Showing papers on "Structure formation published in 1991"

Evolution of Clusters of Galaxies

Abstract: In hierarchical models for structure formation like CDM, the mass scale of clustering evolves rapidly and clusters in the past are predicted to be less massive, but denser and more numerous. The indication from optical searches is that the cluster population is roughly stable in that high velocity dispersion clusters seem to be about as abundant at z ~ 0.5 as at the present while X-ray surveys show a strong negative evolution, there being many fewer highluminosity clusters in the past compared to the present. Using self-similar scaling laws I find that the neutral evolution in the optical agrees quite well with what is expected, but that the negative X-ray evolution is very hard to reconcile with the idea that state of the gas in clusters is the result of gravitational clustering. One way to remedy this discrepancy is to assume that the gas we now see in clusters was heated at some earlier epoch by non-gravitational processes. If one discards the optical observations, an alternative is to invoke primordial fluctuations with a much redder spectrum, and I discuss how one might discriminate between these theories with future observations.

The density field of the local universe

Abstract: An all-sky redshift survey of galaxies detected by IRAS (the Infrared Astronomical Satellite) has been used to map the Universe out to 140h−1 Mpc (the Hubble constant H0 =100h km s−1 Mpc−1). Well-known superclusters and voids are seen, as are others not previously identified. The inferred underlying distribution of density is found to be skewed to high densities (the voids are larger than the superclusters but depart less from the mean density); and there is more structure on large scales than is predicted by the standard cold dark matter theory of galaxy formation.

Observational tests of cosmological inflation

Abstract: I. Inflation Theory.- Fundamental Arguments for Inflation.- Predictions of Inflation.- Classicality of Density Perturbations in the Early Universe.- The Influence of Non-Linear Density Fluctuations on the Microwave Sky.- Quantum Cosmology and the Cosmological Constant.- Lessons from Inflation and Cold Dark Matter.- The Topology of Galaxy Clustering.- Can Non-Gaussian Fluctuations for Structure Formation Arise from Inflation.- Non-Baryonic Dark Matter.- Are Galactic Halos Made of Brown Dwarfs or Black Holes.- II. Cosmological Timescale Test.- Ages of Globular Clusters.- Globular Cluster Ages and Cosmology.- The Local Distance Scale: How Reliable Is It.- Distances to Virgo and Beyond.- The Luminosity-Line-Width Relations and the Value of Ho.- Observational Status of HO.- Calibrating Cepheid Sequences in Nearby Galaxies.- New D-? Results for Coma Ellipticals.- Novae and the Distance Scale.- A High Resolution, Ground Based Observation of a Virgo Galaxy.- Globular Clusters as Extragalactic Distance Indicators.- III. High Redshift Tests of ?o.- High-Redshift Tests of ?.- Cosmology with Galaxies at High Redshifts.- Prospects for Measuring the Deceleration Parameter.- Aligned Radio Galaxies.- K Band Galaxy Counts and the Cosmological Geometry.- Selection Effects in Redshift Surveys.- An Inflationary Alternative to the Big-Bang.- IV. Galaxy Clustering, ?o and the Primordial Spectrum.- Dynamical Estimates of ?o from Galaxy Clustering.- ROSAT Observations of Clusters of Galaxies.- A Deep ROSAT Observation at High Galactic Latitude.- Large Scale Structure and Inflation.- The Structure of the Universe on Large Scales.- Testing the Zeldovich Spectrum.- ? on the Scale of 3Mm/s.- Tests of Inflation Using the QDOT Redshift Survey.- Testing Inflation with Peculiar Velocities.- The Invisible Cosmological Constant.- Support for Inflation from the Great Attractor.- The Angular Large Scale Structure.- Is There Any Observational Evidence for Non- Gaussian Primordial Density Fluctuations.- V. Microwave Background Anisotropy.- COBE DMR Observations of CMB Anisotropy.- Observations of Microwave Background Anisotropy at Tenerife and Cambridge.- Foreground Effects and the Search for Fluctuations in the CMB Radiation.- Microwave Background Anisotropics and Large Scale Structure in the Universe.- Discovery of the Small Scale Sky Anisotropy at 2.7cm: Radio Sources or Relic Emission.- Balloon-Borne Observations of CMB Anisotropics at Intermediate Angular Scales, at Sub-MM and MM Wavelengths.- VI. Poster Papers.- The Durham/UKST Galaxy Redshift Survey.- Time Evolution of Lensed Image Separations.- Deep Galactic Surveys as Probes of the Large Scale Structure of the Universe.- Intergalactic Absorption in the Spectra of High-Redshift QSOs.- A Complete Quasar Sample at Intermediate Redshift.- Radio-Luminosity Dependence of the IR-Radio Alignment Effect in High-z Radio Galaxies.- Density and Peculiar Velocity Fields in the Region of Dressler's Supergalactic Plane Survey.- Scale Invariance Induced by Non-linear Growth of Density Fluctuations.- The Power Spectrum of Galaxy Clustering.- Higher Moments of the IRAS Galaxy Distribution.- Collapse of a Protogalactic Cloud.- Index of Authors.

