Showing papers in "The Astrophysical Journal in 1985"

The evolution of large-scale structure in a universe dominated by cold dark matter

Abstract: The results of numerical simulations of nonlinear gravitational clustering in universes dominated by weakly interacting, 'cold' dark matter are presented. The numerical methods used and the way in which initial conditions were generated are described, and the simulations performed are catalogued. The evolution of the fundamental statistical properties of the models is described and their comparability with observation is discussed. Graphical comparisons of these open models with the observed galaxy distribution in a large redshift survey are made. It is concluded that a model with a cosmological density parameter omega equal to one is quite unacceptable if galaxies trace the mass distribution, and that models with omega of roughly two, while better, still do not provide a fully acceptable match with observation. Finally, a situation in which galaxy formation is suppressed except in sufficiently dense regions is modelled which leads to models which can agree with observation quite well even for omega equal to one.

Photodissociation regions. I - Basic model. II - A model for the Orion photodissociation region

Abstract: A theoretical parameter study of the temperature and chemical structure of dense photodissociation regions and their resultant spectrum is presented. Models are discussed which are relevant not only to the dust and gas between molecular clouds and H II regions, but also apply to any neutral cloud illuminated by intense FUV fluxes. The models relate observed line and continuum emission from these regions to physical parameters such as the gas density and temperature, the elemental and chemical abundances, the local radiation field, and the grain properties. The results are applied to observational data from the OMC-1 region. The model shows that the observed high brightness temperature of the C I 609 microns line can be explained by emission from the C(+)/C/CO transition region. This difference with previous chemical models is due to a higher gas phase elemental abundance of carbon, to the charge exchange reactions of C(+) with S and SiO, and to carbon self-shielding.

Models for high-frequency radio outbursts in extragalactic sources, with application to the early 1983 millimeter-to-infrared flare of 3C 273.

Abstract: The present models for compact radio source variability, with reference to the early 1983 mm-to-IR flare of the quasar 3C 273, indicate that the outburst spectrum's early evolution is most easily explained if the flaring component is expanding. The models encompass the effects of synchrotron, Compton, and expansion losses, as well as variable injection of relativistic electrons and magnetic field. A model based on a uniform expanding source requires comparatively artificial variations of particle injection with source radius, in order to explain the 3C 273 flare data; superior results are noted for a second model in which the outburst is due to a shock wave passing through an adiabatic, conical, relativistic jet. 55 references.

Polycyclic aromatic hydrocarbons and the unidentified infrared emission bands - Auto exhaust along the Milky Way

Abstract: The unidentified infrared emission features (UIR bands) are attributed to a collection of partially hydrogenated, positively charged polycyclic aromatic hydrocarbons (PAHs). This assignment is based on a spectroscopic analysis of the UIR bands. Comparison of the observed interstellar 6.2 and 7.7-micron bands with the laboratory measured Raman spectrum of a collection of carbon-based particulates (auto exhaust) shows a very good agreement, supporting this identification. The infrared emission is due to relaxation from highly vibrationally and electronically excited states. The excitation is probably caused by UV photon absorption. The infrared fluorescence of one particular, highly vibrationally excited PAH (chrysene) is modeled. In this analysis the species is treated as a molecule rather than bulk material and the non-thermodynamic equilibrium nature of the emission is fully taken into account. From a comparison of the observed ratio of the 3.3 to 11.3-micron UIR bands with the model calculations, the average number of carbon atoms per molecule is estimated to be about 20. The abundance of interstellar PAHs is calculated to be about 2 x 10 to the -7th with respect to hydrogen.

Thermal infrared and nonthermal radio: remarkable correlation in disks of galaxies

Abstract: A tight, linear correlation is established between the far-infrared flux measured by IRAS and the nonthermal radio flux density (at 1.4 GHz) from disks of spiral galaxies. This correlation defines a ratio of infrared to radio fluxes that is characteristic of star formation activity. Galaxies with nuclear starbursts seem to follow the correlation. If the far-infrared is reradiated luminosity from young massive stars, then the supernova remnants alone account for less than 10 percent of the radio emission. These results indicate a close coupling between dust heating and cosmic-ray generation and confinement in a wide range of conditions.

The spectra of narrow-line Seyfert 1 galaxies.

Abstract: Measurements are presented of a group of active galactic nuclei with all the properties of Seyfert 1 or 1.5 galaxies, but with unusually narrow H I lines. They include Mrk 42, 359, and 1239 (previously studied by other authors) as well as Mrk 493, 766, 783, and 1126. One other somewhat similar object, Mrk 1388, is also included in the discussion; measurements of its spectrum have been published previously. These narrow-line Seyfert 1 galaxies show a wide variety of deviations from the properties of typical Seyfert 1 objects. They clearly demonstrate that the Seyfert phenomenon is not a simple one-parameter effect. 29 references.

