Engineering topics

Hogan, Craig J.

Publications and source records attributed to Hogan, Craig J..

At least 19 records

NASA NTRS1995

Giant branch mixing and the ultimate fate of primordial deuterium in the Galaxy

The observed cosmic abundances of light elements are most consistent with each other, and with the predictions of big bang nucleosynthesis, if, contrary to the usual assumption, galactic chemical evolution reduces (D = He-3)/H with time. Chemical evolution models which accomplish this require that low-mass stars destroy He-3 in the envelope gas that they return to the interstellar medium. A simple argument based on the rates of limiting nuclear reactions shows that the same giant branch mixing process which appears to be needed to explain the observed C-12/C-13 and C/N ratios in 1-2 solar mass stars would indeed also probably destroy He-3 by a large factor in the bulk of the envelope material. The conclusion is that Galactic He-3/H estimates should not be trusted for setting an upper limit on primordial (D = He-3)/H. This removes the strongest lower bound on the cosmic baryon density from big bang nucleosynthesis and the only argument for abundant baryonic dark matter.

Hogan, Craig J.↗

NASA NTRS1994

Statistical association of QSO's with foreground galaxy clusters

We report a statistically significant overdensity of high redshift quasi-stellar objects (QSO's) in the directions of foreground galaxy clusters. QSO's are taken from the Large Bright QSO Survey (LBQS) between 1.4 less than or equal z less than or equal 2.2 with a limiting magnitude of m(sub B) = 18.5. Foreground clusters are regions within 6 Zwicky radii of small Zwicky clusters at a characteristic redshift of about z approximately = 0.2, covering about 40% of the total area surveyed (304 sq. deg). The overdensity, defined as the ratio of the number density of QSO's in the directions of clusters ('association QSO's) to that in the remainder of the fields ('background QSO's), is equal to 1.7, and formally differs from unity at 4.7 sigma significance. The observed overdensity probably is not due to statistical variation in QSO density, intrinsic QSO-QSO and/or cluster-cluster autocorrelations, or patchy Galactic obscuration. We thus interpret this observation as being due to statistical gravitational lensing of background QSO's by galaxy clusters. However, this amplitude of overdensity behind clusters cannot be accounted for in any cluster lensing model if the background QSO number-magnitude counts are similar to the intrinsic (unlensed) counts, and is implausible in any conventional model of cosmic mass distribution.

Rodrigues-Williams, Liliya L.↗

NASA NTRS1993

Precombination Cloud Collapse and Baryonic Dark Matter

A simple spherical model of dense baryon clouds in the hot big bang 'strongly nonlinear primordial isocurvature baryon fluctuations' is reviewed and used to describe the dependence of cloud behavior on the model parameters, baryon mass, and initial over-density. Gravitational collapse of clouds before and during recombination is considered including radiation diffusion and trapping, remnant type and mass, and effects on linear large-scale fluctuation modes. Sufficiently dense clouds collapse early into black holes with a minimum mass of approx. 1 solar mass, which behave dynamically like collisionless cold dark matter. Clouds below a critical over-density, however, delay collapse until recombination, remaining until then dynamically coupled to the radiation like ordinary diffuse baryons, and possibly producing remnants of other kinds and lower mass. The mean density in either type of baryonic remnant is unconstrained by observed element abundances. However, mixed or unmixed spatial variations in abundance may survive in the diffuse baryon and produce observable departures from standard predictions.

Hogan, Craig J.↗

NASA NTRS1993

The cosmic Doppler instability

The equations governing the behavior of perturbations of a mixture of nearly homogeneous and isotropic matter and radiation are derived, using a diffusion approximation where spatial perturbations in the radiation spectrum are allowed to vary with frequency. A simple model of line opacity leads to dispersion relations which display a new bulk instability. The model is used to derive an approximate dispersion relation for radiation interacting via resonance scattering opacity in atomic hydrogen at low density and low temperature. Possible applications to cosmology are briefly discussed.

Hogan, Craig J.↗

NASA NTRS1993

Equilibrium slab models of Lyman-alpha clouds

Solutions for the equilibrium configuration of a slab with ionizing radiation incident equally from both sides are explored. Radiation effects (photoionization, Ly-alpha photon trapping, and mock gravity) as well as external pressure and self gravity (with and without dark matter) are included. The general formalism is applied to structure growth on small scales at very high z due to mock gravity on dust. Emphasis is placed on the application of slab models at z of less than 5, particularly those that may correspond to Ly-alpha forest, Lyman limit, and damped Ly-alpha systems. The regime with a dominant outward force contributed by trapping of Ly-alpha photons is discussed. General expressions are given for the equilibrium, including dark matter, assuming various relationships between the density of the dark matter halo and the total gas column density.

