Theory Of Active Galactic Nuclei Research Articles

On the Optical-to-silicate Extinction Ratio as a Probe of the Dust Size in Active Galactic Nuclei

Abstract Dust plays a central role in the unification theory of active galactic nuclei (AGNs). Whether the dust that forms the torus around an AGN is micron-sized, like interstellar grains, or much larger, has a profound impact on correcting for the obscuration of the dust torus to recover the intrinsic spectrum and luminosity of the AGN. Here we show that the ratio of the optical extinction in the visual band ( ) to the optical depth of the 9.7 μm silicate absorption feature ( ) could potentially be an effective probe of the dust size. The anomalously lower ratio of / ≈ 5.5 of AGNs compared to that of the Galactic diffuse interstellar medium of / ≈ 18 reveals that the dust in AGN tori could be substantially larger than the interstellar grains of the Milky Way and those of the Small Magellanic Cloud, and therefore, one could expect a flat extinction curve for AGNs.

SILICATE DUST IN ACTIVE GALACTIC NUCLEI

Abstract The unification theory of active galactic nuclei (AGNs) hypothesizes that all AGNs are surrounded by an anisotropic dust torus and are essentially the same objects but viewed from different angles. However, little is known about the dust that plays a central role in the unification theory. There are suggestions that the AGN dust extinction law appreciably differs from that of the Galaxy. Also, the silicate emission features observed in type 1 AGNs appear anomalous (i.e., their peak wavelengths and widths differ considerably from that of the Galaxy). In this work, we explore the dust properties of 147 AGNs of various types at redshifts , with special attention paid to 93 AGNs that exhibit the 9.7 and 18 μm silicate emission features. We model their silicate emission spectra obtained with the Infrared Spectrograph aboard the Spitzer Space Telescope. We find that 60/93 of the observed spectra can be well explained with “astronomical silicate,” while the remaining sources favor amorphous olivine or pyroxene. Most notably, all sources require the dust to be micron-sized (with a typical size of ∼1.5 ± 0.1 μm), much larger than submicron-sized Galactic interstellar grains, implying a flat or “gray” extinction law for AGNs. We also find that, while the 9.7 μm emission feature arises predominantly from warm silicate dust of temperature T ∼ 270 K, the ∼5–8 μm continuum emission is mostly from carbon dust of T ∼ 640 K. Finally, the correlations between the dust properties (e.g., mass, temperature) and the AGN properties (e.g., luminosity, black hole mass) have also been investigated.

DUST IN ACTIVE GALACTIC NUCLEI: ANOMALOUS SILICATE TO OPTICAL EXTINCTION RATIOS?

Dust plays a central role in the unification theory of active galactic nuclei (AGNs). However, little is known about the nature (e.g., size, composition) of the dust which forms a torus around the AGN. In this Letter we report a systematic exploration of the optical extinction (A_V) and the silicate absorption optical depth (\Delta\tau9.7) of 110 type 2 AGNs. We derive A_V from the Balmer decrement based on the Sloan Digital Sky Survey data, and \Delta\tau9.7 from the Spitzer/Infrared Spectrograph data. We find that with a mean ratio of A_V/\Delta\tau9.7 ~ 5.5, the optical-to-silicate extinction ratios of these AGNs are substantially lower than that of the Galactic diffuse interstellar medium (ISM) for which A_V/\Delta\tau9.7 ~ 18.5. We argue that the anomalously low A_V/\Delta\tau9.7 ratio could be due to the predominance of larger grains in the AGN torus compared to that in the Galactic diffuse ISM.