Statistical methods for astronomical data with upper limits. I - Univariate distributions

Abstract: The statistical treatment of univariate censored data is discussed. A heuristic derivation of the Kaplan-Meier maximum-likelihood estimator from first principles is presented which results in an expression amenable to analytic error analysis. Methods for comparing two or more censored samples are given along with simple computational examples, stressing the fact that most astronomical problems involve upper limits while the standard mathematical methods require lower limits. The application of univariate survival analysis to six data sets in the recent astrophysical literature is described, and various aspects of the use of survival analysis in astronomy, such as the limitations of various two-sample tests and the role of parametric modelling, are discussed.

Black hole normal modes - A semianalytic approach

Abstract: A new semianalytic technique for determining the complex normal mode frequencies of black holes is presented. The method is based on the WKB approximation. It yields a simple analytic formula that gives the real and imaginary parts of the frequency in terms of the parameters of the black hole and of the field whose perturbation is under study, and in terms of the quantity (n + 1/2), where n = 0, 1, 2,... and labels the fundamental mode, first overtone mode, and so on. In the case of the fundamental gravitational normal modes of the Schwarzschild black hole, the WKB estimates agree with numerical results to better than 7 percent in the real part of the frequency and 0.7 percent in the imaginary part, with the relative agreement improving with increasing angular harmonic. Carried to higher order the method may provide an accurate and systematic means to study black hole normal modes.

The globular cluster system of the galaxy. IV - The halo and disk subsystems

Abstract: On a examine les distributions spatiales, la cinematique et les metallicites de 121 amas globulaires en vue de definir les sous-populations distinctes du systeme des amas. On a compare les proprietes des amas globulaires a celles d'echantillons d'etoiles variables RR Lyrae et de sous-naines

Neutron Stars Are Giant Hypernuclei

Abstract: Neutron stars are studied in the framework of Lagrangian field theory of interacting nucleons, hyperons, and mesons, which is solved in the mean field approximation The theory is constrained to account for the four bulk properties of nuclear matter; the saturation binding and density, compressibility, and charge symmetry energy The cores of the heavier neutron stars are found to be dominated by hyperons, and the total hyperon population for such stars is 15%--20%, depending on whether pions condense or not The rho-meson, which contributes to the isospin symmetry energy, has an important influence on the baryon populations Lepton populations are strongly suppressed and charge neutrality is achieved among the hadrons A possible consequence for the decay time of the magnetic field of pulsars and hence for their active lifetime is mentioned

Gravitational lensing effects of vacuum strings - Exact solutions

Abstract: Exact interior and exterior solutions to Einstein's field equations are derived for vacuum strings. The exterior solution for a uniform density vacuum string corresponds to a conical space while the interior solution is that of a spherical cap. For Mu equals 0-1/4 the external metric is ds-squared = -dt-squared + dr-squared + (1-4 Mu)-squared r-squared dphi-squared + dz-squared, where Mu is the mass per unit length in the string in Planck masses per Planck length. A maximum mass per unit length for a string is 6.73 x 10 to the 27th g/cm. It is shown that strings cause temperature fluctuations in the cosmic microwave background and produce equal brightness double QSO images separated by up to several minutes of arc. Formulae for lensing probabilities, image splittings, and time delays are derived for strings in a realistic cosmological setting. String searches using ST, the VLA, and the COBE satellite are discussed.

Rotation velocities of 16 SA galaxies and a comparison of Sa, SB and SC rotation properties.

Abstract: Rotational velocities over most of the optical extent of 54 Sa, Sb, and Sc galaxies have been determined. The Sa curves exhibit a similar progression with luminosity as do the Sb and Sc galaxies, and the forms of the rotation curves exhibited by Sa's are markedly similar to those found previously for Sb and Sc galaxies. The overall similarity of forms of rotation curves for spirals of very different morphologies, coupled with the derived values for the dynamical mass-to-luminosity ratios, implies that both the dark halo mass and the disk mass contribute to the total mass distribution at all radii within the optical galaxy. While the approximately four mag range is similar within each Hubble type, the values of V(max) increase with earlier Hubble type. Absolute blue magnitude and absolute infrared magnitude are correlated for all Hubble types. The luminosity-log V(max) correlation has a slope 10 + or - 2 for each Hubble type, but with zero points displaced for each type.

Hot coronae around early type galaxies

Abstract: The study of gas in early type galaxies began at least as early as 1957 when SANDAGE [1] estimated that during the course of normal stellar evolution, the stars of a bright elliptical galaxy would shed ~5×109 M ⊙ of gas. Over the last 25 years, limits on various forms of gas in early type galaxies became increasingly stringent. Models for a galactic wind, like that of MATHEWS and BAKER [2], were developed to explain the absence of gas. However NORMAN and SILK [3] and SARAZIN [4] argued that galaxies with massive halos could maintain gaseous coronae over cosmological times and could overcome the heating from supernovae.

