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  • ASTROPHYSICS  (44)
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  • 11
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    Galaxy formation in an intergalactic medium dominated by explosions (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The evolution of galaxies in an intergalactic medium dominated by explosions of star systems is considered analogously to star formation by nonlinearly interacting processes in the interstellar medium. Conditions for the existence of a hydrodynamic instability by which galaxy formation leads to more galaxy formation due to the propagation of the energy released at the death of massive stars are examined, and it is shown that such an explosive amplification is possible at redshifts less than about 5 and stellar system masses between 10 to the 8th and 10 to the 12th solar masses. Explosions before a redshift of about 5 are found to lead primarily to the formation of massive stars rather than galaxies, while those at a redshift close to 5 will result in objects of normal galactic scale. The model also predicts a dusty interstellar medium preventing the detection of objects of redshift greater than 3, numbers and luminosities of protogalaxies comparable to present observations, unvirialized groups of galaxies lying on two-dimensional surfaces, and a significant number of black holes in the mass range 1000-10,000 solar masses.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 243
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  • 12
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    X-ray emission from M87 - A pressure confined cooling atmosphere surrounding a low mass galaxy (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: A conventional model for the mass of the galaxy M87 (500-billion solar masses) is used to show that a radiative cooling flow agrees well with spectroscopically derived mass flow rates (10 solar masses per year) and surface brightness profiles of X-ray emission around the galaxy. A temperature of 100-million K, and a density of a few x 10 to the -4th/cu cm are obtained without the use of a massive halo. The atmosphere contains 1-trillion solar masses of gas, with a temperature profile increasing outward and gas acting as a reservoir for the flow. The present models indicate that thermal conductivity in M87 is less than 0.002 of its classical value, and does not significantly affect properties of the surrounding gas.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 247
    Format: text
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  • 13
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    The contribution of young galaxies to the X-ray background (1980)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The contribution of young galaxies to the diffuse X-ray background is estimated and two mechanisms by which young galaxies may emit hard X-rays are considered. The first mechanism is direct thermal bremsstrahlung emission from a hot galactic wind powered by supernovae. Estimating mass-loss rates from galaxies by arguments based on metallicity and on the X-ray spectroscopic observations of rich clusters, it is shown that galactic winds may contribute substantially to the background. The second mechanism relies on the decreased metallicity of young galaxies to increase the number of supergiant stars and hence the numbers of hard X-ray binaries. This effect quantitatively explains the relatively large X-ray luminosity of the Magellanic clouds and also indicates that binaries in young galaxies may contribute a significant fraction of the diffuse X-ray background. Both mechanisms require that the epoch of galaxy formation be recent (redshift less than 2-3) in order to account for the observed spectral temperature (45 keV) of the hard X-ray background.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 237
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  • 14
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    A search for expanding supershells of gas around OB associations (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The results of a search for giant, rapidly expanding shells of gas surrounding OB associations are recorded. Two out of thirteen nearby associations (Orion and Carina) show supershells of gas with radii of about 100 pc, expansion velocities of about 100 km/sec, and ages of about 400,000 years. Observations of the newly discovered Carina supershell are presented. It is suggested that these features are formed by supernova explosions in the associations. The birthrate of association supernovae than lies between 1.5 x 10 to the -14th/cu pc per/year and 10 to the -13th/cu pc per/year.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 250
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  • 15
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    On the origin and distribution of C IV and Si IV ions in the neighboring interstellar medium (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: IUE spectrometer observational data are reported on the C IV, Si IV, and N V absorption lines in 46 distant stars at FWHM resolution of 30 km/sec. A simple analytic theory is given for these contributions, including the effects of ionization by a soft X-ray background. While for some of the most distant field stars a photoionized contribution may be ruled out, C IV absorption lines are detected which are shallow and broad. It is suggested that what are being detected in these cases are contributions from the hot interstellar gas that is responsible for the O VI absorption lines. Most of the Si IV and C IV lines probably arise in the H II regions surrounding the observed star or its neighbors.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 248
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  • 16
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    The distribution and morphology of X-ray-emitting gas in the core of the Perseus cluster (1981)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: An unresolved source has been found coincident with the nucleus of NGC 1275 in a high-resolution X-ray image of the core of the Perseus cluster. Absorption in the optical features at high velocity with respect to NGC 1275, which are thought to be associated with a foreground galaxy, does not produce any detectable X-ray absorption. The emission tends to become asymmetric in the presence of the lower-velocity filaments, but no obvious, detailed correlation is found between X-ray enhancements and individual filaments. Deprojection of surface brightness to yield temperature and density profiles of the intracluster gas shows results consistent with a quasi-hydrostatic radiative accretion flow onto NGC 1275, and the pressure-driven mass inflow onto the central galaxy is then 200-400 solar masses/yr. The possibility of a problem in the relation of line-of-sight velocity dispersion to cluster gravitational mass is confirmed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 248
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  • 17
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    Two-dimensional spectrophotometry of the cores of X-ray luminous clusters (1983)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The results of a two-dimensional spectrophotometric survey of the core regions of 11 rich clusters of galaxies are presented. A number of these clusters have spectacular optical emission line systems in their cores. Both morphologically and kinematically, the emission line regions divide into extended, 20-100 kpc systems of long linear filaments associated with the cluster core and more compact, homogeneous elongated regions associated with the dominant central cluster galaxy. It is suggested that the present results can be expected, as hot X-ray emitting gas cools in the cluster center. Luminosities almost entirely agree with expected values. The morphology of the systems can be understood if the filaments form initially in the cooling flow and, in some cases, are subsequently accreted by the central galaxy.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 272; Sept. 1
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  • 18
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    Optical absorption from the high-velocity neutral hydrogen complex C in the spectrum of the RR Lyrae star BT Draconis (1985)
    In:  Other Sources
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    Publication Date: 2019-06-28
    Description: The detection of interstellar Na D absorption lines at a local standard of rest (LSR) velocity of -85 km/s is reported, together with tentative detections of further Na D lines at -133 km/s and -110 km/s, in the spectrum of the RR Lyrae star BT Draconis which lies toward the high-velocity neutral hydrogen complex C (LSR velocity around -115 km/s). This suggests an upper limit of 2.1 kpc on the distance to this high-velocity complex, and indicates that the complex has near-cosmic metallicity. This result strongly favors either a model in which complex C is formed by infalling material in the halo, which has originated in the Galactic disk, or one in which it is part of a large relatively local shell structure.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 2 - Letters to the Editor (ISSN 0004-637X); 293; L15-L18
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  • 19
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    Orion's Cloak - A rapidly expanding shell of gas centered on the Orion OB1 association (1979)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The structure of the interstellar gas surrounding the Orion OB1 association and the neighboring lambda Orionis association is detailed. UV absorption lime spectra of various ionization stages of C, N, Si and S in the directions of 12 stars were obtained by means of the spectrometer on board the Copernicus satellite. The presence of a shell of material surrounding the two associations and expanding at 100 to 120 km/sec, designated Orion's Cloak, was revealed, together with sporadically occurring higher column density matter at lower velocities. Results are interpreted to indicate the presence of a rapidly moving radiative shock outside the H II region of the association stars and inside this feature, a lower velocity, higher column density cloud which appears to be directly ionized by association stars. It is suggested that the gas features are caused by the effects of a recent supernova and of multiple supernovae, stellar winds and rocket-accelerated clouds in addition to stellar ionization.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 230
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  • 20
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    Radiative regulation of gas flow within clusters of galaxies - A model for cluster X-ray sources (1977)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 215
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