Showing papers on "Active galactic nucleus published in 1989"

Astrophysics of Gaseous Nebulae and Active Galactic Nuclei

Abstract: Photoionization equilibrium thermal equilibrium calculation of emitted spectrum comparison of theory with observations internal dynamics of gaseous nebulae interstellar dust H II regions in the galactic context planetary nebulae nova and supernova remnants active galactic nuclei - diagnostic and physics active galactic nuclei - results.

Bars within bars - A mechanism for fuelling active galactic nuclei

Abstract: A mechanism, applicable to AGN and nuclear starburst galaxies in which there is accretion onto a supermassive black hole (SBH), is proposed which brings in gas from large to small scales by successive dynamical instabilities. On the large scale, a stellar bar sweeps the interstellar medium into a gaseous disk a few hundred pc in radius. Under certain conditions, this disk can become unstable again, allowing material to flow inwards until turbulent viscous processes control angular-momentum transport. This flow pattern may feed viscosity-driven accretion flows around an SBH or lead to the formation of an SBH if none was present initially.

The tidal disruption of a star by a massive black hole

Abstract: Results are reported from a three-dimensional numerical calculation of the tidal disruption of a low-mass main-sequence star on a parabolic orbit around a massive black hole (Mh = 10 to the 6th stellar mass). The postdisruption evolution is followed until hydrodynamic forces becomes negligible and the liberated gas becomes ballistic. Also given is the rate at which bound mass returns to pericenter after orbiting the hole once. The processes that determine the time scale to circularize the debris orbits and allow an accretion torus to form are discussed. This time scale and the time scales for radiative cooling and accretion inflow determine the onset and duration of the subsequent flare in the AGN luminosity.

Tidal triggering of starbursts and nuclear activity in galaxies

Abstract: Gas distributed throughout a galaxy responds strongly to the tidal field of a companion during a merger. In some cases dynamical instability will drive a large fraction of the gas into the inner regions of the galaxy. A strong burst of star formation will follow and subsequent evolution may lead to the formation of a black hole. Continued accretion of gas by the black hole may provide sufficient power to explain quasars and nuclear activity in otherwise normal galaxies.

The EXOSAT spectral survey of AGN

Abstract: Results are presented from Exosat observations of 48 hard X-ray selected Seyfert-type active galactic nuclei (AGN). These include all 30 of the emission line AGN in the Piccinotti (1981) sample. Combining Exosat LE and ME data has yielded X-ray spectra over the broad energy range 0.1-10 keV. Spectra in the about 2-10 keV range are found to be well described by a simple power law, with a narrow distribution of spectral indices across the sample about a mean energy index alpha = 0.70. Exosat has also revealed a substantial number of sources with complex soft X-ray spectra. Evidence that soft emission components occur in many Seyferts, together with detection of rapid variability in the soft component, provides a quantitative support for an accretion disk model for AGN. Approximately half of the present sample of AGN show low-energy absorption attributable to substantial cold matter within the host galaxy. A few cases show evidence for column variability and reduced low-energy opacity (by photo-ionization). These results and the observed rarity of intrinsic absorption in the higher luminosity sources suggest the absorbing matter lies close to the central continuum source.

Manifestations of a massive black hole in the galactic center

Abstract: A young star cluster is a less contrived explanation than a massive black hole for many of the features seen in the Galactic center. However from a Copernican point of view, this explanation is less attractive than a black hole. The evidence for a ~ 106M⊙ black hole is becoming progressively less convincing, but the case against it is no stronger. We describe the development of a singular star cluster, as well as the processes of stellar disruption, merging, and gas accretion in such a cluster. Recently merged stars and tidally stripped giants may be detectable within an arcminute of the Galactic Center. We examine the physics of star formation in the inner parsecs of the galaxy, and the problem of maintaining the two parsec molecular torus.