Distribution of dark matter in the spiral galaxy ngc-3198

Abstract: Two-component mass models, consisting of an exponential disk and a spherical halo, are constructed to fit a newly determined rotation curve of NGC 3198 that extends to 11 disk scale lengths. The amount of dark matter inside the last point of the rotation curve, at 30 kpc, is at least 4 times larger than the amount of visible matter, with M/L(B)tot = 18 solar M/L(B). The maximum mass-to-light ratio for the disk is M/L(B) = 3.6. The available data cannot discriminate between disk models with low M/L and high M/L, but arguments are presented which suggest that the true mass-to-light ratio of the disk is close to the maximum computed value. The core radius of the distribution of dark matter is found to satisfy R(core) of between 1.7 and 12.5 kpc. 31 references.

Capture by the sun of a galactic population of weakly interacting, massive particles

Abstract: A previous calculation showed that hypothetical, massive particles, if present in the Sun with the right scattering cross section and in a concentration as low as 10 S by number, could likely explain the solar neutrino discrepancy. We here suppose that the same hypothetical particles make up the missing dynamical mass in the Galaxy, providing a mass density between 0.01 and 0.1 M/sub sun/ pc T and with a velocity dispersion of approx.30 (disk) or 300 (halo) km s . We compute the number of particles which would be captured by the Sun and into bound orbits. For scattering cross sections not too far from the value of 4 x 10 TW cmS, which is optimal for suppressing solar neutrinos, we find that just the right number can be captured. The solutions to the neutrino problem in the Sun and the missing mass problem in the Galaxy may thus be one and the same.

Families of ellipsoidal stellar systems and the formation of dwarf elliptical galaxies

Abstract: Core radii and central surface brightnesses of bulges and elliptical galaxies are measured using CCD photometry obtained with the Canada-France-Hawaii Telescope (scale = 0''.22 pixel/sup -1/; seeing = 0''.45--1''.0 FWHM). The correlations between core parameters are derived and compared for ellipticals, bulges, dwarf spheroidal galaxies, dwarf irregular galaxies, and globular clusters. The results are as follows. 1. Ihe data confirm the existence of well-defined correlations between the core parameters of elliptical galaxies. More luminous ellipticals have larger core radii r/sub c/ and lower central surface brightnesses ..mu../sub 0v/. Galaxies with larger core radii have larger central velocity dispersions. The small, bright core of M32 is normal for a galaxy of M/sub B/ = -15.2. Radio ellipticals and brightest cluster galaxies satisfy the correlations. 2. The bulges of disk galaxies are basically similar to elliptical galaxies. Their cores have slightly smaller r/sub c/ and brighter ..mu../sub 0v/ than ellipticals of the same luminosity, because their nonisothermal profiles rise more rapidly toward the center and because they often contain extra nuclei superposed on their cores. 3. There is a large discontinuity between the parameter correlations for bright ellipticals, including M32, and those for dwarf spheroidals. Seven dE's in the Local Group andmore » three in the Virgo Cluster have core parameters which are correlated, but not as in ordinary ellipticals. More luminous dE's have larger r/sub c/ and brighter ..mu../sub 0v/.« less

Stellar weak interaction rates for intermediate-mass nuclei. IV - Interpolation procedures for rapidly varying lepton capture rates using effective log (ft)-values

Abstract: Simple expressions for continuum electron and positron capture phase space factors and the associated neutrino energy loss integrals are presented in terms of standard Fermi integrals. Continuous approximations to the relevant Fermi integrals and their first derivatives are made. These allow the computation of effective log (ft)--values, at each temperature and density point, for the continuum lepton capture rates considered in the earlier papers in this series. Since the effective log (ft)--values have most of the rapid temperature and density dependence associated with the phase space integrals removed, interpolation in temperature and density to obtain stellar rates is greatly facilitated in speed and accuracy. Computer simulations of stellar evolution will be able to implement more accurately our calculations of the stellar nuclear weak interaction rates of intermediate-mass nuclei. Generalization of the Fermi integral expressions for the lepton continuum capture phase space factors are given for astrophysical environments where there exists an equilibrium distribution of electron-type neutrinos. These allow rough estimates of the effect of neutrino blocking on our tabulated rates and estimates of total neutrino capture rates.