Charlton, Jane C.↗

NASA NTRS1993

Equilibrium Slab Models of Lyman-Alpha Clouds

We model the L(sub y(alpha)) clouds as slabs of hydrogen with an ionizing extragalactic radiation field incident from both sides. In general, the equilibrium configuration of a slab at redshift z approx. less than 5 is determined by a balance of the gas pressure, gravity (including the effects of a dark matter halo), and the pressure exerted by the inter-galactic medium, P(sub ext). These models have been used to make predictions of the number of slabs as a function of the neutral hydrogen column density, N(sub H). A break in the curve is predicted at the transition between regimes where gravity and pressure are the dominant confining forces, with a less rapid decrease at larger N(sub H). The transition from optically thin to optically thick slabs leads to a gap in the distribution, whose location is governed largely by the spectrum of ionizing radiation. There are certain parallels between lines of sight through the outer HI disk of spiral galaxy with increasing radius, and the progression from damped, to Lyman limit, to forest clouds. We discuss briefly the possibility that at least some of the observed low z forest clouds may be a separate population, associated with galaxies, as suggested by the observations of Bahcall et al. This population could dominate the forest at present if the dark matter attached to galaxies should lead to gravity confinement for this disk population, while the isolated clouds remain pressure confined. The formalism developed in this paper will allow a more detailed study. We also discuss a more general parameter study of the equilibrium configuration of slabs, including mock gravity and L(sub y(alpha)) photon trapping.

Charlton, Jane C.↗

NASA NTRS1992

COBE anisotropy from supercluster gas

It is suggested that the microwave background anisotropy detected by the COBE DMR might be dominated not by the direct gravitational effect of primordial fluctuations in the last scattering surface, but by scattering off of moving electrons in optically thin, nearby superclusters. Hot diffuse clouds of ionized gas created during supercluster collapse produce Sunyaev-Zel'dovich and Doppler background anisotropy whose properties may closely mimic those of primordial anisotropy in current data. Strategies for and difficulties in separating the effects are discussed, based on the anisotropy spectrum, autocorrelation, correlation with galaxy catalogs, X-ray emission, and integrated spectral distortions.

Hogan, Craig J.↗

NASA NTRS1992

A photoionization instability in the early intergalactic medium

It is argued that any fairly uniform source of ionizing photons can be the cause of an instability in the pregalactic medium on scales larger than a photon path length. Underdense regions receive more ionizing energy per atom and reach higher temperature and entropy, driving the density down still further. Fluctuations created by this instability can lead to the formation of structures resembling protogalaxies and intergalactic clouds, obviating the need for gas clouds or density perturbations of earlier cosmological provenance, as is usually assumed in theories of galaxy and structure formation. Characteristic masses for clouds produced by the instability, with log mass in solar units plotted against log radius in kpc, are illustrated.

Hogan, Craig J.↗

NASA NTRS1992

Self-organization of cosmic radiation pressure instability. II - One-dimensional simulations

The clustering of statistically uniform discrete absorbing particles moving solely under the influence of radiation pressure from uniformly distributed emitters is studied in a simple one-dimensional model. Radiation pressure tends to amplify statistical clustering in the absorbers; the absorbing material is swept into empty bubbles, the biggest bubbles grow bigger almost as they would in a uniform medium, and the smaller ones get crushed and disappear. Numerical simulations of a one-dimensional system are used to support the conjecture that the system is self-organizing. Simple statistics indicate that a wide range of initial conditions produce structure approaching the same self-similar statistical distribution, whose scaling properties follow those of the attractor solution for an isolated bubble. The importance of the process for large-scale structuring of the interstellar medium is briefly discussed.

Hogan, Craig J.↗

NASA NTRS1991

Discovery of a Ly-alpha galaxy near a damped Ly-alpha absorber at z = 2.3

The detection of a galaxy associated with the damped Ly-alpha absorbing cloud seen at z = 2.309 toward the QSO PHL 957 is reported. In addition to a strong but narrow Ly-alpha emission line and weaker C IV and He II lines, the object shows continuum at V about 24, with a slope rising slightly toward the red. This is similar to what is seen in high-redshift radio galaxies, but this galaxy does not correspond to any known radio source. The detected emission lines and continuum are most easily interpreted as light from hot, recently formed stars, implying a sizable star formation rate and a scarcity of dust. The spatial correlation of the absorbing cloud and the companion galaxy supports the interpretation of damped Ly-alpha clouds as objects fundamentally different from the lower column density Ly-alpha forest clouds, which show weak or no clustering.

Lowenthal, James D.↗

NASA NTRS1991

Formation of cosmic structure by Doppler instability

A new mechanism is described which can create an instability in homogeneous gaseous matter at very low density. When an isotropic background radiation field has, near an electronic resonance, a spectral feature for which photon occupation number increases with frequency, moving atoms increase their speed by taking energy from the photon distribution. In a cosmological setting, a sufficiently intense spectral feature can interact with neutral atomic gas, after recombination, to generate protogalactic perturbations of the scale and magnitude needed to explain large-scale cosmic structure.

