Unification Scheme Of Active Galactic Nuclei Research Articles

Determining Quasar Orientation

Abstract Since the discovery of active galactic nuclei (AGN) and their subclasses, a unification scheme of AGN has been long sought. Orientation-based unified models predict that some of the diversity within AGN subclasses can be explained by the different viewing angles of the observer. Several orientation categorisations have been suggested, but a widely applicable measure has yet to be found. Using the properties of the ultraviolet and optical broad emission lines of quasars, in particular the velocity offsets and line widths of high-ionisation C iv and low-ionisation Mg ii lines, a correlation has been measured. It is postulated that this correlation is due to the viewing angle of the observer. Comparison with other orientation tracers shows consistency with this interpretation. Using a simulation of a wide angle disk-wind model for the broad emission line region, we successfully replicate the observed correlation with inclination. Future more detailed modelling will not only enable improved accuracy in the determination of the viewing angle to individual AGN, but will also substantially increase our understanding of the emitting regions of AGN.

Compact symmetric objects and supermassive binary black holes in the VLBA Imaging and Polarimetry Survey

We present multi-frequency Very Long Baseline Array (VLBA) follow-up observations of VLBA Imaging and Polarimetry Survey sources identified as likely compact symmetric objects (CSOs) or super-massive binary black holes (SBBHs). We also present new spectroscopic redshifts for 11 sources observed with the Hobby-Eberly Telescope. While no new SBBHs can be confirmed from these observations, we have identified 24 CSOs in the sample, 15 of which are newly designated, and refuted 52 candidates leaving 33 unconfirmed candidates. This is the first large uniform sample of CSOs which can be used to elicit some of the general properties of these sources, including morphological evolution and environmental interaction. We have detected polarised emission from two of these CSOs the properties of which are consistent with Active Galactic Nuclei unification schemes.

FORMATION OF RAMAN SCATTERING WINGS AROUND Hα, Hβ, AND PAαIN ACTIVE GALACTIC NUCLEI

Powered by a supermassive black hole with an accretion disk, the spectra of active galactic nuclei (AGNs) are characterized by prominent emission lines including Balmer lines. The unification schemes of AGNs require the existence of a thick molecular torus that may hide the broad emission line region from the view of observers near the equatorial direction. In this configuration, one may expect that the far UV radiation from the central engine can be Raman scattered by neutral hydrogen to reappear around Balmer and Paschen emission lines which can be identified with broad wings. We produce H$\alpha$, H$\beta$ and Pa$\alpha$ wings using a Monte Carlo technique to investigate their properties. The neutral scattering region is assumed to be a cylindrical torus specified by the inner and outer radii and the height. While the covering factor of the scattering region affects the overall strengths of the wings, the wing widths are primarily dependent on the neutral hydrogen column density $N_{\rm HI}$ being roughly proportional to $N_{\rm HI}^{1/2}$. In particular, with $N_{\rm HI}=10^{23}{\rm cm^{-2}}$ the H$\alpha$ wings typically show a width $\sim 2\times 10^4{\rm km s^{-1}}$. We also find that H$\alpha$ and Pa$\alpha$ wing profiles are asymmetric with the red part stronger than the blue part and an opposite behavior is seen for H$\beta$ wings.

VLTI/AMBER observations of the Seyfert nucleus of NGC 3783

Context. The putative tori surrounding the accretion disks of active galactic nuclei (AGNs) play a fundamental role in the unification scheme of AGNs. Infrared long-baseline interferometry allows us to study the inner dust distribution in AGNs with unprecedented spatial resolution over a wide infrared wavelength range. Aims. Near- and mid-infrared interferometry is used to investigate the milli-arcsecond-scale dust distribution in the type 1.5 Seyfert nucleus of NGC 3783. Methods. We observed NGC 3783 with the VLTI/AMBER instrument in the K-band and compared our observations with models. Results. From the K-band observations, we derive a ring-fit torus radius of 0.74 +/- 0.23 mas or 0.16 +/- 0.05 pc. We compare this size with infrared interferometric observations of other AGNs and UV/optical-infrared reverberation measurements. For the interpretation of our observations, we simultaneously model our near- and mid-infrared visibilities and the SED with a temperature/density-gradient model including an additional inner hot 1400 K ring component.

