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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1993

Primeval galaxies in the sub-mm and mm

Although the results of COBE's FIRAS experiment 1 constrain the deviation in energy from the CMB blackbody in the 500-5000 micron range to be delta E/E, sub cmb less than 0.005, primeval galaxies can still lead to a brilliant sub-mm sky of non-Gaussian sources that are detectable at 10 inch resolution from planned arrays such as SCUBA on the James Clerk Maxwell Telescope and, quite plausibly, at sub-arcsecond resolution in planned mm and sub-mm interferometers. Here, we apply our hierarchical peaks method to a CDM model to construct sub-mm and mm maps of bursting PG's appropriate for these instruments with minimum contours chosen to correspond to realistic observational parameters for them and which pass the FIRAS limits.

Bond, J. Richard↗
NASA NTRS1993

Cluster evolution as a probe of primordial density fluctuations

Although COBE's detection of large angle microwave background anisotropies fixes the amplitude of density fluctuations on length scales k exp -1 approximately = (300-6000) h(exp -1)Mpc, what is crucial for the level of large scale clustering is the amplitude of density fluctuations on scales (5-50) h(exp -1)Mpc. The level of dynamical clustering is usually parameterized by the size of the mass fluctuations in 8 h exp -1 Mpc spheres, sigma sub 8. For the cold dark matter model, COBE gives sigma sub 8 approximately = 1, while models with extra large scale power give sigma sub 8 approximately = 1/2. The most massive clusters of galaxies (greater than or approximately = 10 exp 15 solar mass) form from rare 'peak patches' found in the initial mass density distribution. Their abundance as a function of redshift is a sensitive probe of the wave number band k(exp -1) approx. (3-8) h(exp -1)Mpc, hence of sigma sub 8, and so cluster evolution can discriminate among models allowed by the COBE results. We use our Hierarchical Peaks Method, which accurately reproduces the results of P3M N-body simulations, to calculate the evolution of cluster x-ray flux counts, luminosity, and temperature functions as a function of sigma sub 8 for CDM models and those with more large scale power. We find that the EMSS and Edge et al. cluster samples support sigma sub 8 in the range from approx. 0.6-0.9, and that models with more large scale power (and hence flatter fluctuation spectra in the cluster regime) fit the x-ray bright end better.

Bond, J. Richard↗
NASA NTRS1993

The effect of tidal fields on the shapes and kinematics of dark halos

We have carried out a series of N-body simulations to investigate the effect of tidal shear on the structure and kinematics of dark halos. We simulate the collapse of density perturbations using a tree code as described in Dubinski & Carlberg (1991). Density peaks are selected from a random realization of a CDM density field and used as the initial conditions for N-body simulations. We use an experimental approach to examine the effects of tidal shear on collapse. The cosmological tidal field is treated as an external time dependent potential whose strength and orientation can be varied freely. We examine the effects of the tidal field with two experiments. In the first experiment, we simulate a sample of 14 dark halos from the collapse of density peaks in the presence of a 1(sigma) tidal field. In the second experiment, we use the same initial conditions though the tidal field is turned off allowing an experimental control for comparison to highlight the influence of tidal shear on the development of the structure and kinematics of the dark halos.

Dubinski, John↗
NASA NTRS1993

Faint blue counts from formation of dwarf galaxies at z approximately equals 1

The nature of faint blue objects (FBO's) has been a source of much speculation since their detection in deep CCD images of the sky. Their high surface density argues against them being progenitors of present-day bright galaxies and since they are only weakly clustered on small scales, they cannot be entities that merged together to form present-day galaxies. Babul & Rees (1992) have suggested that the observed faint blue counts may be due to dwarf elliptical galaxies undergoing their initial starburst at z is approximately equal to 1. In generic hierarchical clustering scenarios, however, dwarf galaxy halos (M is approximately 10(exp 9) solar mass) are expected to form at an earlier epoch; for example, typical 10(exp 9) solar mass halos will virialize at z is approximately equal to 2.3 if the power-spectrum for the density fluctuations is that of the standard b = 2 cold dark matter (CDM) model. Under 'ordinary conditions' the gas would rapidly cool, collect in the cores, and undergo star-formation. Conditions at high redshifts are far from 'ordinary'. The intense UV background will prevent the gas in the dwarf halos from cooling, the halos being released from their suspended state only when the UV flux has diminished sufficiently.