活动星系核中的尘埃

Dust is the cornerstone of the unification theory of active galactic nuclei (AGNs). This theory proposes that all AGNs are essentially born equal: all types of AGNs are surrounded by an optically thick dust torus and are basically the same object but viewed from different lines of sight. The large diversity in the observational properties of AGNs is simply caused by the viewing-angle-dependent obscuration of the nucleus: those viewed face-on are unobscured (allowing for a direct view of their nuclei) and recognized as 1 AGNs, while those viewed edge-on are AGNs with most of their central engine and broad line regions being hidden by the obscuring dust. However, little is known about the dust in the circumnuclear torus of AGNs. There is evidence suggesting that the size and composition of the dust in AGNs may differ substantially from that of the Galactic interstellar dust, as reflected by the flat gray extinction curves, the 2175 A extinction bump is not (or at least rarely) seen in AGNs, and the anomalous silicate emission or absorption features observed respectively in type1 and type 2 AGNs. This review summarizes our current understanding of the extinction and infrared emission properties of the circumnuclear dust in AGNs as well as the inferred dust composition and size distribution.

Gamma Ray and Infrared Emission from the M87 Jet and Torus

The existence of intrinsic obscuration of Fanaroff-Riley I objects is a controversial topic. M87, the nearest such object, is puzzling in that although it has very massive central black hole it has a relatively low luminosity, suggesting it is in a dormant state. Despite of its proximity to us (16 Mpc) it is not known with certainty whether or not M87 has a dusty torus. Infrared observations indicate that if a torus exists in M87 it must have a rather low infrared luminosity. Using arguments from unification theory of active galactic nuclei, we have earlier suggested that the inner parsec-scale region of M87 could still harbour a small torus sufficiently cold such that its infrared emission is dwarfed by the jet emission. The infrared emission from even a small cold torus could affect through photon-photon pair production interactions the escape of 100 GeV to TeV energy gamma rays from the central parsec of M87. The TeV gamma-ray flux from the inner jet of M87 has recently been predicted in the context of the Synchrotron Proton Blazar (SPB) model to extend up to at least 100 GeV (Protheroe, Donea, Reimer, 2002). Subsequently, the detection of gamma-rays above 730 GeV by the HEGRA Cherenkov telescopes has been reported. We discuss the interactions of gamma-rays produced in the inner jet of M87 with the weak infrared radiation expected from a possible dusty small-scale torus, and show that the HEGRA detection shows that the temperature of any torus surrounding the gamma-ray emission region must be cooler than about 250 K. We suggest that if no gamma-rays are in future detected during extreme flaring activity in M87 at other wavelength, this may be expected because of torus heating.

Radiation Fields in Blazars — a Possible Extension of the Small Scale Symbiosis (Disk/Jet) into a Large Scale (Dust/Dust) Symbiosis

AbstractIn blazar models both protons and electrons may be efficiently accelerated in jets and produce γ-rays. Here we discuss the interactions of these γ-rays with different radiation fields. The external radiation fields within a few parsecs from the black hole involved in such interactions could be the direct radiation from the accretion disk coupled with the jet, the infrared radiation from a dusty torus, and the emission line radiation from the broad line region surrounding the accretion disk. The optical thickness for absorption of γ-ray photons in the external radiation fields is analysed for blazars and quasars.Based on the unification theory of active galactic nuclei we briefly review the evidence for the existence of small scale dust tori in blazars/FR I. We propose that the existing jet–accretion disk symbiosis extrapolates to a large scale symbiosis between other important dusty constituents of the blazar/FR I family.

The interacting subsystems theory of active galactic nuclei -- I. The broad-absorption-line QSO theory

The theory of broad-absorption-line (BAL) QSOs is worked out on the basis of the radiation hydrodynamics equation solution for the two-phase media of active galactic nuclei. We suppose that the BAL QSO physics depends on some ‘hidden’ AGN parameters, such as the mass and size of the compact stellar system. We therefore approach this problem in the more general framework of the ‘interacting subsystems theory’, which includes these parameters. We compare the results of the numerical model calculations with the observed spectra and show that the BAL QSOs (and the radio-quiet quasars as well) contain massive compact stellar kernels in their central regions. We show that the line-locking effect is determined by the radiation pressure, and is also favoured by the drag force of the hot gas acting on the line-absorbing clouds. We derive some general conclusions about the physics of AGN. In particular, we show that the radio-quiet versus radio-loud dichotomy can be explained by using two types of hot gas outstreams in quasars.