Quasars and galaxy formation. I - The z greater than 4 objects

Abstract: The physical properties of the known quasars with z greater than 4 are examined with particular reference to theories of cosmic structure formation. A 'standard' massive accreting black hole model for quasars is used to calculate the masses, radiative efficiencies, and accretion rates of the observed objects. The masses, densities, cosmological overdensities, and sizes of the accretion fuel reservoirs associated with the quasar black holes are considered, and several indirect arguments are used to connect these quantities to the masses and overdensities of the quasar host objects. Finally, constraints on the epoch of formation of the host objects are discussed. The general conclusion is that the observed quasars with z greater than 4 suggest that the cosmic structure formation was already well advanced at z = 5, when the universe was a small fraction of its present age.

Strings, texture, and inflation

Abstract: We examine mechanisms, several of which are proposed here, to generate structure formation, or to just add large-scale features, through either gauged or global cosmic strings or global texture, within the framework of inflation. We first explore the possibility that strings or texture form if there is no coupling between the topological theory and the inflaton or spacetime curvature, via (1) quantum creation, and (2) a sufficiently high reheat temperature. In addition, we examine the prospects for the inflaton field itself to generate strings or texture. Then, models with the string/texture field coupled to the curvature, and an equivalent model with coupling to the inflaton field, are considered in detail. The requirement that inflationary density fluctuations are not so large as to conflict with observations leads to a number of constraints on model parameters. We find that strings of relevance for structure formation can form in the absence of coupling to the inflaton or curvature through the process of quantum creation, but only if the strings are strongly type I, or if they are global strings. If formed after reheating, naturalness suggests that gauged cosmic strings correspond to a type-I superconductor. Similarly, gauged strings formed during inflation via conformal couplingmore » {xi}=1/6 to the spacetime curvature (in a model suggested by Yokoyama in order to evade the millisecond pulsar constraint on cosmic strings) are expected to be strongly type I.« less

Do Clusters of Galaxies Affect the Spectrum of the Microwave Background

Abstract: The distortion, averaged over the sky, due to the Compton scattering of background photons with electrons in the hot gas in clusters of galaxies is calculated. Using an existing sample of X-ray clusters, various values of the density parameter Omega, and plausible models for cluster evolution, Monte Carlo realizations of the microwave sky are generated. The spatial structure of these simulations shows a network of discrete sources whose properties can be a strong function of both evolution and Omega. The amount of spectral distortion in the models is greatest for models characterized by self-similar cluster evolution in an open universe and is within an order of magnitude of the current upper limits. Thus, improved observational sensitivity must inevitably detect some deviation from a blackbody spectrum.

Non scale-invariant density perturbations from chaotic extended inflation

Abstract: Chaotic inflation is analyzed in the frame of scalar-tensor theories of gravity. Fluctuations in the energy density arise from quantum fluctuations of the Brans-Dicke field and of the inflation field. The spectrum of perturbations is studied for a class of models: it is non scale-invarient and, for certain values of the parameters, it has a peak. If the peak appears at astrophysically interesting scales, it may help to reconcile the Cold Dark Matter scenario for structure formation with large scale observations.

Superclusters are warm, not hot - The predicted flux distribution of soft X-rays

Abstract: Arguments showing that gas present in superclusters should have a characteristic temperature of approximately 10 6 K are presented. In the most popular models of large-scale structure formation, including hot and cold dark matter, peculiar velocities of matter relative to the background are 1000 km/s. This would imply temperatures exceeding 10 7 K. However, most of the peculiar motions can be attributed to large-scale, coherent velocities. Moreover, gas inside superclusters which is not concentrated in groups or clusters will cool adiabatically with the Hubble expansion. A temperature of 10 6 K implies that radiation from gas in superclusters should not be seen in X-rays of energy > 500 eV

Cosmic rays in the Small Magellanic Cloud

Abstract: The SMC is investigated in order to determine whether this galaxy is in dynamic quasi-stable equilibrium between the expansive pressure of the hot cosmic ray gas, the magnetic fields, and the kinetic motion of interstellar gas and the gravitational attraction of matter. It is argued that the cosmic ray density level is three to five times smaller than that which would exist if the galaxy were in approximate dynamic equilibrium and cosmic ray sources were adequate. The scale of coupling between the cosmic rays and matter in the projected dimension that can be observed is less than 0.5 kpc, which is rather small to be consistent with a stable cosmic ray distribution. The observed matter distribution and the calculated cosmic ray density distribution are used to determine the expected gamma ray intensity distribution.

