Active Galactic Nuclei Torus Research Articles

Towards a physical model of dust tori in Active Galactic Nuclei

Towards a physical model of dust tori in Active Galactic Nuclei

Evolutionary Consequences of Dusty Tori in Active Galactic Nuclei

Deep surveys of {\em Chandra} and {\em HST} (Hubble Space Telescope) show that active galactic nucleus (AGN) populations are changing with hard X-ray luminosities. This arises an interesting question whether the dusty torus is evolving with the central engines. We assemble a sample of 50 radio-quiet PG quasars to tackle this problem. The covering factors of the dusty tori can be estimated from the multiwavelength continuum. We find they are strongly correlated with the hard X-ray luminosity. Interestingly this correlation agrees with the fraction of type II AGNs discovered by {\em Chandra} and {\em HST}, implying strong evidence for that the AGN population changing results from the evolution of the tori. We also find that the frequencies of the dips around 1$\mu$m in the continuum correlate with the covering factors in the present sample, indicating the dip frequencies are adjusted by the covering factors. In the scenario of fueling black hole from the torus, the covering factor is a good and the dip frequency is a potential indicator of the torus evolution.

The Detection of Silicate Emission from Quasars at 10 and 18 Microns

We report the spectroscopic detection of silicate emission at 10 and 18 mm in five PG quasars, the first detection of these two features in galaxies outside the Local Group. This finding is consistent with the unification model for active galactic nuclei (AGNs), which predicts that an AGN torus seen pole-on should show a silicate emission feature in the mid-infrared. The strengths of the detected silicate emission features range from 0.12 to 1.25 times the continuum at 10 mu m and from 0.20 to 0.79 times the continuum at 18 mu m. The silicate grain temperatures inferred from the ratio of 18 mu m to 10 mm silicate features under the assumption of optically thin emission range from 140 to 220 K.

Simultaneous Optical and Near‐Infrared Spectropolarimetry of Type 2 Seyfert Galaxies

We present optical and near-infrared spectropolarimetry of the nuclei of four type 2 Seyfert galaxies, Mrk 463E, Mrk 1210, NGC 1068, and NGC 4388. The data were obtained simultaneously, covering the wavelength range of 0.46-2.5 μm. We model the polarizations from two dust-scattering components: (1) scattering in dusty regions in ionization cones and (2) scattering in a torus surrounding a type 1 nucleus. The polarizations from electron scattering in the cones and dichroic absorption by aligned dust grains in the torus are also compared with the observations. We confirmed that a combination of electron and dust scattering in the ionization cones is the preferred mechanism for the optical continuum polarization. For the near-infrared, dichroic absorption by aligned grains can explain the continuum polarization of Mrk 463E and Mrk 1210 as well as NGC 1068. Visual optical depths of the order of 10-20 are estimated for dichroic absorption in these nuclei. Dust scattering in the torus, whose grain size distribution is assumed to be the same as in the Galactic diffuse interstellar medium, cannot reproduce the observed spectral slope of the near-infrared polarization and total nuclear flux simultaneously. However, this might only indicate that the grain size distribution in the torus of active galactic nuclei (AGNs) is different, and dust scattering with moderate optical depth and dominated by large grains might provide a reasonable explanation for the near-infrared radiation from AGNs.

Binary black holes and tori in AGN

Observations of HST and groundbased data strongly suggest that most galaxies harbour central supermassive black holes and that most galaxies merge with others. Consequently a black hole binary emerges as the two black holes are spiraling into the center towards each other. In our work we are investigating two basic questions of our understanding of the central activity of galaxies and find that both can be answered with "yes" : (1) Do the black holes actually merge? (2) And does the effect of the torque of the black hole binary on the surrounding stellar distribution help to explain the presence of the ubiquitous torus of molecular material surrounding apparently all active galactic nuclei? The first question is the topic of the present article, while the second question will be subject of a subsequent paper. Simulating the evolution of a stellar cluster in the potential of such a binary by solving the equations of the restricted three body problem we obtained the following results: provided that the cluster is about as massive as the black hole binary the two black holes coalesce after due to ejection of stars and finally via emission of gravitational radiation. Whether a star is ejected or not crucially depends on its angular momentum. Almost all stars whose angular momentum is twice as large as that of a star circulating around the binary in a distance corresponding to that between the black holes, stay bound to the binary. In a sequence of models where the mass of the secondary black hole increases while M1 is fixed, a bigger fraction of stars is ejected. For a more massive binary also the cluster has to be more massive in order to allow the two black holes to coalesce. The merger then proceeds on smaller time scales. The cluster is depleted in the central region and the final distribution of stars assumes a torus-like structure, peaking at three times the initial distance of the two black holes. The relationship of the bound stars to the obscuring torus in active galactic nuclei will be investigated in a subsequent paper.