Babul, Arif↗
NASA NTRS1993

The evolution of the intergalactic medium and the origin of the galaxy luminosity function

The coupling of the Press and Schechter prescription with the CDM scenario and the Hoyle-Rees-Ostriker cooling criterion leads to a galaxy formation scenario in which galaxies are overproduced by a large factor. Although star formation might be suppressed in the smaller halos, a large amount of energy per galactic mass is needed to account for the present number density of galaxies. The evolution of the intergalactic medium (IGM) provides a simple criterion to prevent galaxy formation without requiring feedback, since halos with small virial temperatures are not able to retain the infalling hot gas of the IGM. If the ionizing background has decreased since z is approximately 1 - 2, then this criterion explains the slope of the luminosity function at the faint end. In addition, this scenario predicts two populations of dwarf galaxies, well differentiated in age, gas content, stellar populations, and clustering properties, which can be identified with dE and dIm galaxies.

Valls-Gabaud, David↗
NASA NTRS1993

On the evolution of Lyman alpha clouds based upon the Minihalo model

Intergalactic clouds confined by the gravity of cold dark matter (CDM), the so called mini-halo, is considered. Assuming a simplified evolution law of UV flux and mass function of gas clouds, number density evolution and H1 column density distribution can be reproduced by mini-halo model. Considering interaction between supersonic flow and a mini-halo, spatial correlation and the proximity effect of Ly-alpha forest can be investigated.

Murakami, Izumi↗
NASA NTRS1991

The best-fit universe

Inflation provides very strong motivation for a flat Universe, Harrison-Zel'dovich (constant-curvature) perturbations, and cold dark matter. However, there are a number of cosmological observations that conflict with the predictions of the simplest such model: one with zero cosmological constant. They include the age of the Universe, dynamical determinations of Omega, galaxy-number counts, and the apparent abundance of large-scale structure in the Universe. While the discrepancies are not yet serious enough to rule out the simplest and most well motivated model, the current data point to a best-fit model with the following parameters: Omega(sub B) approximately equal to 0.03, Omega(sub CDM) approximately equal to 0.17, Omega(sub Lambda) approximately equal to 0.8, and H(sub 0) approximately equal to 70 km/(sec x Mpc) which improves significantly the concordance with observations. While there is no good reason to expect such a value for the cosmological constant, there is no physical principle that would rule out such.

Turner, Michael S.↗
NASA NTRS1992

The gravitational wave contribution to cosmic microwave background anisotropies and the amplitude of mass fluctuations from COBE results

A stochastic background of primordial gravitational waves may substantially contribute, via the Sachs-Wolfe effect, to the large-scale cosmic microwave background (CMB) anisotropies recently detected by COBE. This implies a bias in any resulting determination of the primordial amplitude of density fluctuations. We consider the constraints imposed on n is less than 1 ('tilted') power-law fluctuation spectra, taking into account the contribution from both scalar and tensor waves, as predicted by power-law inflation. The gravitational wave contribution to CMB anisotropies generally reduces the required rms level of mass fluctuation, thereby increasing the linear bias parameter, even in models where the spectral index is close to the Harrison-Zel'dovich value n = 1. This 'gravitational wave bias' helps to reconcile the predictions of CDM models with observations on pairwise galaxy velocity dispersion on small scales.

Lucchin, Francesco↗
NASA NTRS1993

Textures and clusters

We discuss the properties of galaxy clusters expected in a texture-seeded, CDM-dominated, Omega = 1 universe. Assuming that the textures are spherical, we use the spherical collapse model to compute the cluster velocity dispersion (or temperature) distribution function. For objects of mass 10 exp 11 to 10 exp 15 solar masses, we find v varies as M super gamma with gamma of about 0.25. An unbiased (b = 1) texture model predicts too many high-velocity dispersion clusters. A biased texture model appears to be compatible with cluster properties inferred from optical and X-ray observations. In the texture model, the cluster velocity distribution functio does not evolve rapidly; thus, the model predicts the existence of rich clusters at moderate redshift (about 1-2).

Bartlett, James G.↗
NASA NTRS1993

Experiment-specific cosmic microwave background calculations made easier - Approximation formula for smoothed delta T/T windows

Simple and easy to implement elementary function approximations are introduced to the spectral window functions needed in calculations of model predictions of the cosmic microwave backgrond (CMB) anisotropy. These approximations allow the investigator to obtain model delta T/T predictions in terms of single integrals over the power spectrum of cosmological perturbations and to avoid the necessity of performing the additional integrations. The high accuracy of these approximations is demonstrated here for the CDM theory-based calculations of the expected delta T/T signal in several experiments searching for the CMB anisotropy.

