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At least 217 records · Page 12
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

The 158 micrometer (CII) line: A measure of global star formation activity in galaxies

Some 158 micron (CII) fine structure line observations from a sample of fourteen gas rich galaxies are reported. These measurements confirm and generalize previous basic results that the (CII) line is bright amounting to approximately 0.1 to 1 percent of the Far Infra Red (FIR) luminosity of the nuclear regions of galaxies; the (CII) line is formed in the warm (temperature of the gas is greater than 200 K), dense (n sub H greater than 1000/cu cm) photodissociated gas at the interfaces between giant molecular clouds and ionized gas regions and is therefore associated with the molecular gas component in spiral galaxies; the (CII) line tracks the FIR continuum in a manner consistent with the PDR models; the integrated (CII) to isotope (12)CO (transition 1 to 0) line ratio is large (greater than or equal to 1000) in all galaxies studied, and is similarly large for galactic molecular clouds; the (CII) line is therefore energetically very important for the study of giant molecular clouds. Conclusions obtained from these results are given.

Stacey, G. J.↗
NASA NTRS1990

A comparison of the radial distribution of molecular gas and non-thermal radio continuum in spiral disks

The present study includes 65 spiral galaxies selected from the Five College Radio Astronomy Observatory (FCRAO) Extragalactic CO Survey for which the major axis distributions of CO emission and 1.49 GHz radio continuum emission are well determined. The radial distribution of the CO emission has been measured with the FCRAO at positions along the major axis that are spaced by one half power beam width (HPBW) (45 seconds). The radial profile of the 1.49 GHz radio continuum emission was constructed by determining the radio emission at the location of the CO measurements from the 1.49 GHz maps of Condon (1987). Large, greater than a factor of ten, radially decreasing gradients in the star formation efficiency are observed for a small percentage, approx. 10 percent, of the spirals in this sample. The majority of spirals, however, are associated with small gradients in the star formation efficiency that do not systematically increase or decrease with radius. That the star formation efficiency does not systematically decrease with radius tends to argue against a global dynamical mechanism, such as a spiral density wave, for being the dominant mechanism triggering disk star formation for the majority of spirals in this sample. The results tend to support the view that the star formation in spiral disks is dominated by a local process that depends more on the molecular cloud properties than the dynamical structure of a galaxy.

Devereux, Nicholas A.↗
NASA NTRS1984

Research in extragalactic astrophysics

The spatial distribution of clusters of galaxies and the large scale structure of the universe is investigated. Correlations in the spatial cluster distribution are much stronger than those seen for galaxies, and over scales much larger than previously anticipated. The following main studies are completed and published; (1) The determination of the correlation functions of rich clusters of galaxies, and its dependence on richness and other parameters. Very large scale structure exists in the universe, and rich clusters participate in this structure more efficiently than galaxies. (2) A complete catalog of superclusters, is determined. The selection criteria is that of a volume density enhancement, and is done in 3 dimensions. (3) The giant galaxy void in Bootes is found to be surrouned by rich, large superclusters from the catalog described above. (4) A very large scale void of rich clusters of galaxies is found in the study of the spatial distribution of rich clusters.

Bahcall, N.↗
NASA NTRS1993

The structure of the nearby universe traced by theIRAS galaxies

One of the most important discoveries of the Infrared Astronomical Satellite (IRAS) has been the detection of about 20,000 galaxies with 60 microns fluxes above 0.5 Jy. From the observational point of view, the IRAS galaxies are ideal tracers of density, since they are homogeneously detected over most of the sky, and their fluxes are unaffected by galactic extinction. The nearby universe was mapped by the IRAS galaxies to a distance of approximately 200 h(exp -1) Mpc for the absolute value of b less than 5 deg. The ability to map down to such low galactic latitudes has proven to be particularly imporant, since some of the most important nearby large-scale structures, such as the Great Attractor, the Perseus-Pisces region, and the Shapley concentration, all lie there. Two major results of the U.S. IRAS redshift survey are discussed.

Yahil, Amos↗
NASA NTRS1978

The cosmic X-ray experiment aboard HEAO-1

The HEAO-1 A-2 experiment, designed to study the large scale structure of the galaxy and the universe at X-ray energies is described. The instrument consists of six gas proportional counters of three types nominally covering the energy ranges of 0.15-3 keV, 1.2-20 keV, and 2.5-60 keV. The two low energy detectors have about 400 sq cm open area each while the four others have about 800 sq cm each. Dual field of view collimators allow the unambiguous determination of instrument internal background and diffuse X-ray brightness. Instrument characteristics and early performance are discussed.

