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  • 11
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    Numerical techniques for large cosmological N-body simulations (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: Techniques for carrying out large N-body simulations of the gravitational evolution of clustering in the fundamental cube of an infinite periodic universe are described and compared. The accuracy of the forces derived from several commonly used particle mesh schemes is examined, showing how submesh resolution can be achieved by including short-range forces between particles by direct summation techniques. The time integration of the equations of motion is discussed, and the accuracy of the codes for various choices of 'time' variable and time step is tested by considering energy conservation as well as by direct analysis of particle trajectories. Methods for generating initial particle positions and velocities corresponding to a growing mode representation of a specified power spectrum of linear density fluctuations are described. The effects of force resolution are studied and different simulation schemes are compared. An algorithm is implemented for generating initial conditions by varying the number of particles, the initial amplitude of density fluctuations, and the initial peculiar velocity field.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal Supplement Series (ISSN 0067-0049); 57; 241-260
    Format: text
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  • 12
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    An observational view of large scale structure (1986)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A summary of recent observations of galaxy clustering is presented, including a brief review of redshift maps and galaxy clustering statistics. Simple arguments are presented that argue the underlying mass fluctuations are most likely associated with a clustering scale no larger than that of individual galaxies. The acceleration of the local group from the comoving frame of the universe and its connection to the microwave dipole anisotropy are also discussed. A final topic for consideration is the existence of large voids and clusters, and whether they are consistent with Gaussian initial conditions. The extreme size and depth of the Bootes void, if real, do present a puzzle. Finally, future directions for observational study of large scale structure are discussed.
    Keywords: ASTROPHYSICS
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  • 13
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    Cosmological velocity correlations - Observations and model predictions (1989)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: By applying the present simple statistics for two-point cosmological peculiar velocity-correlation measurements to the actual data sets of the Local Supercluster spiral galaxy of Aaronson et al. (1982) and the elliptical galaxy sample of Burstein et al. (1987), as well as to the velocity field predicted by the distribution of IRAS galaxies, a coherence length of 1100-1600 km/sec is obtained. Coherence length is defined as that separation at which the correlations drop to half their zero-lag value. These results are compared with predictions from two models of large-scale structure formation: that of cold dark matter and that of baryon isocurvature proposed by Peebles (1980). N-body simulations of these models are performed to check the linear theory predictions and measure sampling fluctuations.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 344; 1-19
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  • 14
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    Research in particle and gamma-ray astrophysics (1988)
    In:  CASI
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    Publication Date: 2019-07-13
    Description: Research activities in cosmic rays, gamma rays, and astrophysical plasmas are covered. Each activity is described, followed by a bibliography. The research program is directed toward the investigation of the astrophysical aspects of cosmic rays and gamma rays and of the radiation and electromagnetic field environment of the earth and other planets. These investigations were performed by means of energetic particle and photon detector systems flown on spacecraft and balloons.
    Keywords: ASTROPHYSICS
    Type: NASA-CR-183126 , NAS 1.26:183126
    Format: application/pdf
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  • 15
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    Research in particles and fields (1987)
    In:  CASI
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    Publication Date: 2019-07-13
    Description: The astrophysical aspects of cosmic rays and gamma rays and the radiation and electromagnetic field environment of the Earth and other planets are investigated. These investigations are carried out by means of energetic particle and photon detector systems flown on spacecraft and balloons. Particle astrophysics is directed toward the investigation of galactic, solar, interplanetary, and planetary energetic particles and plasmas. The emphasis is on precision measurements with high resolution in charge, mass, and energy. Gamma ray research is directed toward the investigation of galactic, extragalactic, and solar gamma rays with spectrometers of high angular resolution and moderate energy resolution carried on spacecraft and balloons.
    Keywords: ASTROPHYSICS
    Type: NASA-CR-180173 , NAS 1.26:180173
    Format: application/pdf
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  • 16
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    H2 spectroscopy as an agent for extinction determinations The near-infrared curve for the Orion molecular cloud (1986)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A near-infrared (1.8 to 3.5) microns extinction curve for the Orion molecular cloud is presented. The curve is derived from high-resolution spectra of the Orion H2 source recorded from the Kuiper Airborne Observatory. The data reveal that the Orion extinction law is indistinguishable from a 1/lambda form in the near-infrared, except for strongly enhanced extinction near a wavelength of about 3 microns. The implications of these results, in the context of current interstellar grain models, are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 304; 481-489
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  • 17
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    Massive superclusters as a probe of the nature and amplitude of primordial density fluctuations (1985)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: It is pointed out that correlation studies of galaxy positions have been widely used in the search for information about the large-scale matter distribution. The study of rare condensations on large scales provides an approach to extend the existing knowledge of large-scale structure into the weakly clustered regime. Shane (1975) provides a description of several apparent massive condensations within the Shane-Wirtanen catalog, taking into account the Serpens-Virgo cloud and the Corona cloud. In the present study, a description is given of a model for estimating the frequency of condensations which evolve from initially Gaussian fluctuations. This model is applied to the Corona cloud to estimate its 'rareness' and thereby estimate the rms density contrast on this mass scale. An attempt is made to find a conflict between the density fluctuations derived from the Corona cloud and independent constraints. A comparison is conducted of the estimate and the density fluctuations predicted to arise in a universe dominated by cold dark matter.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 297; 365-370
    Format: text
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  • 18
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    Interstellar absorption features toward the compact infrared source W33A (1985)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: A high-resolution (lambda/Delta lambda of approximately 2000) broad-bandwidth (3.3-5.0 microns) spectrum of the compact infrared source W33A is presented. Four absorption features are associated with compositionally distinct components in the interstellar medium: solid-phase C3, CN, or CH3OH at 4.9 microns, solid-phase CO and either solid or gaseous CH3NC near 4.6 microns and the well-known dirty ice feature near 3 microns. The analysis of this spectrum supports chemically complex models of grain mantle evolution in cold, quiescent molecular clouds.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 299; 873-880
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  • 19
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    Post-perihelion observations of water in comet Halley (1986)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: An ambitious postperihelion observational program has been carried out which significantly extends the scope of the Halley water investigation. The spectral bandwidth of the postperihelion data included essentially all of the nu3 band. Fifteen spectral lines of water were detected, including three previously undetected lines in the nu3 band and three lines in the (011-010) band. The observed relative intensities illustrate that water in the cometary coma is rotationally relaxed, as predicted by recent nonthermal equilibrium models. An ortho/para ratio of about three is consistent with both pre- and postperihelion data. The water spatial brightness profile can be fit by that of a parent molecule, and a significant sunward-tailward brightness asymmetry was observed which suggests ejection primarily into the sunlit hemisphere. Comet Halley exhibited marked temporal activity on time scales as short as two hours. The largest water production rate measured was 2.3 x 10 to the 30th mol/s on 24:18 UT in March 1986, which is about an order of magnitude or more larger than the production rates measured preperihelion at the same heliocentric distance, indicating a large pre- to postperihelion asymmetry in gas production.
    Keywords: ASTROPHYSICS
    Type: Nature (ISSN 0028-0836); 324; 441-444
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  • 20
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    Velocity-resolved observations of water in comet Halley (1986)
    In:  Other Sources
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    Publication Date: 2019-07-12
    Description: High-resolution observations of H2O in comet Halley are presented. The positions and widths of the H2O emission line profiles are indicative of the coma's velocity field characteristics. Spectral line shape changes are used to infer temporal variations in the kinematic properties of the outflow. The results suggest that H2O is released into the coma by way of multiple jets.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 309; L95-L99
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