Radiative Transfer Simulations of AGN Dust Tori

The presence of dust tori in active galactic nuclei (AGN) lies at the heart of AGN unification schemes. The dominating inner part of the tori can only be resolved by infrared interferometry. Here we discuss the present state of torus models and the radiative transfer methods to simulate the infrared SEDs and visibilities for comparison with actual observations. We highlight the differences between smooth and clumpy tori and present a model for the Seyfert 2 nucleus NGC 1068.

On the relativistic iron line and soft excess in the Seyfert 1 galaxy Markarian 335

Abstract We report a 133-ks XMM–Newton observation of the Seyfert 1 galaxy Markarian 335. The 0.4–12 keV spectrum contains an underlying power-law continuum, a soft excess below 2 keV, and a double-peaked iron emission feature in the 6–7 keV range. We investigate the possibility that the double-peaked emission might represent the characteristic signature of the accretion disc. Detailed investigations show that a moderately broad accretion disc line is most likely present, but that the peaks may be due to narrower components from more distant material. The peaks at 6.4 and 7 keV can be identified, respectively, with the molecular torus in active galactic nucleus unification schemes, and very highly ionized, optically thin gas filling the torus. The X-ray variability spectra on both long (∼100 ks) and short (∼1 ks) time-scales do not support the recent suggestion that the soft excess is an artefact of variable, moderately ionized absorption.

Polycyclic Aromatic Hydrocarbons as a Tracer of Star Formation?

Infrared (IR) emission features at 3.3, 6.2, 7.7, 8.6, and 11.3 ?m are generally attributed to IR fluorescence from (mainly) far-ultraviolet (FUV) pumped large polycyclic aromatic hydrocarbon (PAH) molecules. As such, these features trace the FUV stellar flux and are thus a measure of star formation. We examined the IR spectral characteristics of Galactic massive star-forming regions and of normal and starburst galaxies, as well as active galactic nuclei (AGNs) and ultraluminous infrared galaxies (ULIRGs). The goal of this study is to analyze whether PAH features are a good qualitative and/or quantitative tracer of star formation, and hence to evaluate the application of PAH emission as a diagnostic tool in order to identify the dominant processes contributing to the infrared emission from Seyfert galaxies and ULIRGs. We develop a new mid-infrared (MIR)/far-infrared (FIR) diagnostic diagram based on our Galactic sample and compare it to the diagnostic tools of Genzel and coworkers and Laurent and coworkers, with these diagnostic tools also applied to our Galactic sample. This MIR/FIR diagnostic is derived from the FIR normalized 6.2 ?m PAH flux and the FIR normalized 6.2 ?m continuum flux. Within this diagram, the Galactic sources form a sequence spanning a range of 3 orders of magnitude in these ratios, ranging from embedded compact H II regions to exposed photodissociation regions (PDRs) and the (diffuse) interstellar medium (ISM). However, the variation in the 6.2 ?m PAH feature-to-continuum ratio is relative small. Comparison of our extragalactic sample with our Galactic sources revealed an excellent resemblance of normal and starburst galaxies to exposed PDRs. While Seyfert 2 galaxies coincide with the starburst trend, Seyfert 1 galaxies are displaced by at least a factor of 10 in 6.2 ?m continuum flux, in accordance with general orientation-dependent unification schemes for AGNs. ULIRGs show a diverse spectral appearance. Some show a typical AGN hot dust continuum. More, however, either are starburst-like or show signs of strong dust obscuration in the nucleus. One characteristic of the ULIRGs also seems to be the presence of more prominent FIR emission than either starburst galaxies or AGNs. We discuss the observed variation in the Galactic sample in view of the evolutionary state and the PAH/dust abundance and discuss the use of PAHs as quantitative tracers of star formation activity. Based on these investigations, we find that PAHs may be better suited as a tracer of B stars, which dominate the Galactic stellar energy budget, than as a tracer of massive star formation (O stars).

What Do theHubble Space TelescopeandChandraTell Us about the Jet and the Nuclear Region of the Radio Galaxy 3C 270?