Rothschild, R. E.↗
NASA NTRS1979

On the variability of extragalactic sources in the decimeter range and their correlation with galactic structures

It is shown that all of the extragalactic radio sources presently known are variable in the decimeter range (lambda or = 30 cm) and are projected on the large continuum radio structure of the galaxy: loops, spurs, ridges. Probability that coordinates could coincide is or = 10 to the minus 7 power. The variations in the intensity are explained by scintillations (regime of focusing radiation) on the large-scale irregularities of electron density in the medium of loops, spurs and ridges with the dimension a magnitude of approximately 10 to the 13th power cm. A correlation of the characteristics of radiation of the sources with their position relative to the galactic loop is considered. Based on the known experimental data, it is shown that the angle of scattering of extragalactic radiation and the dispersion measures of pulsars projecting on the loops is considerably larger than those of the sources lying outside the loops.

Shapirovskaya, N. Y.↗
NASA NTRS1980

Future prospects for gamma-ray

Astrophysical phenomena discussed are: the very energetic and nuclear processes associated with compact objects; astrophysical nucleo-synthesis; solar particle acceleration; the chemical composition of the planets and other bodies of the solar system; the structure of our galaxy; the origin and dynamic pressure effects of the cosmic rays; the high energy particles and energetic processes in other galaxies, especially active ones; and the degree of matter antimater symmetry of the universe. The gamma ray results of GAMMA-I, the gamma ray observatory, the gamma ray burst network, solar polar, and very high energy gamma ray telescopes on the ground provide justification for more sophisticated telescopes.

Fichtel, C.↗
NASA NTRS1981

Future prospects for gamma-ray astronomy

As gamma-ray astronomy moves from the discovery to the exploratory phase, the promise of gamma-ray astrophysics noted by theorists in the late 1940s and 1950s is beginning to be realized. In the future, satellites should carry instruments that will have over an order of magnitude greater sensitivity than those flown thus far, and, for at least some portions of the gamma-ray energy range, these detectors will also have substantially improved energy and angular resolution. The information to be obtained from these experiments should greatly enhance our knowledge of several astrophysical phenomena including the very energetic and nuclear processes associated with compact objects, astrophysical nucleosynthesis, solar particle acceleration, the chemical composition of the planets and other bodies of the solar system, the structure of our galaxy, the origin and dynamic pressure effects of the cosmic rays, high energy particles and energetic processes in other galaxies especially active ones, and the degree of matter-antimatter symmetry of the universe. The gamma-ray results of the forthcoming programs such as Gamma-I, the Gamma Ray Observatory, the gamma-ray burst network, Solar Polar, and very high energy gamma-ray telescopes on the ground will almost certainly provide justification for more sophisticated telescopes. These advanced instruments might be placed on the Space Platform currently under study by N.A.S.A.

Fichtel, C.↗
NASA NTRS1982

The future of high energy gamma ray astronomy and its potential astrophysical implications

Future satellites should carry instruments having over an order of magnitude greater sensitivity than those flown thus far as well as improved energy and angular resolution. The information to be obtained from these experiments should greatly enhance knowledge of: the very energetic and nuclear processes associated with compact objects; the structure of our galaxy; the origin and dynamic pressure effects of the cosmic rays; the high energy particles and energetic processes in other galaxies; and the degree of matter-antimatter symmetry of the universe. The relevant aspects of extragalactic gamma ray phenomena are emphasized along with the instruments planned. The high energy gamma ray results of forthcoming programs such as GAMMA-1 and the Gamma Ray Observatory should justify even more sophisticated telescopes. These advanced instruments might be placed on the space station currently being considered by NASA.

Fichtel, C. E.↗
NASA NTRS1983

Status and future of high energy diffuse gamma-ray astronomy

There are two distinctly different high energy diffuse gamma-ray components, one well correlated with broad galactic features and the other apparently isotropic and presumably extragalactic. The observed diffuse galactic high energy gamma radiation is generally thought to be produced in interactions between the cosmic rays and the interstellar matter and photons. It should then ultimately be possible to obtain from the diffuse galactic emission a detailed picture of the galactic cosmic-ray distribution, a high contrast view of the general structure of the galaxy, and further insight into molecular clouds. Two of the candidates for the explanation of the extragalactic diffuse radiation are the sum of emission from active galaxies and matter-antimatter annihilation. A major advancement in the study of the properties of both galactic and extragalactic gamma radiation should occur over the next decade.

Fichtel, C. E.↗
NASA NTRS1985

Nonlinear spiral density waves - An inviscid theory

It is pointed out that the theory of spiral density waves, invented to explain the spiral structure of disk galaxies, has also been found useful for the study of planetary rings. The linear theory is by now well developed, while the nonlinear theory is less complete. Analytical calculations which include self-gravitation have, so far, obtained results only in the slightly nonlinear regime, or have concentrated on partial effects which are not of primary importance to the physical problem at hand. In the present paper, it is attempted to remedy these shortcomings. The simplest asymptotic ordering which can still yield useful results is adopted. Attention is given to the reduction to a nonlinear integral equation in a single variable, the use of the Wentzel-Kramers-Brillouin-Jeffreys theory, and the replacement of an equation by another which is easier to handle numerically.

