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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1980

The structure and evolution of X-ray clusters of galaxies

Einstein Observatory observations of the structures of nearby X-ray clusters of galaxies are discussed in relation to dynamic cluster evolution. Examples of Virgo-type clusters, in which cool and hot gas associated with cluster members are presented and variations in surface brightness profile and the location of the cluster center are used to classify the observed clusters. The types of clusters observed are interpreted in terms of dynamic cluster evolution, with Virgo-type clusters with broad, highly clumped emission and low velocity dispersion representing early evolutionary stages, clusters containing a cD galaxy formed as the cluster evolves, and clusters with dominant galaxies and Coma-type clusters representing an equilibrium stage. X-ray emission from poor clusters of galaxies is also considered, and similarities between the presumably collisionally formed cD galaxies in rich and poor clusters are noted. Finally, observations of distant clusters are discussed, and it is noted that their analysis will lead to a better determination of the evolutionary sequence.

Jones, C.↗
NASA NTRS1993

The warped disk of Centaurus A in the near-infrared

We present infrared images of Cen A (NGC 5128) in the J, H, and K bands. The infrared morphology is primarily determined by the presence of a thin absorptive warped disk. By integrating the light of the underlying prolate galaxy through such a disk, we construct models which we compare with infrared and X-ray data. The geometry of the warped disk needed to fit the IR data is consistent with a warped disk which has evolved as a result of differential precession in a prolate potential. The disk has an inclination, with respect to the principal axis of the underlying elliptical galaxy, that is higher at larger radii than in the inner region. A scenario is proposed where a small gas-rich galaxy infalling under the force of dynamical friction is tidally stripped. Stripping occurs at different times during its infall. The orientation of the resulting gas disk depends upon the angular momentum of the infalling galaxy. We find that the resulting precession angle of the disk is well described by the precession model, but that the inclination angle may vary as a function of radius. We propose an orbit for the infalling galaxy that is consistent with the geometry of the warped disk needed to fit our infrared data, and rotation observed in the outer part of the galaxy.

Quillen, A. C.↗
NASA NTRS1977

Clusters of galaxies

Observational properties of clusters of galaxies and some theoretical interpretations are reviewed. The major subjects of the review include catalogs of clusters, the static properties of rich clusters, cluster dynamics, X-ray emission from clusters of galaxies, and cluster radio emission. Observed static properties discussed are: richness, classification schemes, galactic content, cD galaxies, density profiles, sizes, and the optical luminosity function. Several characteristic time scales of clusters of galaxies are examined along with mass segregation, the dynamical evolution of clusters, cluster masses, mass-to-light ratios, the missing-mass problem, and the mean galactic and cluster mass densities in the universe. The main characteristics of clusters that contain X-ray emission are summarized, emphasizing the relation of the X-ray emission to optical properties. The most important properties of cluster radio emission are outlined, the relation of this emission to the other cluster properties studied is indicated, and evidence suggesting the influence of an intracluster gas on the radio galaxies and their relativistic electrons is noted.

Bahcall, N. A.↗
NASA NTRS1980

Galaxy collisions - A preliminary study

Collisions of spherical galaxies were studied in a series of numerical experiments to see what happens when galaxies collide. Each experiment starts with two model galaxies, each consisting of 50,000 stars, moving toward each other along a specified orbit. The series of experiments provides a systematic sampling of the parameter space spanned by the initial orbital energy and the initial angular momentum. Deeply penetrating collisions are emphasized. The collisions reported here scale to relative velocities as great as 500 km/s, well into the range for collisions within clusters of galaxies. It is found that: (1) the galaxies contract momentarily to about half their original sizes shortly after close passage; and (2) the initial galaxies blend into a single dynamical system while they are near each other.

Miller, R. H.↗
NASA NTRS1982

On the contribution of a stochastic background of gravitational radiation to the timing noise of pulsars

The influence of a stochastic and isotropic background of gravitational radiation on timing measurements of pulsars is investigated, and it is shown that pulsar timing noise may be used to establish a significant upper limit of about 10 to the -10th on the total energy density of very long-wavelength stochastic gravitational waves. This places restriction on the strength of very long wavelength gravitational waves in the Friedmann model, and such a background is expected to have no significant effect on the approximately 3 K electromagnetic background radiation or on the dynamics of a cluster of galaxies.

Mashhoon, B.↗
NASA NTRS1984

Studies of X-ray clusters of galaxies/intergalactic plasmas

Intergalactic plasmas were investigated from both an observational and theoretical point of view. A multiobject spectrometer, the MX spectrograph was used to obtain detailed dynamical information on clusters of galaxies; this information was then compared with X ray emission from hot gas in these clusters. Several spectra of galaxies are presented, and data reduction of the spectra was discussed. The existence of quasar winds in Seyfert galaxies and the interaction between such a wind and the interstellar medium also were considered.