Gas-Induced Primary and Secondary CMB Anisotropies

Abstract: On scales less than a few degrees the primary CMB anisotropies (from linear processes occurring at photon decoupling) and the secondary CMB anisotropies (from nonlinear processes occurring later) arise mainly from gas-driven mechanisms such as Thomson scattering, inhomogeneous Compton cooling, and radiant emission by dust. We describe some of the current experiments which probe these anisotropies and the constraints on models that can be derived from them. We pay special attention to hierarchical Gaussian models of structure formation such as the Ω = 1, H0 = 50, adiabatic CDM model. For these theories, observations currently restrict the biasing factor defining the amplitude of the perturbations to be b p ≳ 0.7 (for ΩB ≳ 0.03) assuming standard recombination. If there is early reionization, the best limits currently come from all-sky surveys probing the Sachs-Wolfe effect, giving b p ≳ 0.3. The currently preferred value of the biasing factor (apart from the ‘extra large scale power conundrum’ of deep redshift surveys) is b p ~ 1 – 1.5, so modest experimental sensitivity improvements will test these theories. We also present our methods for generating large catalogues of galaxy groups and clusters using a hierarchical Gaussian peaks formulation. These are used to generate simulated maps of secondary CMB anisotropies due to the Sunyaev-Zeldovich effect and the associated soft X-ray emission viewable with ROSAT. The non-Gaussian nature of the resulting image statistics and the ability to superpose various backgrounds and foregrounds are essential for optimally designing observational tests of these models. Both aspects are well-handled by the map synthesis technique. The prospects for ambient SZ observation depend strongly on the primordial amplitude of the perturbations but radio source confusion will make it difficult at Rayleigh-Jeans wavelengths. We also sketch how the hierarchical peaks method can be applied to the construction of primeval galaxy catalogues and display illustrative far infrared maps of dust-laden star-bursting galaxies at z ~5 that may be detectable with ground-based sub-mm telescopes.

Microwave Background Anisotropies and Large Scale Structure in the Universe

Abstract: Recent observations show that the Universe contains more structure on scales ≳ 10h-1Mpc* than expected in the standard cold dark matter (CDM) theory of structure formation This may indicate that the initial fluctuations were not scale-invariant I investigate how this would affect microwave background anisotropics if the Universe is spatially flat with Ω = 1, as seems most natural if the Universe went through an inflationary phase If we are allowed arbitrary freedom in choosing the shape of the initial fluctuation spectrum, then it is possible to construct models which can account for observed structure in the galaxy distribution but which result in small (ΔT/T ≲ 10-6) anisotropics in the microwave background radiation on angular scales ≳ 5° However, it is very difficult to avoid producing temperature fluctuations of order 10-5 on angular scales of ~ 1°, even if the Universe were reionised If the extra large-scale power detected in galaxy surveys really reflects clustering in the mass distribution, then experiments at θ ~ 1° should soon see fluctuations in the microwave background

Non-Gaussian microwave background fluctuations from nonlinear gravitational effects

Abstract: Whether the statistics of primordial fluctuations for structure formation are Gaussian or otherwise may be determined if the Cosmic Background Explorer (COBE) Satellite makes a detection of the cosmic microwave-background temperature anisotropy delta T(sub CMB)/T(sub CMB). Non-Gaussian fluctuations may be generated in the chaotic inflationary model if two scalar fields interact nonlinearly with gravity. Theoretical contour maps are calculated for the resulting Sachs-Wolfe temperature fluctuations at large angular scales (greater than 3 degrees). In the long-wavelength approximation, one can confidently determine the nonlinear evolution of quantum noise with gravity during the inflationary epoch because: (1) different spatial points are no longer in causal contact; and (2) quantum gravity corrections are typically small-- it is sufficient to model the system using classical random fields. If the potential for two scalar fields V(phi sub 1, phi sub 2) possesses a sharp feature, then non-Gaussian fluctuations may arise. An explicit model is given where cold spots in delta T(sub CMB)/T(sub CMB) maps are suppressed as compared to the Gaussian case. The fluctuations are essentially scale-invariant.

Large Scale Cosmic Instability

Abstract: Spectral distortions in cosmic background radiation can create instabilities in matter, whose possible role in generating large scale structure is outlined here. A promising physical mechanism is sketched, based on Lyα resonance scattering, which exponentially amplifies perturbations in neutral hydrogen density on large scales after recombination.

Cosmic rays in other galaxies

Abstract: A survey is given of the present knowledge of the properties of cosmic rays in nearby galaxies through studies of their emission. The queston of the origin of cosmic rays is addressed through relationships between observations at radio and other wavelengths. Intense star-formation appears to be responsible for the active phases of cosmic ray production. This star-formation is induced by galaxy interactions and the effects of galactic bars. Studies of the radio brightness and spectral index distributions across galaxies at a range of inclinations shows that cosmic rays diffuse away from their place of origin in the galactic disk with a lifetime of a few times 10 7 years.