Fitting improved accretion disk models to the multiwavelength continua of quasars and active galactic nuclei

Abstract: Fits to the ultraviolet-optical-infrared spectra of 60 quasars and active galactic nuclei (AGNs) are investigated, using black hole accretion disk models. The disk is assumed to be geometrically thin and optically thick. The observed spectrum from a black hole accretion disk, rotating (Kerr) or nonrotating (Schwarzschild), is transformed by the geometric inclination effect, Doppler shifts, gravitational redshifts, and gravitational focusing. These effects alter the locally emitted fluxes and produce harder spectra for an observer at higher inclination angles. The relativistic corrections are much more significant in the Kerr geometry since the inner edge of the disk is much closer to the rotating black hole. A simple inverse relation between the inferred black hole mass and the assumed inclination angle was found in the Kerr case, while the inferred accretion rate (solar masses/yr) remains independent of the angle. In the Schwarzschild case, the ratio of the inferred accretion rate to the black hole mass remains roughly constant when the viewing angle changes. In both geometries, low-redshift Seyfert galaxies have relatively low accretion rates, only a few percent of their Eddington luminosities, while the most luminous quasars are accreting near their Eddington limits.

Structure of line-emitting accretion disks in active galactic nuclei - Arp 102B

Abstract: The prime objects of the present self-consistent model of a line-emitting accretion disk able to account for the properties observed in a small class of AGNs are Arp 102B and 3C 390.3, whose double-peaked emission lines have been attributed to a Keplerian disk. Improved calculations of the line profile of a relativistic Keplerian disk, generalized to include a variety of emissivity laws as well as local broadening due to electron scattering or turbulence, are noted to fit Arp 102B; analytic and numerical calculations of the solid angle presented by the outer thin disk to an extended isotropic source of illumination demonstrate that the energy budget requirements for line emission from the disk are also satisfied.

Accurate galactic N(H) values towards quasars and AGN

Abstract: Integrated Galactic 21-cm column densities toward about 174 quasars and AGN are presented. The data are corrected for stray radiation, using the technique of Lockman et al. (1986). Because of the 21-arcmin beam size of the 140-ft telescope used in the study, the uncertainty in N(H) due to angular variations in the H I of the Galaxy at high latitudes is minimized. The column densities are accurate to about 1 X 10 to the 19th atoms/sq cm, or 5 percent, whichever is larger. Errors above N(H) of about 4 X 10 to the 20th atoms/sq cm are dominated by opacity uncertainties.

Theory of accretion disks

Abstract: Conference Photograph.- Preface.- List of Participants.- Welcoming Address.- Observational Evidence for Accretion Disks in Galactic Nuclei.- The Low-Frequency Spectra of Accretion Disks in Active Galactic Nuclei: The Dark Side of the Big Blue Bump.- Observational Constraints on Viscosity in the AGN Accretion Discs.- Accretion Disk Magnetohydrodynamics and the Origin of Jets.- Formation of Relativistic MHD Jets in the Magnetosphere of Accretion Disks.- Nonsteady MHD Jets from Magnetized Accretion Disks - Sweeping-Magnetic-Twist Mechanism -.- Statistical Modelling of IR/UV Spectra in AGN.- Formation and Evolution of the Solar Nebula.- Angular Momentum Transport in Protostellar Disks.- Protostellar Discs.- Formation of Viscous Protostellar Accretion Disks.- Formation and Structure of Protostellar Accretion Disks.- Merging White Dwarfs, Disk Formation and Type I Super-Novae.- Variability of Active Galactic Nuclei and Galactic QPO Sources: A Diagnosis.- Nonlinear Pulsation in the Transonic Region of Geometrically Thin Accretion Disks.- One-Armed Oscillations Of Disks and Their Applications.- Thermal and Tidal Instabilities in Accretion Disks of Dwarf Novae.- Hydrodynamic Simulation of Accretion Disks in Cataclysmic Variables.- Simulations of Accretion Flow in Close Binary Stars.- Viscous Evolution of Accretion Discs in the Quiescence of Dwarf Novae.- Black Hole Accretion Disc Instability and Soft X-Ray Transients.- Non-Axisymmetric Shear Instabilities in Thick Accretion Disks.- Instabilities of Accretion Flows Caused by the Interaction of Internal Gravity Modes.- Simulations of Three-Dimensional Slender Tori.- Accretion Disks in Low Mass X-Ray Binaries.- Reflected Accretion Disk Emission Lines in Cataclysmic Variables.- The Structure of the Boundary Layer of Accretion Disks in Cataclysmic Binaries.- The Dynamics of Twisted Accretion Discs.- A Free Accretion Disk in Sn 1987A?.- The Spectra of Relativistic Accretion Disks.- The Compact Object in SS433: Neutron Star or Black Hole?.- Physics of Accretion by Spiral Shock Waves.- Spiral Shocks in Accretion Disks: A Preliminary Numerical Study.- Mass Transfer by Tidally Induced Spiral Shocks in an Accretion Disc.- Accretion Disks and the Link Between an AGN and its Host Galaxy.- Large-Scale Accretion Flows in AGN.- The Disk Accretion of a Tidally Disrupted Star onto a Massive Black Hole.- Line Radiation from Stationary Accretion Disks.- Continuum Spectra of Accretion Discs.- Influence of Radiative Transfer on the Vertical Structure of Accretion Disks.- Dusty Disks and the Infrared Emission from AGN.