Hogan, Craig J.↗

NASA NTRS1991

Self-organization of cosmic radiation pressure instability

Under some circumstances the absorption of radiation momentum by an absorbing medium opens the possibility of a dynamical instability, sometimes called 'mock gravity'. Here, a simplified abstract model is studied in which the radiation source is assumed to remain spatially uniform, there is no reabsorption or reradiated light, and no forces other than radiative pressure act on the absorbing medium. It is shown that this model displays the unique feature of being not only unstable, but also self-organizing. The structure approaches a statistical dynamical steady state which is almost independent of initial conditions. In this saturated state the absorbers are concentrated in thin walls around empty bubbles; as the instability develops the big bubbles get bigger and the small ones get crushed and disappear. A linear analysis shows that to first order the thin walls are indeed stable structures. It is speculated that this instability may play a role in forming cosmic large-scale structure.

Hogan, Craig J.↗

NASA NTRS1991

Large scale cosmic instability

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 Ly-alpha resonance scattering, with exponentially amplifies perturbations in neutral hydrogen density on large scales after recombination.

Hogan, Craig J.↗

NASA NTRS1990

Cosmic instability from radiation pressure

The Cosmic Background Explorer has recently confirmed the blackbody character of the microwave background to high accuracy (Mather et al., 1990), and will have the capability to detect other cosmic backgrounds throughout the infrared. A detection of cosmic background radiation dating from the pregalactic era would have important consequences for theories of cosmic structure. During the creation of such a background the pressure of the radiation itself causes an instability which leads inevitably to the growth of large-scale structure in the matter distribution. In contrast to conventional gravitational-instability models, the statistical properties of this structure are determined primarily by the self-organizing dynamics of the instability rather than details of cosmological initial conditions. The behavior of the instability is described here.

Hogan, Craig J.↗

NASA NTRS1989

Superclustering of quasi-stellar object absorption clouds

The clustering of C IV absorption line systems along the lines of sight to the 55 quasars in the survey of Sargent, Boksenberg, and Steidel is examined. Their original result that there is strong clustering of these systems at separations less than 1000 km/s is confirmed. The peculiar velocities of the clouds that combine with the one-dimensional nature of the sampling is shown to preclude any detailed comparison of the observed clustering strength on these scales with that found for nearby systems such as galaxies. The sample also contains significant clustering on larger scales, 1000-10,000 km/s. This probably reflects true spatial clustering of an amplitude exceeding that observed for any nearby systems, even rich clusters of galaxies. The signal is due mainly to high equivalent width absorbers and is dominated by a single very large 'absorption supercluster' in the quasar 0237-233. It seems likely that this structure was produced by processes other than gravitational instability.

Heisler, Julia↗

NASA NTRS1989

Mock gravity and the cosmic structure

The process of generating large-scale cosmic structure from the radiation-pressure or 'mock gravity' instability is studied with particular emphasis on the implications of the Berkeley-Nagoya rocket data for the submillimeter background. The linear theory of perturbations in an absorbing medium embedded in an expanding universe of radiation sources is presented. The instability sets up collapse velocities in linear perturbation theory which far exceed those from gravitational instability, so growth continues even after the instability switches off. Perturbation due to this effect is analyzed and related to the growth of large-scale cosmic structures at recent times; large-scale structure is shown to evolve very little from 1 + z about 5 to the present. Nonlinear small-scale effects of the instability are analyzed; it is shown that radiation pressure would compress gas into small, dense pressure-confined clouds with tau much greater than one.

Hogan, Craig J.↗

NASA NTRS1989

Cosmic structure from radiation-blown bubbles

Absorbing material in an expanding universe filled with sources of radiation is subject to an instability driven by radiation pressure. In the optically thick limit, this instability takes the form of rapidly growing cavities or bubbles in the absorbing material. No matter how small they are when they begin, such bubbles grow to approach a limiting size which depends only on the ratio of photon pressure to absorber inertia. For a nonprimordial submillimeter radiation background as strong as that recently detected by the Berkely-Nagoya group, the bubbles grow to a comoving radius of about 20 Mpc, comparable in scale to the voids seen in the large-scale galaxy distribution.

Hogan, Craig J.↗

NASA NTRS1989

Large-scale galaxy flow from a non-gravitational impulse

A theory is presented describing linear perturbations of an expanding universe containing multiple, independently perturbed, collisionless, gravitationally coupled constituents. Solutions are found in the limit where one initially unperturbed component dominates the total density. The theory is applied to perturbations generated by a nongravitational process in one or more of the light components, as would occur in explosive or radiation-pressure-instability theories of galaxy formation. The apparent dynamical density parameter and correlations between density and velocity amplitude for various populations, are evaluated as a function of cosmic scale factor.

Hogan, Craig J.↗