Scott, J. S.↗
NASA NTRS1984

Waves in planetary rings

The Voyager spacecraft revealed the rings of Saturn to have an unexpected richness of structure. Many of the observed features have now been identified as collective effects arising from the self-gravity of the ring material. These effects include spiral density waves and spiral bending waves, the main topics of this review. Both kinds of waves were first discussed in the astronomical literature in connection with the dynamics and structure of spiral galaxies, and the present discussion contrasts the similarities and differences between the disks of galaxies and planetary rings. After developing the theory of free and forced waves of both types, the way in which the observed waves can be used as diagnostics to obtain crucial parameters that characterize the physical state of the rings is discussed.

Shu, F. H.↗
NASA NTRS1987

Starbursts and their dynamics

Detailed mechanisms associated with dynamical process occurring in starburst galaxies are considered including the role of bars, waves, mergers, sinking satellites, self gravitating gas and bulge heating. The current understanding of starburst galaxies both observational and theoretical is placed in the context of theories of galaxy formations, Hubble sequence evolution, starbursts and activity, and the nature of quasar absorption lines.

Norman, Colin↗
NASA NTRS1989

Numerical experiments on the oscillations of a rotating, axisymmetric galaxy

Modes of oscillation in six rotating, axisymmetric N-body systems are studied in a sequence of self-consistent, three-dimensional numerical experiments. The experimental systems are realizations of theoretical models of galaxies which are stellar-dynamical counterparts of uniformly rotating polytropes of index equal to 0.5. The ratio of the rotational kinetic energy to the gravitational potential energy ranges from 0.13 to 0.20. The systems oscillate axisymmetrically; the oscillations are interpreted as superpositions of a mode of radial pulsation and a Kelvin-like mode of oscillation. The experimental frequencies of these modes agree very well with theoretical predictions. When these modes are suppressed, the states of the experimental systems are very steady. The systems are dynamically unstable with respect to a toroidal mode when the ratio of the rotational kinetic energy to the gravitational potential energy exceeds a value lying between 0.16 and 0.17.

Miller, R. H.↗
NASA NTRS1990

The diffuse galactic gamma ray emission

The EGRET (Energetic Gamma-Ray Experiment Telescope) detector will provide a much more detailed view of the diffuse galactic gamma ray intensity in terms of higher resolution, greater statistical significance, and broader energy range than earlier missions. These observations will furnish insight into a number of very important questions related to the dynamics and structure of the Galaxy. A diffuse emission model is being developed that incorporates the latest information on matter distribution and source functions. In addition, it is tailored to the EGRET instrument response functions. The analysis code of the model maintains flexibility to accommodate the quality of the data that is anticipated. The discussion here focuses on the issues of the distributions of matter, cosmic rays, and radiation fields, and on the important source functions that enter into the model calculation of diffuse emission.

Bertsch, David L.↗
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 NTRS2000

Can Massive Stars Be Formed by Accretion?

Although massive stars play a critical role in the production of turbulent energy in the ISM, in the formation and destruction of molecular clouds, and ultimately in the dynamical and chemodynamical evolution of galaxies, our understanding of the sequence of events which leads to their formation is still rather limited.

Star formation accretion disks radiation hydrodyna↗
NASA NTRS1994

Multiple Core Galaxies

Nuclei of galaxies often show complicated density structures and perplexing kinematic signatures. In the past we have reported numerical experiments indicating a natural tendency for galaxies to show nuclei offset with respect to nearby isophotes and for the nucleus to have a radial velocity different from the galaxy's systemic velocity. Other experiments show normal mode oscillations in galaxies with large amplitudes. These oscillations do not damp appreciably over a Hubble time. The common thread running through all these is that galaxies often show evidence of ringing, bouncing, or sloshing around in unexpected ways, even though they have not been disturbed by any external event. Recent observational evidence shows yet another phenomenon indicating the dynamical complexity of central regions of galaxies: multiple cores (M31, Markarian 315 and 463 for example). These systems can hardly be static. We noted long-lived multiple core systems in galaxies in numerical experiments some years ago, and we have more recently followed up with a series of experiments on multiple core galaxies, starting with two cores. The relevant parameters are the energy in the orbiting clumps, their relative.masses, the (local) strength of the potential well representing the parent galaxy, and the number of cores. We have studied the dependence of the merger rates and the nature of the final merger product on these parameters. Individual cores survive much longer in stronger background potentials. Cores can survive for a substantial fraction of a Hubble time if they travel on reasonable orbits.

Miller, R.H.↗
NASA NTRS1990

The extragalactic radiation field and sharp edges to HI disks in galaxies

Observations of neutral hydrogen (HI) are widely used as a probe of the interstellar medium in galaxies and of galactic kinematics and dynamics. The 21-cm line can be used to determine galactic rotation curves far beyond the optical disk, and is one of the prime sources of evidence for the existence of dark haloes in spiral galaxies. However, a recent attempt to measure the 21-cm emission from NGC 3198 to very low column densities (N sub H approx. equals 5 times 10 to the 18th power) found that the HI disk is very sharply truncated at a column density of a few times 10 to the 19th power (Sancisi 1989, private communication). This discovery reinforces the earlier suggestion (Briggs et al. 1980) that extended, low-column density envelopes of neutral hydrogen are not common around galaxies. Here the author suggests that the sharp edge seen in the HI disk in NGC 3198 is consistent with a model in which a self-gravitating neutral HI disk is photoionized by the extragalactic radiation field. The possibility that the extragalactic background would produce sharp edges to HI disks was first suggested by Silk and Sunyaev (1976).