Evolution of self-gravitating accretion disks in active galactic nuclei

Abstract: The evolution of self-gravitating gaseous disks in active galactic nuclei on scales of about 10-1000 pc is investigated. Star formation is a plausible outcome of the Jeans instability operating in a disk which violates the criterion for local stability. Even a low efficiency of star formation would deplete the gaseous disk on a short time scale and create a flat stellar system. These systems can evolve (sphericalize) secularly by means of stellar encounters but this process appears to be too slow to be important. Such flattened stellar systems may be common in the circumnuclear regions of disk galaxies. Conventional viscosities are inefficient in building anew the accretion process even in a cosmological time. Strongly self-gravitating disks are unstable to global nonaxisymmetric modes, which can induce radial inflow of gas in a short dynamical time. The latter effect is studied in a separate paper.

Small Dense Broad-Line Regions in Active Nuclei

Abstract: Recent observational studies indicate that the broad-line region (BLR) of AGN may be much smaller than previously thought. One consequence is that the flux of ionizing radiation is considerably larger and the clouds may be denser than assumed in standard photoionization models for these objects. This paper attempts to investigate these questions by exploring realistic cloud distributions over a range of radii and densities. Some simple pressure laws are proposed and integrated over the radial distribution of clouds to obtain the contribution to the line and continuum emission for all parts of the BLR. Photoionization calculations are made at very high densities up to 10 to the 13th/cu cm, and the most important processes that are included are discussed in detail. Hydrogen and helium are approximated at about 100 level systems, three-body recombination contribution to all elements is included, and optical depth effects are fully taken account of. Line profiles are predicted. 87 refs.

Hot dust on the outskirts of the broad-line region in Fairall 9

Abstract: The Seyfert galaxy Fairall 9 (F9) has been observed in the far-UV and optical range with the IUE, and at 27 different epochs at J, H, K, and L. The UV continuum underwent dramatic variations, its intensity decreasing by a factor 33 from a maximum in 1978 to a deep minimum in mid-1984. The near-IR and optical fluxes changed by a factor of about three and in the same sense as the UV. The K and L emission are interpreted as thermal radiation from dust lying at about 1 lt-yr from the UV source. The Ly-alpha 1216, C IV 1550, and Mg II 2800 emission-line intensities also vary in the same sense as the UV continuum but with a lag of 155 + or - 45 days. This strongly suggests that the broad-line region (BLR) gas is photoionized and lies inside the dust shell. Strong evidence is presented that the Mg II 2800 line emissivity depends primarily on the density of soft X-ray photons, as postulated in optically thick photoionization models of the BLR. 59 references.

Extended Optical Line Emitting Gas in Powerful Radio Galaxies: What is the Radio Emission-Line Connection?