Shu, F. H.↗
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 NTRS1985

Galactic interstellar abundance surveys with IUE. I - Neutral hydrogen

A survey is initiated of interstellar densities, abundances, and cloud structure in the Galaxy, using the International Ultraviolet Explorer. From high-resolution (0.1-A) spectra, interstellar column densities are derived toward 244 early-type stars by fitting the damping wings of the Ly-alpha absorption line. Published B-V photometry is used to derive E(B-V) color excesses, stellar distances r, and the mean hydrogen densities, n-bar = N(H I)/r. The data set includes stars out to r = 8.5 kpc and E(B-V) = 0.86, with 68 stars in the galactic halo (absolute b greater than 20 deg). The statistical sample consists of 205 stars of type O3-B2.5 within 5 kpc (53 stars in the halo). The column densities, log N(H I), range from less than 19.44 per square centimeter for alpha Vir to 21.81 for HD 37061. A statistical analysis of the E(B-V) distribution suggests variations in the line-of-sight cloud density and the mean reddening per cloud.

Shull, J. M.↗
NASA NTRS1986

IUE/IRAS studies of metal abundances and infrared cirrus

A survey is reported of interstellar densities, abundances, and cloud structure in the Galaxy, using the IUE and IRAS satellites. Heavy element depletions are discussed along with their correlations with mean density, reddening, and galactic location. Interesting correlations between the Fe/Si abundance ratio and the infrared diffuse cirrus is also reported, which may provide information on the history and formation of grains in the galactic halo.

Vansteenberg, M. E.↗
NASA NTRS1986

Continuum emission in active galactic nuclei; Proceedings of the Workshop, Tucson, AZ, January 11-14, 1986

Papers are presented on IRAS observations of active galactic nuclei, 0.3-100 micron continua of Seyfert 1 galaxies, the diverse soft X-ray slopes of QSOs, and the continuum variability of the optically-active radio-quiet quasar GQ Comae. Also considered are the polarization signatures of accretion disks in active galactic nuclei, X-ray spectra of BL Lacertae objects, diffusive shock acceleration and quasar variability, and radio-optical broadband spectra and variability of quasars. Other topics include the radio properties of optically selected Seyfert galaxies, and the structure of the emission region in blazars.

Sitko, Michael L.↗
NASA NTRS1988

Galactic interstellar abundance surveys with IUE. II - The equivalent widths and column densities

This paper continues a survey of interstellar densities, abundances, and cloud structure in the Galaxy, using the International Ultraviolet Explorer (IUE) satellite. Equivalent widths of 18 ultraviolet resonance transitions are presented and column densities for Si II, Mn II, Fe II, S II, and Zn II toward 261 early-type stars are derived. These equivalent widths and column densities agree within the stated errors of earlier Copernicus, BUSS, or IUE surveys of Mn II, Fe II, S II, and Zn II for 45 stars in common. The column densities are derived from single-component curves of growth with a common b-value based on that of Fe II and Si II.

Van Steenberg, Michael E.↗
NASA NTRS1988

Galactic interstellar abundance surveys with IUE. III - Silicon, manganese, iron, sulfur, and zinc

This paper continues a survey of intestellar densities, abundances, and cloud structure in the Galaxy using the IUE satellite. A statistical data set of 223 O3-B2.5 stars is constructed, including 53 stars in the Galactic halo. It is found that S II lines in B stars, of luminosity classes IV and V, have possible contamination from stellar S II, particular for stars with v sin i less than 200 km/s. The mean logarithmic depletions are -1.00, -1.19. -0.63, and -0.23 (Si, Mn,Fe,S, Zn). Depletions of Si, Mn, and Fe correlate with the mean hydrogen density n-bar along the line of sight, with a turnover for n-bar greater than 1/cm. Sulfur depletions correlate with n-bar along the line of sight. The slight Zn depletion correlation also appears to be statistically insignificant. No correlation of depletion is found with the physical density derived from H2 rotational states in 21 lines of sight. Depletion variations in the disk are consistent with a Galactic abundance gradient or with enhanced mean depletions in the anticenter region.

Van Steenberg, Michael E.↗
NASA NTRS1988

Overview of measurements from the infrared telescope on Spacelab 2

A small helium cooled IR telescope flown on Spacelab-2 in July/August 1985 was used to make infrared measurements between 2 microns and 120 microns. New data were obtained on the structure of the Galaxy at 2 microns and 7 microns showing it to be much broader at these wavelengths than at longer wavelengths. The IR emission due to contamination from the Shuttle was found to be greater than anticipated, indicating the induced environment to be much higher than the planned limits. Aspects of superfluid helium management in zero-G and of a cryogenically cooled telescope design were also tested.

Koch, D. G.↗