Gorski, Krzysztof M.↗
NASA NTRS1994

Small-scale cosmic microwave background anisotropies as probe of the geometry of the universe

We perform detailed calculations of cosmic microwave background (CMB) anisotropies in a cold dark matter (CDM)-dominated open universe with primordial adiabatic density perturbations for a variety of reionization histories. The CMB anisotropies depend primarily on the geometry of the universe, which in a matter-dominated universe is determined by Omega and the optical depth to the surface of last scattering. In particular, the location on the primary Doppler peak depends primarily on Omega and is fairly insensitive to the other unknown parameters, such as Omega(sub b), h, Lambda, and the shape of the power spectrum. Therefore, if the primordial density perturbations are adiabatic, measurements of CMB anisotropies on small scales may be used to determine Omega.

Kamionkowski, Marc↗
NASA NTRS1994

Internal velocity and mass distributions in simulated clusters of galaxies for a variety of cosmogonic models

The mass and velocity distributions in the outskirts (0.5-3.0/h Mpc) of simulated clusters of galaxies are examined for a suite of cosmogonic models (two Omega(sub 0) = 1 and two Omega(sub 0) = 0.2 models) utilizing large-scale particle-mesh (PM) simulations. Through a series of model computations, designed to isolate the different effects, we find that both Omega(sub 0) and P(sub k) (lambda less than or = 16/h Mpc) are important to the mass distributions in clusters of galaxies. There is a correlation between power, P(sub k), and density profiles of massive clusters; more power tends to point to the direction of a stronger correlation between alpha and M(r less than 1.5/h Mpc); i.e., massive clusters being relatively extended and small mass clusters being relatively concentrated. A lower Omega(sub 0) universe tends to produce relatively concentrated massive clusters and relatively extended small mass clusters compared to their counterparts in a higher Omega(sub 0) model with the same power. Models with little (initial) small-scale power, such as the hot dark matter (HDM) model, produce more extended mass distributions than the isothermal distribution for most of the mass clusters. But the cold dark matter (CDM) models show mass distributions of most of the clusters more concentrated than the isothermal distribution. X-ray and gravitational lensing observations are beginning providing useful information on the mass distribution in and around clusters; some interesting constraints on Omega(sub 0) and/or the (initial) power of the density fluctuations on scales lambda less than or = 16/h Mpc (where linear extrapolation is invalid) can be obtained when larger observational data sets, such as the Sloan Digital Sky Survey, become available.

Cen, Renyue↗
NASA NTRS1994

The evolution of X-ray clusters in a cold plus hot dark matter universe

We present the first self-consistently computed results on the evolution of X-ray properties of galaxy clusters in a cold + hot dark matter (CHDM) model. We have performed a hydrodynamic plus N-body simulation for the COBE-compatible CHDM model with standard mass components: Omega(sub hot) = 0.3, Omega (sub cold) = 0.6 and Omega(sub baryon) = 0.1 (h = 0.5). In contrast with the CDM model, which fails to reproduce the observed temperature distribution function dN/dT (Bryan et al. 1994b), the CHDM model fits the observational dN/dT quite well. Our results on X-ray luminosity are less firm but even more intriguing. We find that the resulting X-ray luminosity functions at redshifts z = 0.0, 0.2, 0.4, 0.7 are well fit by observations, where they overlap. The fact that both temperatures and luminosities provide a reasonable fit to the available observational data indicates that, unless we are missing some essential physics, there is neither room nor need for a large fraction of gas in rich clusters: 10% (or less) in baryons is sufficient to explain their X-ray properties. We also see a tight correlation between X-ray luminosity and gas temperature.

