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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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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1968

Spiral structure in galaxies.

Density wave with quasi-stationary spiral structure in galactic disk, discussing associated gravitational effects and hydromagnetic forces

Lin, C. C.↗
NASA NTRS1979

Gravitational collapse in dust lanes and the appearance of spiral structure in galaxies

The possibility is considered that there are two types of spiral for each spiral galaxy: an underlying spiral of relatively old stars seen in red and IR photographs and a spiral outlined by H II regions and young bright blue stars. The origin of the first spiral is attributed to large-scale spiral density waves, and the second spiral is identified as a stochastic spiral. It is suggested that there is a physical or causal connection between the two spirals and that dust lanes play the role of intermediary. An analysis is performed which shows that such a model can account for the formation and destruction of giant molecular cloud complexes and for observed spiral structure in one simple unified theory.

Elmegreen, B. G.↗
NASA NTRS1980

The photometric properties of brightest cluster galaxies. II - SIT and CCD surface photometry

Surface photometry of the first-ranked galaxy in 108 Abell clusters is presented. Galaxy structure, as parameterized by simple Hubble law models, is found to correlate with galaxy absolute magnitude and cluster structure. All the structure data support the dynamical friction evolution model. Twenty-eight percent of the galaxies have multiple component nuclei; the short lifetimes of such systems provide the best available evidence that ongoing evolution actually occurs. Average magnitude and structure evolution rates are derived from the data.

Hoessel, J. G.↗
NASA NTRS1979

The structure and evolution of X-ray clusters of galaxies

The structure of clusters is described from observations at the Einstein Observatory. It was concluded that the nature of the X-ray emission is complex and varies from broad and highly clumped to smooth and centrally peaked. The clusters whose emission is clumped tend to be rich in spirals and to have X-ray temperatures in the few kilovolt range and low velocity dispersions. The smooth centrally peaked clusters are spiral poor, and have higher temperatures and larger velocity dispersions. For many of the clusters, the emission is irregular and cannot be described by the simple, spherically symmetric models for a hot isothermal or adiabatic gas. For these clusters, the low density, intracluster gas is influenced by the potential of individual bright galaxies.

Jones, C.↗
NASA NTRS1979

X-ray structure of clusters of galaxies

Observations of the X-ray structure of the Virgo and Coma clusters of galaxies are described which were made with a two-dimensional imaging X-ray telescope in a series of sounding-rocket flights. It is shown that the X-ray structure of these two clusters exhibits considerable variations in surface brightness, that the central region of the Coma cluster exhibits local variations in surface brightness or clumpiness, and that the centroid of the X-ray emission from the Coma cluster is near the supergiant elliptical galaxy NGC 4874 and is a local minimum in intensity. Positive evidence is obtained for the Fe XXVII-XXIV complex of lines in the spectrum of M87 in the Virgo cluster. It is suggested that the gas in the halo of M87 is isothermal and contains an iron abundance that is uniformly close to the cosmic value.

Gorenstein, P.↗
NASA NTRS1981

The X-ray structure of a galaxy cluster at z = 0.54 - Implications for cluster evolution and cosmology

High-resolution X-ray observations of the rich cluster 0016+16 at a redshift of 0.541 are presented. The emitting gas in this cluster is hot and extremely luminous, and its structure resembles that seen in the brightest nearby cluster sources. In most of its properties, 0016+16 resembles a richer version of the Coma cluster, and it offers little support to the hypothesis that clusters at z greater than 0.5 differ fundamentally from nearer objects.

White, S. D. M.↗
NASA NTRS1971

The small-scale structure of radio galaxies and quasi-stellar sources at 3.8 centimeters.

Observation of fringes from 31 compact radio sources, including eight known or suspected galaxies and 20 known or suspected QSSs, by using the Goldstack interferometer at lambda = 3.8 cm (d/lambda = 10 to the 8th power). Fringe visibility curves were obtained for nine sources showing structure on a scale of .001 sec of arc, and simple models are fitted to the data. Results for 3C 273 and 3C 279 are compared with data taken by Knight et al. (1971) at an earlier epoch. The apparent changes in brightness distribution of 3C 273 and 3C 279 are difficult to explain.

Cohen, M. H.↗
NASA NTRS1977

No Organic Compounds on Mars? Understanding the Structure of Spiral Galaxies

A prime goal of the Viking missions to Mars is to search for life on that planet. Each of the two landers incorporate three specific life-detection experiments, and all have operated successfully. However, as any newspaper reader knows, the results are ambiguous, in that some experiments suggest a highly active martian biology while others appear to indicate that the samples are sterile. It would be premature to conclude from the results of the biological experiments that martian life forms have definitely been detected. In addition, the picture is clouded by unexpected results from another Viking experiment, which is designed to detect organic and inorganic chemical compounds in the martian soil. In Science for 1 October 1976, K. Biemann of MIT and ten of his colleagues report the first results from the Viking 1 Gas-Chromatograph/Mass Spectrometer (GCMS) experiment.