Abstract: The results of the emission-line and radio imaging survey of radio galaxies presented in Baum et al (1988) and Baum and Heckman (1989) are used here to look for statistical evidence of energetic and spatial relationships between the extended emission-line gas and the radio source in powerful radio galaxies It is found that radio luminosity correlates with the optical narrow emission-line luminosity over roughly four orders of magnitude in line luminosity and five in radio luminosity The total emission-line luminosity of the powerful radio galaxies in the sample is roughly half of the luminosity of the associated radio source There is a better correlation between the radio and narrow-line luminosity than between the radio and the total emission-line luminosities in radio galaxies Statistical evidence is found for a spatial relationship between the very extended emission-line gas and the radio source 88 references

Radio studies of the Galactic center. I. The Sagittarius A complex

Abstract: VLA observations of the Sgr A complex at 90 cm, 20 cm, and 6 cm are combined to obtain spectral index and optical depth distributions across the source and are used to constrain the three-dimensional structure at the Galactic center. At 90 cm the 'spiral' structure of the thermal source Sgr A West is clearly seen in absorption against the nonthermal emission of Sgr A East, showing conclusively that the former is in front of the latter. The entire shell of Sgr A East shows a low-frequency turnover corresponding to an optical depth of 1-2. This turnover corresponds to free-free absorption with an emission measure of about 100,000 pc/cm exp 6 if the electron temperature is about 5000 K. The approximately 7 arcmin halo surrounding Sgr A East and West also shows a low-frequency turnover. It is suggested that the halo contains a mixture of thermal and nonthermal gas and that the thermal component is responsible for the absorption extending across Sgr A East. 48 refs.

The 12 micron galaxy sample. I: Luminosity functions and a new complete active galaxy sample

Abstract: An all-sky 12 micron flux-limited sample of active galaxies was selected from the IRAS Point Source Catalog. Most of the sample galaxies are in existing catalogs, and 99 percent have measured redshifts. The 12-micron and the far-infrared luminosity functions of active and normal galaxies are derived using IRAS co-added data. A total of 22 percent of the sample galaxies harbor active nuclei. The sample consists almost equally of Seyfert 1, Seyfert 2, and LINER nuclei. The derived luminosity fuctions for Seyfert 1 and Seyfert 2 galaxies are indistinguishable from those of the optically selected CfA sample. Thus, 12 micron selection is the most efficient available technique for finding complete active galaxy samples. 28 refs.

Anisotropic ionizing radiation in NGC5252

Abstract: THE different kinds of active galactic nuclei (AGNs) and quasars may be fundamentally similar, and owe their observed differences to anisotropy in the distribution of their emitted radiation: apparently different objects may then be the same thing seen from different viewpoints. In particular, it has been proposed that narrow-line Seyfert 2 galaxies are Seyfert 1 galaxies for which the broad-line and continuum emitting regions are hidden by obscuring material along our line of sight1,2 Here we present narrow-band imaging observations of the Seyfert 2 galaxy NGC5252 which strongly support such unifying hypotheses. Images in the light of the [O III] line at λ = 5,007 A reveal a sharply defined bi-conical structure extending to a maximum radius of ∼33 kpc (H0 = 50 km s−1 Mpc−11) along the radio-emission axis. This is just the type of structure expected if the ionizing-radiation field from the nucleus is anisotropic. A map of the ratio of the [O III] and Hα lines shows a central band perpendicular to the cones that we identify as the disk or torus responsible for collimating the radiation field.

Active galactic nuclei

Abstract: Recent observational and theoretical investigations of AGN are reviewed. The basic AGN characteristics (broad emission lines of abundant ions, X-ray emission, and a sharp starlike appearance) are summarized, and particular attention is given to the spectral classification of Seyfert galaxies, the AGN velocity fields and their measurement, low-ionization narrow-emission-line regions (LINERs), observations supporting a torus model of Seyfert AGN, extended gas and ionization, and the black-hole/accretion-disk model of the AGN energy source. It is suggested that no one model can explain the physical processes in all types of AGN. 43 refs.