11.7. Signature of accretion disks in active galactic nuclei

One of the fundamental components of any theory of Active Galactic Nuclei (AGNs) is an accretion disk surrounding a supermassive blackhole. While most theories predict the formation of such a disk, fewer say anything about the observational consequences of the disk. Those studies which have addressed the observational signatures of accretion disks have found that the Lyman edge at 912Å is a powerful diagnostic feature for the physical characteristics of the disk. One of the important consequences of such disks is a feature at the Lyman edge (912Å). Spectroscopic investigations have found scant evidence for Lyman edge features arising in an accretion disk (Koratkar, Kinney and Bohlin 1992). Proving the existence of radiating disks in even a few AGNs would be a major step in our understanding of the phenomenon.

On possible signatures of heavy neutrino balls in active galactic nuclei

After the discovery of quasars, theories of active galactic nuclei (AGN) were constructed which focused on the investigation of gas dynamics in the gravitational potential of a black hole. In this paper we study the influence of a possible dark matter neutrino ball on the accretion process in AGN, and investigate possible observable signatures of such a ball which is supported by degeneracy pressure. We find that, if such balls exist in the vicinity of AGN, they will cause a bump in the IR region of the emitted spectrum which is, in fact, observed for several AGN. We also find that, if the bump corresponding to ∼ 1000 K in the spectrum of the typical radio-loud quasar 3C 273 is due to a neutrino ball, then our model implies a mass of the black hole of 108 M⊙, a ball mass of 4 × 109 M⊙ and, in turn, a neutrino of mass 3 keV/c2.

Unified theories of active galactic nuclei: a hard X-ray sample of Seyfert 2 galaxies

Unified theories of active galactic nuclei: a hard X-ray sample of Seyfert 2 galaxies

The difference between radio-loud and radio-quiet active galaxies

The recent development of unified theories of active galactic nuclei (AGN) has indicated that there are two physically distinct classes of these objects - radio-loud and radio-quiet. The primary observational distinctions between the two types are: (1) The radio-loud objects produce large scale radio jets and lobes, with the kinetic power of the jets being a significant fraction of the total bolometric luminosity. On the other hand, the weak radio ejecta of the radio-quiet objects are energetically insignificant. (2) The radio-loud objects are associated with elliptical galaxies which have undergone recent mergers, while the radio-quiets prefer spiral hosts. (3) The space density of the radio-louds at a given optical luminosity is $\approx$ 10 times lower than that of the radio-quiets. Despite these differences, the (probably) thermal emissions from the AGN (continua and lines from X-ray to infrared wavelengths) are quite similar in the two classes of object. We argue that this last result suggests that the black hole masses and mass accretion rates in the two classes are not greatly different, and that the difference between the classes is associated with the spin of the black hole. We assume that the normal process of accretion through a disk does not lead to rapidly spinning holes, and propose instead that galaxies (e.g. spirals) which have not suffered a recent major merger event contain non-rotating or only slowly rotating black holes. When two such galaxies merge, the two black holes are known to form a binary and we assume that they eventually coalesce. In the small fraction of mergers in which the two ``parent'' galaxies contain very massive holes of roughly equal mass, a rapidly spinning, very massive hole results. It is proposed that such mergers are the progenitors of powerful radio

Unified theories of active galactic nuclei: the hard X-ray spectrum of NGC 1068

We have reanalysed the Ginga observation of the archetypal Seyfert 2 galaxy NGC 1068. The X-ray spectrum can be decomposed into four components: a hard X-ray power law, iron Kα emission-line radiation, reflection from an accretion disc or molecular torus, and emission-line radiation at ∼ 8 keV. Although reflection is not required, its inclusion is consistent with the data and increases the underlying intrinsic power-law energy index to α E ∼ 0.9, similar to the typical slope found for Seyfert 1 galaxies. Reflection is unlikely to produce all the observed equivalent width of the Ginga Fe Kα emission line, but can account for all of the component seen at 6.4 keV by BBXRT

Unified Theories of Active Galactic Nuclei.

Unified Theories of Active Galactic Nuclei.

Accretion discs in active galactic nuclei: tell-tale signs of the nuclear star cluster?

