reviewed. Constraints on the amount of energy injection allowed in the early universe are discussed. Predictions of fluctuation levels are given for small and large angle anisotropy experiments for theories of structure formation in which the fluctuations are initially Gaussian and scale-invariant, as expected in inflationary models. The texture of maps of the CMB fluctuations is discussed using the theory of two-dimensional Gaussian random fields. The implications of the recently reported large scale streaming velocities for CMB fluctuations on scales of a few degrees are considered. Using a simple model for the fuzziness of recombination, simple equations are derived which give a physical overview of the various mechanisms which lead to anisotropy in the microwave background. Secondary anisotropies that result from Compton upscat-tering of the CMB photons off inhomogeneous hot gas and redshifted dust emission from primeval galaxies are estimated.

Cosmic strings and superconducting cosmic strings

Abstract: The possible consequences of forming cosmic strings and superconducting cosmic strings in the early universe are discussed Lecture 1 describes the group theoretic reasons for and the field theoretic reasons why cosmic strings can form in spontaneously broken gauge theories Lecture 2 discusses the accretion of matter onto string loops, emphasizing the scenario with a cold dark matter dominated universe In lecture 3 superconducting cosmic strings are discussed, as is a mechanism which leads to the formation of structure from such strings

N-Body Methods and the Formation of Large-Scale Structure

Abstract: I review the N-body techniques that have been used to study the evolution of large-scale structure in the Universe After a brief summary of the initial conditions expected in universes dominated by weakly interacting massive particles, I discuss the nonlinear structure found in neutrino-dominated and cold dark matter dominated models The cold dark matter model currently appears the most attractive possibility and is able to reproduce observed structures from galaxy halos up to rich galaxy clusters The distribution of galaxy formation sites in this model may be biased in the manner required to make a flat universe consistent with dynamical constraints from galaxy groups and clusters and from the Local Supercluster

Yet Another Scenario for Galaxy Formation

Abstract: I consider the prospects for developing a model for the formation of galaxies and clusters of galaxies using only what we are fairly sure is present-baryons and radiation. This leads to a primeval entropy perturbation scenario in vrtiich galaxies form at z ~ 100 as the last generation to be substantially held up by Compton drag. The consequences for galaxy formation seem attractive; the situation for cluster formation is unclear because of ambiguities in the scenario developed so far.

Cosmic Strings and Galaxy Formation

Abstract: Recently it has repeatetly been claimed [1] that there is now a successful “standard model” for galaxy formation. This model involves: (i) cold dark matter WithΩ0 = 1, (ii) primordial Gaussian fluctuations With a Harrison-Zel’dovich spectrum, and (iii) biasing, which presumes that galaxies only form in the rare peaks of density perturbations.

Galaxy Formation from Tepid Inflation

Abstract: It is argued that the primordial density fluctuations needed for galaxy formation may have been of thermal origin in a quasi-exponential state of inflation.

Cosmic strings and the formation epoch of large scale structure

Abstract: The formation of galaxies, clusters and superclusters (SCs) is discussed based on a cosmic string model, and the constraints are given for the corresponding loops which form these structures by the accretion of the loops. Only those loops that came into horizon after teq (the time of equal matter and radiation) may be the seeds of clusters and SCs.

Galaxy Formation with Neutrinos and cosmic strings

Abstract: Galaxy and structure formation in a neutrino dominated universe with cosmic strings are investigated. Unlike in the usual adiabatic scenario strings survive neutrino free streaming to seed galaxies and clusters. The effective maximal Jeans mass is about 1.5 x 1014h50 -4Mo. Hence cluster formation is only marginally different than in the cola dark matter (COM) and strings model. Galaxy masses are lower than with strings and COM. The mass spectrum is flatter and the density profile about an individual loop is less steep, in better agreement with observations.

Cosmic strings and galaxy formation

Abstract: After reviewing the most important aspects of the cosmic-string model of formation of structure, recent results on galaxy formation are summarized. The origin of galactic angular momentum via tidal torquing is discussed, and the evolution of the mass function of galaxies is studied.

Decreasing density perturbations and the isotropy of the cosmic microwave background.

Abstract: We compute the effect on the small-scale anisotropy of the cosmic background radiation induced by decreasing density fluctuations in a spatially flat Robertson-Walker universe.