Accretion disks and magnetic fields in astrophysics

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Spectropolarimetry and variability of Seyfert 1.8 and 1.9 galaxies

Abstract: The phenomenon which produces the spectra classified as Seyfert 1.8 or 1.9 is investigated through CCD spectropolarimetry and through analysis of three highly variable objects. The Seyfert 1.9 galaxy IRAS 1958-183 has a highly polarized continuum and a broad H-alpha line which is 30 percent polarized. The variability of NGC 2622, NGC 7603 (= Mrk 530), and Mrk 1018 are studied. The changes in flux of the broad lines and the continuum near H-alpha and H-beta are consistent with changes in the extinction in all cases. Improved IRAS photometry supports the conclusion that most Seyfert 1.8s and 1.9s are normal Seyfert 1s seen through a screen of dust located in or just outside of the broad-line regions. Variability is due to changes in the optical depth of this screen.

Implications of Ca II emission for physical conditions in the broad-line region of active galactic nuclei

Abstract: This paper summarizes observations of Ca II emission from AGN and discusses their implications for conditions in the emitting gas. Observations of Ca II K and H, the infrared triplet, and the forbidden lines near 7300 A are compiled to give a standard set of line ratios for AGN with relatively strong Fe II emission. A series of photoionization calculations is presented which examines the conditions needed to produce the observed emission. Special attention is paid to the inclusion of heating due to free-free and H(-) absorption processes which couple the near-infrared to millimeter continuum with the emitting gas. It is found that, regardless of density or ionization parameters, thin clouds cannot produce the observed Ca II emission. The semiforbidden C III 1909 A/C IV 1549 A intensity ratio is not a good indicator of the ionization parameter for this continuum. Clouds with large column densities and ionization parameters largely reproduce the observed carbon, calcium, and hydrogen spectrum. 98 refs.

The radiative deceleration of ultrarelativistic jets in active galactic nuclei

Abstract: A detailed study of the dynamical interaction between a highly relativistic jet and the thermal radiation field from an AGN accretion disk is reported, and the Comptonized spectrum arising from this interaction is self-consistently determined. A simple model that captures the essential radiative and geometrical features of realistic disk configurations is presented, and the disk radiation field is calculated. The results confirm Phinney's (1987) suggestion that the thermal radiation field produced by accretion in an AGN could be very effective in decelerating ultrarelativistic jets that are accreted by electromagnetic or hydromagnetic forces closer to the central black hole. Terminal Lorentz factors are consistent with the values inferred in superluminal radio sources are readily produced in this model for plausible disk and jet parameters without additional acceleration in the interaction zone. A new interpretation of the hard X-ray component detected in BL Lac spectra is proposed.

The ``Seyfert 1'' Optical Spectra of the Type II Supernovae 1987F and 1988I

Abstract: Observations of the type II supernovae 1987F and 1988I are discussed The overall optical spectroscopic properties of SN 1987F are shown to resemble those of type I Seyfert nuclei and quasars The properties of SN 1987F are compared with the spectrum of 3C 48 Similar results are obtained for SN 1988I It is suggested that if either of these supernovae had occurred in the nucleus of a normal galaxy, the nucleus would have been classified as a type 1 Seyfert, if the only data available were low-resolution optical spectra Thus, it is concluded that some low-luminosity active galaxies might actually be due to violent bursts of star formation rather than accretion onto a supermassive black hole 80 references

Near-infrared images of NGC 1068: bar-driven star formation and the circumnuclear composition

Abstract: Consideration is given to differences in images of the core of the Seyfert galaxy NGC 1068 at different wavelength regimes dominated by stellar photospheric emission. A stellar bar is observed at 1.6 and 2.2 microns, but not at shorter visual wavelength observations. Two explanations for these differences are examined: substantial stellar population variations with position and the effects of dust obscuration. The relation of the bar to active star formation is discussed. The mass of the central disk and bar is estimated at 2-3 X 10 to the 10th solar masses. It is found that the rate of star formation (100 solar masses/yr) is due to a large mass of molecular gas. 57 refs.