Morrison, Nancy D.↗
NASA NTRS1979

General properties of HII regions in galaxies

The structure, electron density, and dimensions of HII regions in galaxies are discussed. These parameters are correlated to the chemical composition gradient along the galactic radius, the dimensions of the three largest HII regions in the galaxy, and the amount of hydrogen in the galaxy, as well as the mass, dimensions, and total optical luminosity of the galaxy. The relationships of HII regions to star formation and galactic nucleus activity are discussed and the kinematic properties of the SB and Sab galaxies are related to the size of HII regions.

Smirnov, M. A.↗
NASA NTRS1981

The structure of giant elliptical galaxies in poor clusters of galaxies

Detailed photographic surface photometry is presented of the nine first brightest galaxies in poor clusters suspected of being cD galaxies. The surface photometry is put on an absolute scale, using multiaperture photoelectric photometry. Oemler (1976) and de Vaucouleurs (1948) laws are fitted to the profiles to derive structural parameters, and the structural properties of the galaxies are compared with those of the cD galaxies in rich clusters studied by Oemler (1976) and those of normal ellipticals studied by Kormendy (1977). It is found that (1) the profiles of first brightest galaxies are well fitted by an r to the 1/4 power law over a range of more than nine magnitudes and down to the faintest light levels observed and (2) structural parameters characterizing only the central part of first brightest galaxies in poor and rich clusters, and excluding the envelope component in the case of rich clusters, smoothly extend properties exhibited by normal ellipticals. These observations are consistent with the idea that the central part of first brightest galaxies in both poor and rich clusters is made up by the mergers of smaller galaxies.

Thuan, T. X.↗
NASA NTRS1976

The structure and content of the galaxy and galactic gamma rays. A review

The International Gamma Ray Symposium entitled 'The Structure and Content of the Galaxy and Galactic Gamma Rays', held at the NASA Goddard Space Flight Center in June, 1976, is briefly reviewed. Highlights from presentations on galactic structure, discrete sources, and low energy gamma rays are described. New data and their interpretations are also summarized.

Crannell, C. J.↗
NASA NTRS1972

Computer experiments on the structure and dynamics of spiral galaxies

The evolution of an initially balanced rotating disk of stars with an initial velocity dispersion given by Toomre's local criterion was investigated by means of a computer model for isolated disks of stars. It was found that the disk is unstable against very large scale modes. A stable axisymmetric disk with a velocity dispersion much larger than that given by Toomre's criterion was generated. The final mass distribution for the disk gives a high density central core and a disk population of stars that is closely approximated by an exponential variation. Various methods and rates of cooling the hot axisymmetric disks were investigated. It was found that the cooling resulted in the development of two-arm spiral structures which persisted as long as cooling continued. An experiment was performed to induce spiral structure in a galaxy by means of the close passage of a companion galaxy. Parameters similar to those expected for M51 and its companion were used. It was found that because of the high velocity dispersion of the disturbed disk galaxy, only a weak two-arm spiral structure appeared. The evolution of a uniformly rotating disk galaxy which is a stationary solution of the collisionless Boltzmann equation was investigated for various values of the initial rms velocity dispersion. It was found that the disk becomes stable at a value of the velocity dispersion predicted by theory.

Hohl, F.↗
NASA NTRS1970

Galactic astronomy; Proceedings of the Second Summer Institute for Astronomy and Astrophysics, State University of New York, Stony Brook, N.Y., June 19-July 17, 1968. Volumes 1 & 2.

Basic problems on the structure of our galaxy are presented. The essentially spiral structure of the galaxies is examined by a theory of the spiral shape, and by a study of the spiral structure with the 21-cm line of neutral hydrogen. Irregular spirals are discussed, and some problems that arise as a result of large deviations from the regular spiral structure are treated. The density wave theory of spiral structure is compared with the observed spiral structure of the galaxies. A section on the star migration technique, which is used to test the density wave theory of galactic spirals, is included. The interstellar medium is described in terms of such phenomena as supernova explosions and cosmic rays, both of which contribute to the energy balance of the medium. Other topics that are germaine to a description of the interstellar medium are the UV spectrum, kinematics of interstellar clouds, and ionization equilibrium. The implications of spiral galactic shock pattern on star formation along spiral arms are investigated, and by means of a blast wave model, it is shown that a hydrodynamic shock of galactic scale exists in galaxy M82.

Chiu, H.-Y.↗