The Hubble Space Telescope (HST)/Space Telescope Imaging Spectrograph ultraviolet image of the FR I radio galaxy 3C 270 shows the presence of a jetlike structure emerging from the position of the nucleus. This feature, which represents the first jetlike component ever detected in the UV in a radio galaxy with jets lying almost on the plane of the sky, has the same position angle as the jet in the radio and X-ray images. We propose two different scenarios for the origin of the emission: (1) nonthermal synchrotron from a mildly relativistic component of the jet and (2) scattered light from the nucleus, where a BL Lac source may be hosted. Either of these pictures would have important consequences for the active galactic nuclei unification schemes and for our knowledge of the jet structure. In the Chandra image, a faint counterjet is also present. From a comparative analysis of the HST images and Chandra X-ray spectrum, we find that the nucleus is only moderately obscured. The obscuring structure might well reside in the geometrically thin dark disk observed on large scales. This fits the scenario in which a standard geometrically and optically thick torus is not present in FR I radio galaxies.

Radiation fields of disk, BLR and torus in quasars and blazars: implications for γ-ray absorption

The radiation fields external to the jets and originating from within a few parsecs from the black hole, are discussed in this paper. They are the direct radiation from an accretion disk in symbiosis with jets, the radiation field from the broad line region (BLR) surrounding the accretion disk, and the infrared radiation from a dusty torus. The jet/disk symbiosis modifies the energetics in the central parsec of active galactic nuclei (AGN) such that for a given accretion rate, a powerful jet would occur at the expense of the disk luminosity, and consequently the disk would less efficiently ionize the BLR clouds or heat the dust in the torus, thereby affecting potentially important target photon fields for interactions of γ-rays, accelerated electrons and protons along the jet. Motivated by unification schemes of AGN, we briefly review the evidence for the existence of BLRs and small-scale dust tori in BL Lacs and Fanaroff-Riley Class I (FR-I) radio galaxies. We propose that an existing jet-accretion disk symbiosis can be extrapolated to provide a large-scale symbiosis between other important dusty constituents of the blazar/FR-I family. In the present paper, we discuss in the context of this symbiosis interactions of GeV and TeV γ-rays produced in the jet with the various radiation fields external to the jet in quasars and blazars, taking account the anisotropy of the radiation.

Disk-driven hydromagnetic winds as a key ingredient of active galactic nuclei unification schemes

Centrifugally driven winds from the surfaces of magnetized accretion disks have been recognized as an attractive mechanism of removing the angular momentum of the accreted matter and of producing the bipolar outflows and jets that are often associated with compact astronomical objects. As previously suggested in the context of young stellar objects, such winds have unique observational manifestations stemming from their highly stratified density and velocity structure and from their exposure to the strong continuum radiation field of the compact object. We have applied this scenario to active galactic nuclei (AGNs) and investigated the properties of hydromagnetic outflows that originate within approximately 10(M(sub 8)) pc of the central 10(exp 8)(M(sub 8)) solar mass black hole. On the basis of our results, we propose that hydromagnetic disk-driven winds may underlie the classification of broad-line and narrow-line AGNs (e.g., the Seyfert 1/Seyfert 2 dichotomy) as well as the apparent dearth of luminous Seyfert 2 galaxies. More generally, we demonstrate that such winds could strongly influence the spectral characteristics of Seyfert galaxies, QSOs, and BL Lac objects (BLOs). In our picture, the torus is identified with the outer regions of the wind where dust uplifted from the disk surfaces by gas-grain collisions is embedded in the outflow. Using an efficient radiative transfer code, we show that the infrared emission of Seyfert galaxies and QSOs can be attributed to the reprocessing of the UV/soft X-ray AGN continuum by the dust in the wind and the disk. We demonstrate that the radiation pressure force flattens the dust distribution in objects with comparatively high (but possibly sub-Eddington) bolometric luminosities, and we propose this as one likely reason for the apparent paucity of narrow-line objects among certain high-luminosity AGNs. Using the XSTAR photoionization code, we show that the inner regions of the wind could naturally account for the warm (greater than or approximately equal to 10(exp 5) K) and hot (greater than or approximately equal to 10(exp 6) K) gas components that have been inferred to exist on scales less than or approximately equal to 10(exp 2) pc in several Seyfert galaxies. We suggest that the partially ionized gas in the inner regions of the wind, rather than the dusty, neutral outflow that originates further out in the disk, could account for the bulk of the X-ray absorption in Seyferts observed at relatively small angles to their symmetry axes. Finally, we discuss the application of this model to the interpretation of the approximately 0.6 keV X-ray absorption feature reported in several BLOs.