The properties of infrared color-selected quasars

Abstract: The properties of a sample of color-selected quasars from the IRAS Point Source Catalog have been compared with those of a larger sample of PG quasars detected by the IRAS. Fe II emission is shown to be prominent in quasars with strong IR luminosity relative to optical luminosity. Two of the seven quasars observed by the IRAS have energy distributions which are similar to those of the typical PG quasars, while the others are much redder. 12 refs.

Hot spots in extragalactic radio sources : proceedings of a Workshop held at Ringberg Castle, Tegernsee, FRG, February 8-12, 1988

Abstract: Hot spot radio galaxies - an introduction.- Radio observation of hot spots.- Compact hotspots, double hotspots, and source asymmetry.- Cygnus a and the Williams model.- A tale of two hot spots.- 3C332: A source with an "exceptional" multiple hot spot morphology.- Constraints on the hotspot advance speed in the core-dominated quasar 1510-089.- The extended structure of the radio galaxy PKS 0521-36: radio polarization and optical emission lines.- New VLA results on M87.- Digital photometry of the jet in M 87.- Imaging polarimetry of the jet in m 87.- Continuum observations of hot spots at wavelengths < 1 cm.- Multifrequency flux determination in the hot spot of 3C33 South.- A blue and polarized source along the radio axis of PKS 2152-69.- Extended optical line emission in radio galaxies.- Highly ionized gas in PKS 2152-69.- Models of hot spots.- The terminal shock in jets.- Giant radio galaxies via inverse Compton weakened jets.- Jet speed, beaming & sidedness, and all that.- How important are currents and fields in radio sources?.- Numerical simulations of hot spots.- Walljets.- Simulations of synchrotron loss in hotspots.- The influence of magnetic fields on the propagation of supersonic jets.- Fermi acceleration.- First-order fermi acceleration at relativistic shock fronts.- Spectral indices from relativistic and non-relativistic shocks.- Shock acceleration theory applied - The spectra of radio hot spots.- Synchrotron-emission - photons and neutrinos - from shockwave regions in active galactic nuclei, jet and hot spots.- Remarks about diffusive shock pave acceleration.- Particle acceleration in hotspots.- Magnetic reconnection and particle acceleration.- Stochastic acceleration of relativistic electrons in synchroton sources with turbulently reconnecting magnetic fields.

The origin of coronal lines in Seyfert galaxies

Abstract: This paper examines the possibility that the coronal line region in Seyfert galaxies may be the result of an interstellar medium (ISM) exposed to, and subsequently photoionized by, a 'bare' Seyfert nucleus. It is shown that a 'generic' AGN continuum illuminating the warm-phase of the ISM of a spiral galaxy can produce the observed emission. In this picture the same UV-radiation cone that is responsible for the high-excitation extended narrow-line emission clouds observed out to 1-2 kpc or farther from the nuclei of some Seyfert galaxies also produces the coronal lines. Soft X-rays originating in the nucleus are Compton-scattered off the ISM, thus producing extended soft X-ray emission, as observed in NGC 4151. The results of the calculations show a basic insensitivity to the ISM density, which explains why similar coronal line spectra are found in many Seyfert galaxies of varying physical environments. 60 refs.

The ratio of molecular to atomic gas in infrared luminous galaxies

Abstract: In infrared luminous galaxies the ratio of the CO(1 - 0) to H I integrated fluxes increases with the far-infrared excess, f(fir)/f(b). All infrared active galaxies with f(fir)/f(b) greater than 2 have molecular to atomic gas mass fractions greater than 0.5. Among the galaxies with the higher infrared excesses there are systems with strikingly small atomic mass fractions, where less than 15 percent of the total mass of interstellar gas is in atomic form. The optical morphology of luminous infrared galaxies indicates that the majority, if not all, of these objects are interacting systems. These observations suggest that the overall mass fraction of molecular to atomic gas, and the infrared luminosities per nucleon of interstellar gas are enhanced during galaxy-galaxy interactions.