Ionization, Extinction, and Spiral Structure in the Inner Disk of NGC 1068

We present our analysis of Hubble Space Telescope/Planetary Camera narrow-bandpass and broadband imagery of the inner 3 kpc region of NGC 1068. Our analysis of F160BW and F547M broadband continuum imagery suggests that roughly 40% of the scattered active galactic nucleus (AGN) continuum emission originates from an unobscured single cloud complex largely free from dust with total number densities typical of diffuse clouds in our own Milky Way. The net emission-line fluxes are extracted from continuum-subtracted narrow-bandpass imagery for Ha + [N ], Hβ, S II λλ6717, 6731, and [O III] λ5007. Although the [O ]/(Hα + [N ]) flux ratio shows a sharp drop-off at distances beyond ~4'' northeast of the nucleus, the [O ]/Hb ratio indicates no such decrease. This implies that the ionization of these species is not strongly influenced by shocks associated with the expanding radio lobes as inferred from a previous study. The sharp drop-off seen in the [O ]/(Ha + [N ]) appears to be due to high interstellar reddening immediately beyond the scattering bright clouds near the nucleus, as further evidenced by the high Ha/Hb ratio in this region. The presence of a faint inner spiral arm interior to the sharply defined star formation ring, possibly driven by an outer-inner Linblad resonance, may provide a means of fueling the central AGN, as recently suggested by Yuan and Kuo. Because IR observations do not support the presence of a true AGN torus in NGC 1068, we present a qualitative model in which the radio ejecta has carved out an ionization cone in the high concentration of dense molecular clouds surrounding the nucleus. This picture also implies that the extended conical emission region to the northeast of the nucleus lies near the galactic plane and is surrounded by lower density ambient gas than that surrounding the highly ionized gas southwest of the nucleus.

Magneto hydrodynamic simulations of magnetically driven mass accretion in active galactic nuclei

When a nuclear gas torus in active galactic nuclei (AGN) is threaded by large-scale magnetic fields, magnetic braking can drive mass accretion from the torus to the central black hole. Through two dimensional resistive MHD simulations, we studied the dependence of the mass accretion rate on magnetic diffusivity η 0 = η (v K0r 0) , where r 0 is the reference radius and v K0 is the Keplerian rotation speed at r = r 0. We found three regimes, (a) when 0 ≤ η 0 ≤ 0.003 accretion takes place episodically, (b) when 0.003 ≤ η 0 ≤ 0.01 accretion rate increases monotonically with time and approaches to a constant value, (c) when η 0 > 0.01 the saturation level of the accretion rate decreases with η 0. If we assume the mass of the central black hole as 10 9 M ⊙ and the mass of the circum-nuclear gas torus as 10 8 M ⊙, the mass accretion rate is larger than 1M ⊙ yr when η 0 ≤ 0.02.

Global three-dimensional MHD simulations of accretion disks and jet formation in AGNS

We present the results of three-dimensional global magnetohydrodynamic (MHD) simulations of a gas torus in active galactic nuclei. When the torus is threaded by large-scale vertical magnetic fields, a magnetically driven jet is ejected from the torus. Since magnetic braking extracts angular momentum from the torus, the surface layer of the torus accretes like an avalanche. Due to the growth of global non-axisymmetric MHD modes, the avalanching flow breaks up into spiral channels and creates a helical structure inside the jet. We found that when the torus is initially threaded by weak toroidal magnetic fields, it evolves toward a quasi-steady state with β = P gas P mag ∼ 10 . As the angular momentum is re-distributed, the torus becomes flattened and the angular momentum distribution approaches that of Keplerian disks.

8.10. Three-dimensional global MHD simulations of jet formation in active galactic nuclei

Magnetically driven jets from accretion disks are considered to be the most promising models of astrophysical jets. Uchida & Shibata (1985) and Shibata & Uchida (1986) first carried out two-dimensional nonlinear MHD simulations of jet formation from a magnetized disk. Matsumoto et al. (1996) applied the Uchida-Shibata model to a gas torus in active galactic nuclei and showed that the surface layer of the torus accretes faster than the equatorial region like an avalanche because magnetic braking most effectively extracts angular momentum from that layer. A magnetized torus subjects to global non-axisymmetric instabilities (Curry & Pudritz 1996) and local magnetorotational instability (Balbus & Hawley 1991). We carried out three-dimensional global MHD simulations to show the non-axisymmetric effects on the torus, avalanche flow and jet formation.

Thick Tori around Active Galactic Nuclei: The Case for Extended Tori and Consequences for Their X‐Ray and Infrared Emission

Two families of models of dusty tori in active galactic nuclei (AGNs; moderately thick and extended versus very thick and compact) are tested against available observations. The confrontation suggests that the former class better explains the infrared (IR) broadband spectra of both broad- and narrow-line AGNs, the anisotropy of the emission deduced by comparing IR properties of Seyfert 1 and 2 nuclei, and the results of IR spectroscopy and those of high spatial resolution observations. There is, however, clear evidence for a broad distribution of optical depths. We also examine the relationship between IR and X-ray emission. The data support a view in which the matter responsible for the X-ray absorption is mostly dust free, lying inside the dust sublimation radius. The consequences of these results for the hard X-ray background as well as IR counts and background are discussed.

Theoretical perspectives on obscuring tori

A review is presented of contributions theoretical work has already made, and can make in the future, to our understanding of obscuring tori in active galactic nuclei. These fall most naturally under the headings: geometry, energy budget and spectral output, dynamics, and “natural history”. The greatest progress has been made in the first two categories; most ideas in the latter two are still little more than speculation.

On the stability and evolution of relativistic radiation tori: equations and speculations

Equations describing the general relativistic evolution of hypothetical radiation-supported tori in active galactic nuclei are presented and discussed. Equations describing global, non-axisymmetric perturbations of radiation tori are also given. The role of instabilities in transporting angular momentum and heat is examined and some simple prescriptions for use in numerical calculation of the secular evolution of these structures are suggested. In particular, if a sufficiently optically thick torus evolves to become dynamically unstable then a gyrotropic, marginally stable convection zone with angular momentum constant on isentropes may be established. Goldreich–Schubert–Fricke instability should be adequate to maintain a radiative zone in a barotropic state.