Bryan, Greg L.↗
NASA NTRS1994

Gravitational collapse of small-scale structure as the origin of the Lyman-alpha forest

If gravitational clustering is a hierarchical process, the present large-scale structure of the galaxy distribution implies that structures on smaller scales must have formed at high redshift. We simulate the formation of small-scale structure (average cell mass: (delta) bar m(sub b) = 10(sup 4.2) solar mass) and the evolution of photoionized gas, in the specific case of the CDM + Lambda model. The photoionized gas has a natural minimal scale of collapse, the Jeans scale (m(sub b,J) is approximately equal to 10(exp 9) solar mass). We find that low column density (N(sub HI) is less than or equal to 10(exp 14)/sq cm) lines originate in regions resembling Zel'dovich pancakes, where gas with overdensities in the range 3-30 is enclosed by two shocks but is typically reexpanding at approximately the Hubble velocity. However, higher column density lines stem from more overdense regions where the shocked gas is cooling. We show that this model can probably account for the observed number of lines, their distribution in column density and b-parameters, as well as the cloud physical sizes as observed in gravitationally lensed quasars. We find a redshift evolution that is too steep; however, this may be due to insufficient dynamical range in the simulation or because the specific model is incorrect. The model predicts that high signal-to-noise observations should find systematic deviations from Voigt profiles, mainly in the form of broad wings in the line profiles, and that a fluctuating Gunn-Peterson effect will be detected, which can be modeled as a superposition of weak lines with a wide range of b-parameters.

Cen, Renyue↗
NASA NTRS1991

The cosmic microwave background

Recent limits on spectral distortions and angular anisotropies in the cosmic microwave background are reviewed. The various backgrounds are described, and the theoretical implications are assessed. Constraints on inflationary cosmology dominated by cold dark matter (CDM) and on open cosmological models dominated by baryonic dark matter (BDM), with, respectively, primordial random phase scale-invariant curvature fluctuations or non-gaussian isocurvature fluctuations are described. More exotic theories are addressed, and I conclude with the 'bottom line': what theorists expect experimentalists to be measuring within the next two to three years without having to abandon their most cherished theories.

Silk, Joseph↗
NASA NTRS1995

A count probability cookbok: Spurious effects and the scaling model

We study the errors brought by finite volume effects and dilution effects on the practical determination of the count probability distribution function P(sub N)(n,l), which is the probability of having N objects in a cell of volume l cubed for a set of average number density n. Dilution effects are particularly revelant to the so-called sparse sampling strategy. This work is mainly done in the framework of the Bailan & Schaeffer scaling model, which assumes that the Q-body correlation functions obey the scaling relation Xi(sub Q)(lambda r(sub l),....lambda r(sub Q) = lambda(exp -(Q-1)gamma) Xi(sub Q)(r(sub 1),....r(sub Q)). We use three synthetic samples as references to perform our analysis: a fractal generated by a Rayleigh-Levy random walk with approximately 3 x 10(exp 4) objects, a sample dominated by a spherical power-law cluster with approximately 3 x 10(exp 4) objects and a cold dark matter (CDM) universe involving approximately 3 x 10(exp 5) matter particles.

Colombi, S.↗
NASA NTRS1995

Simulated cosmic microwave background maps at 0.5 deg resolution: Basic results

We have simulated full-sky maps of the cosmic microwave background (CMB) anisotropy expected from cold dark matter (CDM) models at 0.5 deg and 1.0 deg angular resolution. Statistical properties of the maps are presented as a function of sky coverage, angular resolution, and instrument noise, and the implications of these results for observability of the Doppler peak are discussed. The rms fluctuations in a map are not a particularly robust probe of the existence of a Doppler peak; however, a full correlation analysis can provide reasonable sensitivity. We find that sensitivity to the Doppler peak depends primarily on the fraction of sky covered, and only secondarily on the angular resolution and noise level. Color plates of the simulated maps are presented to illustrate the anisotropies.

Hinshaw, G.↗
NASA NTRS1995

Large-scale motions in the universe: Using clusters of galaxies as tracers

Can clusters of galaxies be used to trace the large-scale peculiar velocity field of the universe? We answer this question by using large-scale cosmological simulations to compare the motions of rich clusters of galaxies with the motion of the underlying matter distribution. Three models are investigated: Omega = 1 and Omega = 0.3 cold dark matter (CDM), and Omega = 0.3 primeval baryonic isocurvature (PBI) models, all normalized to the Cosmic Background Explorer (COBE) background fluctuations. We compare the cluster and mass distribution of peculiar velocities, bulk motions, velocity dispersions, and Mach numbers as a function of scale for R greater than or = 50/h Mpc. We also present the large-scale velocity and potential maps of clusters and of the matter. We find that clusters of galaxies trace well the large-scale velocity field and can serve as an efficient tool to constrain cosmological models. The recently reported bulk motion of clusters 689 +/- 178 km/s on approximately 150/h Mpc scale (Lauer & Postman 1994) is larger than expected in any of the models studied (less than or = 190 +/- 78 km/s).

Gramann, Mirt↗