X-ray secondary heating and ionization in quasar emission-line clouds

Abstract: Accurate Monte Carlo computations of the X-ray secondary electron heating, ionization, and excitation of H and He gas in interstellar space and in quasar emission-line clouds, are presented. The fraction of energy deposited in each form is sensitive to the background ionization fraction, x = n(H+)/n(Htot), and can affect the temperature, ionization state, and line emissivities at large depths in X-ray photoionized clouds. Analytic fits are provided for these energy fractions over the range 0.0001-1 for primary electron energies up to many keV. In both broad-line and narrow-line clouds, emission lines sensitive to the energy budget and electron density may be strongly affected.

Shock acceleration of electrons and ions in solar flares

Abstract: The simultaneous first-order Fermi shock acceleration of electrons, protons, and alpha particles are compared to observations of solar energetic particle events. For each event, a unique shock compression ratio in the range of approximately 1.6-3 produces spectra in good agreement with observation. The range in compression ratios predicts that the more than five orders of magnitude spread in electron-to-proton intensity ratios observed at MeV energies is compressed to about three orders of magnitude at an assumed injection energy of 100 keV. The remaining spread can be accounted for with a modest range of injection conditions. The model predicts that the acceleration time to a given energy will be approximately equal for electrons and protons, and for reasonable solar parameters, can be on the order of 1 s to approximately 100 MeV. 37 references.

Semidynamical models of radio jets: relativistic beaming and source counts.

Abstract: VLBI observations of compact radio sources exhibit core-jet structure and apparent superluminal expansion. Both features can be produced by a relativistic jet of radio-emitting plasma beamed toward the observer. This model implies that there should be several fainter unbeamed sources for every beamed source. Partial luminosity functions (the probability distribution functions for the observed flux from a source at a given distance viewed from different directions) are calculated for different relativistic jet models. Kinematical models that approximate relativistic shock fronts are emphasized. These differ from the kinematically simpler models which are usually invoked, in that the velocity of the emitting fluid, which is responsible for the Doppler boosting, is distinct from the velocity of the pattern, which is responsible for the superluminal motion. Numerical calculations of the emission from semidynamical shock models demonstrate that a wide range of partial luminosity functions can be produced. It is concluded that source counts cannot be used as precise probes of relativistic beaming and are of limited use in testing the beaming hypothesis. 27 references.

On model-dependent bounds on H 0 from gravitational images : application to Q 0957+561 A, B.

Abstract: On considere les images d'une source de l'arriere plan formees par une distribution de masse de premier plan dans une cosmologie de Friedmann dominee par la matiere On presente une transformation qui laisse invariantes les proprietes observables de ces images gravitationnelles En utilisant une distribution de masse composee pour representer une lentille galaxie-amas, on peut rendre compte des positions et des grossissements relatifs des images A et B de Q0957+561 On en deduit des valeurs limites pour la constante de Hubble

Gas deficiency in cluster galaxies: a comparison of nine clusters

Abstract: The available 21 cm line data in the literature for galaxies in nine clusters is combined with new high-sensitivity observations of 51 galaxies in five of the nine clusters in order to test for discriminating circumstances between those clusters which show H I deficiency among their spiral population and those which do not. An H I deficiency for the complete cluster sample is derived employing a comparison sample of galaxies chosen from the Catalog of Isolated Galaxies. The deficiency and its radial dependence is summarized for each cluster and a composite. A comparison of the environments in different clusters leads to the conclusion that the occurrence of H I deficiency is correlated with the presence of a hot X-ray intracluster medium, and that an ongoing interaction process is active through the cores of X-ray clusters.

Broad emission features in QSOs and active galactic nuclei. II - New observations and theory of Fe II and H I emission

Abstract: Seven QSOs of low to intermediate redshift are characterized on the basis of: 320-850-nm spectra obtained with the UVITS spectrograph and image-dissector scanner at the f/9 Cassegrain focus of the 2.7-m reflecting telescope at McDonald Observatory; improved model calculations for the Fe II, Mg II, and Balmer lines and the Balmer continuum; and the theoretical results of Netzer and Wills (1983). The data are presented in tables, spectra, and difference spectra and described in detail. Fe II is found to be the largest contributor to the emission line spectrum, with strong UV Fe II lines in all QSOs (regardless of the strength of the optical Fe II lines) and an abundance ratio suggesting a factor-of-three overabundance of Fe relative to Mg and probably H. A discrepancy between the observed Fe II strength and the predictions of dustless standard photoionization models is noted, and the addition of a high-density component to the models is suggested.

A theory for the origin of globular clusters

Abstract: In the present theory for the origin of globular clusters during protogalaxy collapse, a thermal instability promotes the development of a two-phase structure in the gas in those cases where the cooling time is comparable to that of free fall. The hot component, which remains near the virial temperature, compresses the cold component into discrete clouds with temperatures near 10,000 K and mean densities of 1 to 10 solar masses/cu pc. These predictions are compared with the observed properties of globular clusters; satisfactory agreement is obtained. 57 references.