Maloney, Philip Richard↗
NASA NTRS2001

Understanding the Puzzling X-Ray Spectrum of the SO Galaxy NGC 4382; NGC 43819: Spectral Analysis of the Prototypical Early Merger and ASCA Observations of a Dynamically Young Elliptical: NGC 4125

We have analyzed the ASCA observations of NGC 4382, NGC 4038/9, NGC 4125 and produced papers for publication. NGC 4382 is one of the E and SO galaxies detected with the lowest X-ray to optical luminosity ratio. These galaxies have a peculiar X-ray (0.1-3 keV) spectrum, with a significant excess of counts in the lowest spectral channels (less than 1 keV) relative to the spectral count distributions of X-ray brihter E and SO galaxies. Analyzing the ROSAT PSPC observation of NGC 4382 it was unclear whether this soft excess was due to a real very soft component in a multi-component spectrum, or reflected an extremely low metal abundance in an isothermal hot gas. Our ASCA observations show that the low-abundance single-temperature model does not fit well to the X-ray spectrum, in agreement with our previous suggestions. A better explanation is a composite spectrum with a very soft component (0.3 keV) in addition to a harder (5 keV) component from X-ray binaries. In this model, the abundance cannot be constrained. More complex spectral models are also possible. The ASCA observations of The Antennae - (NGC4038/9) show that at least two spectral components are required to describe the emission-thermal emission from a plasma at 0.8 keV, and a component at higher energies. The hot gas contributes about half of the flux in the 0.5 to 6 keV band. If the column density to the higher energy component is greater than 2 x 10 (exp 21) per square centimeter, then the fitted abundance in the hot gas component is less than 0.2 solar. This low abundance is not expected for the hot interstellar medium in NGC4038/9 in which supernovae and star formation (expected to enrich and heat the gas) are ongoing. We do not detect any spatial variations in the spectrum. We relate these findings to data obtained by other satellites (Einstein, ROSAT) for this interacting galaxy pair. NGC4125's ASCA data was analyzed jointly with its Beppo-SAX observation. A hard component (kT /sim 4-10 keV) is observed together with a thermal component with line emission.

Fabbiano, Giuseppina↗
NASA NTRS1991

On the velocity fields of elliptical galaxies

A family of self-consistent maximum entropy dynamical models is presented for the triaxial 'perfect ellipsoids'. These models are projected in different viewing geometries to explore the possible morphologies of the 2D radial-velocity fields of elliptical galaxies. It is found that, typically, about half the time an 'ordinary' velocity field is seen with a mixture of major- and minor-axis rotation. For other lines of sight, the velocity fields are more complicated, showing multiple peaks, steep gradients, strongly twisted contours, or kinematically distinct or counter-rotating cores. The origin of each of these velocity field morphologies is explained in terms of competing contributions from the major families of tube orbits.

Statler, Thomas S.↗
NASA NTRS1986

High dynamic range VLBI observations of NGC 6251

The large radio galaxy NGC 6251 has been observed with an 11-telescope VLBI array, including the full VLA, at 18 cm. High-dynamic-range maps of both the VLBI-scale and VLA-scale radio structure from these observations, as well as hybrid maps from earlier VLBI experiments at 6 and 13 cm, are presented. The 18-cm VLBI hybrid map has an unusually high dynamic range of about 300 to 1 (ratio of the peak brightness to five times the rms noise level) and is limited by thermal noise rather than calibration errors. A lower limit of 80 to 1 for the ratio of jet/counterjet brightness at parsec scales can be set. This is the strongest such limit produced by VLBI observations of any source to date. If the absence of a detectable counterjet is caused by relativistic beaming, then the jet must be aligned within 45 deg of the line of sight. The VLA map shows a complex structure along the previously detected kiloparsec-scale counterjet. The physical conditions in the inner few parsecs of the NGC 6251 jet and their relation to the conditions in the extraordinarily long and well collimated kiloparsec-scale jets are discussed, and it is shown that both small-scale and large-scale morphology can be explained in terms of a simple model and relativistic beaming.

Jones, D. L.↗
NASA NTRS1983

The center for astrophysics redshift survey

Major advances in the art of redshift measurements have improved the obtainable accuracy to better than 30 km/s. It is now posible to obtain a redshift for almost any galaxy brighter than 15th magnitude on a 60-inch telescope in 60 minutes or less. These advances were utilized in an observation program initiated in the spring of 1978. This program represents a survey of radial velocities for all 2400 galaxies brighter than 14.5 at high galactic latitude in the northern hemisphere. The obtained data set has been employed as a basis for a derivation of a good measure of the local mean mass density. In addition, information was obtained concerning the overdensity and the dynamics of the local supercluster, and an analysis was conducted of the dynamics of groups and clusters of galaxies within the sample volume.

Davis, M.↗