Can Non-Gaussian Fluctuations for Structure Formation Arise from Inflation?

Abstract: Non-Gaussian fluctuations for structure formation may be generated during the inflationary epoch from the nonlinear interaction of two scalar fields with gravity. Semi-analytical calculations are given describing nonlinear long wavelength evolution in 3+1 dimensions. Long wavelength fields are governed by a single equation, the separated Hamilton-Jacobi equation (SHJE). I discuss complete analytic solutions of the SHJE for two scalar fields with a potential whose logarithm In V(Φj) is linear. More complicated potential surfaces may be approximated by continuously joining various linear In V(Φj) potentials. Typically, non-Gaussian fluctuations arise when one passes over several sharp ridges in the potential surface. One can input this richer class of initial conditions into N-body codes to see the effects on the large scale structure in the Universe. The cleanest test of non-Gaussian fluctuations will hopefully occur in the near future from large angle microwave background anisotropy experiments.

Stochastic inflation lattice simulations - Ultra-large scale structure of the universe

Abstract: Non-Gaussian fluctuations for structure formation may arise in inflation from the nonlinear interaction of long wavelength gravitational and scalar fields Long wavelength fields have spatial gradients a -1∇ small compared to the Hubble radius, and they are described in terms of classical random fields that are fed by short wavelength quantum noise Lattice Langevin calculations are given for a ‘toy model’ with a scalar field interacting with an exponential potential where one can obtain exact analytic solutions of the Fokker-Planck equation For single scalar field models that are consistent with current microwave background fluctuations, the fluctuations are Gaussian However, for scales much larger than our observable Universe, one expects large metric fluctuations that are non-Gaussian This example illuminates non-Gaussian models involving multiple scalar fields which are consistent with current microwave background limits

Motion of a Spherical Domain Wall and the Large-Scale Structure Formation

Abstract: The evolution of a wall· like structure in the universe is investigated by assuming a simplified model of a domain wall. The domain wall is approximated as a thin spherical shell with domain wall-like matter, which is assumed to interact with dust-like dark matter in an entirely inelastic manner, and its motion in an expanding universe is numerically studied in the general-relativistic treatment. We evaluate the lifetime of the wall, which is defined as the characteristic time for the wall to shrink due to its own tension. It is necessary that this time is not smaller than the cosmic age, in order that the walls avoid the collapse to the present time and play an important role in the structure formation of the universe. It is shown that, in spite of the above interaction, the strong restriction is imposed on the surface density of the domain walls and the allowed values are too small to have any influences on the background model.

Cosmological structure formation

Abstract: A summary of the current forefront problem of physical cosmology, the formation of structures (galaxies, clusters, great walls, etc.) in the universe is presented. Solutions require two key ingredients: (1) matter; and (2) seeds. Regarding the matter, it now seems clear that both baryonic and non-baryonic matter are required. Whether the non-baryonic matter is hot or cold depends on the choice of seeds. Regarding the seeds, both density fluctuations and topological defects are discussed. The combination of isotropy of the microwave background and the recent observations indicating more power on large scales have severly constrained, if not eliminated, Gaussian fluctuations with equal power on all scales, regardless of the eventual resolution of both the matter and seed questions. It is important to note that all current structure formation ideas require new physics beyond SU(3) x SU(2) x U(1).

High energy physics and cosmology

Abstract: This research will focus on the implications of recent theories and experiments in high energy physics for the evolution of the early Universe, and on the constraints that cosmological considerations can place on such theories. Several problems are under investigation, including the development of constraints on the inflationary predictions of scale-free primordial fluctuations in a universe at critical closure density by studying their linear and non-linear evolution after they re-enter the particle horizon. We will examine the observable imprint of primordial density fluctuations on the cosmic microwave background radiation in curved cosmological models. Most astronomical evidence points to an open universe: one of our goals is to reconcile this conclusion with the particle physics input. We will investigate the response of the matter distribution to a network of cosmic strings produced during an early symmetry--breaking transition, and compute the resulting cosmic microwave background anisotropies. We will simulate the formation of large--scale structures whose dynamics are dominated by weakly interacting particles such as axions massive neutrinos or photinos in order to model the formation of galaxies, galaxy clusters and superclusters. We will study the distortions in the microwave background radiation, both spectral and angular, that are produced by ionized gas associatedmore » with forming clusters and groups of galaxies. We will also study constraints on exotic cooling mechanisms involving axions and majorons set by stellar evolution and the energy input into low mass stars by cold dark matter annihilation in galactic nuclei. We will compute the detailed gamma ray spectrum predicted by various cold dark matter candidates undergoing annihilation in the galactic halo and bulge.« less