Early theories of active galactic nuclei concentrated on the physics of gas in the region dominated by the gravitational potential of a central black hole. Recently, attention has increasingly focussed on the role of individual stars in the environment of an active galactic nucleus. We investigate some aspects of accretion disc structure in active galactic nuclei containing a gravitationally significant stellar component. Because such discs radiate the gravitational binding energy released locally by viscous processes, their spectra probe the mass distributions of the regions where the discs reside

Altered luminosity functions for relativistically beamed objects. II - Distribution of Lorentz factors and parent populations with complex luminosity functions

view Abstract Citations (71) References (2) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Altered Luminosity Functions for Relativistically Beamed Objects. II. Distribution of Lorentz Factors and Parent Populations with Complex Luminosity Functions Urry, C. M. ; Padovani, P. Abstract Relativistic beaming introduces strong selection effects into flux- limited samples because of the considerable Doppler enhancement. In populations where relativistic beaming is important, the ratio of number densities between beam-dominated and parent objects becomes a function of luminosity. Urry and Shafer previously calculated this effect assuming a single Lorentz factor for the bulk motion and a single power-law parent luminosity function with sharp cutoffs. Here we investigate more realistic luminosity functions, required for actual tests of unified theories of active galactic nuclei (AGNs), including double power-law parent luminosity functions and distributions of bulk Lorentz factors for the relativistically beamed component. The observed luminosity function of the beam-dominated objects is generally flatter than the parent luminosity function, so that the number density ratio is a strong function of luminosity and can easily be greater than unity at high luminosities, even for gradual low-luminosity cutoffs in the parent luminosity function. Several characteristic break points can be identified, depending on the details of the parent luminosity function. Our calculations can be used to test unified theories by predicting the observed luminosity function for aligned objects from the luminosity function of the proposed parent population. This procedure is essential when samples are defined by a limit on the total flux, as is frequently the case. Publication: The Astrophysical Journal Pub Date: April 1991 DOI: 10.1086/169870 Bibcode: 1991ApJ...371...60U Keywords: Bl Lacertae Objects; Lorentz Transformations; Luminosity; Relativistic Electron Beams; Active Galactic Nuclei; Spectral Emission; Astrophysics; BL LACERTAE OBJECTS; GALAXIES: JETS; LUMINOSITY FUNCTION full text sources ADS |

Toward a Unified Theory of Active Galactic Nucleia

A unified theory of active galactic nuclei, based on the physics of accretion flows around massive black holes, is described. The accretion modes are dependent on the ratio of accretion rate to the mass of the hole. The spectral characteristics and radiative efficiency for the ion tori, thin disks, and radiation tori modes of accretion are examined. The properties of the three accretion modes are compared with the observed characteristics of various types of active galactic nuclei (AGN) and the data reveal that the accretion modes affect the behavior of AGNs. 49 references.

THEORY OF ACTIVE GALACTIC NUCLEI

ABSTRACT The involvement of accretion disks around supermassive black holes in the theory of active galactic nuclei (AGN) is discussed. The physics of thin and thick accretion disks is discussed and the partition between thermal and nonthermal energy production in supermassive disks is seen as uncertain. The thermal limit cycle may operate in supermassive disks (Shields, 1985), with accumulation of gas in the disk for periods of 10 to the 4th to 10 to the 7th years, punctuated by briefer outbursts during which the mass is rapidly transferred to smaller radii. An extended X-ray source in AGN is consistent with observations (Tennant and Mushotsky, 1983), and a large wind mass loss rate exceeding the central accretion rate means that only a fraction of the mass entering the disk will reach the central object; the rest being lost to the wind. Controversy in the relationship between the broad lines and the disk is also discussed.

Theories of the nuclei of active galaxies

The underlying source of power in active galaxies and quasars is still debated. X-ray observations of these objects may relate more directly with this power source than observations in other wavebands. Theories of active galactic nuclei are reviewed, together with X-ray production mechanisms, both thermal and non-thermal. Constraints imposed on these theories by present and future observations are discussed and interpreted in terms of some models for specific objects.