Active Galactic Nuclei Continuum Research Articles

Mid‐InfraredSpitzerSpectra of X‐Ray–Selected Type 2 QSOs: QSO2s Are Not Ultraluminous Infrared Galaxies

We have performed a spectroscopic study of seven Type 2 QSOs using the mid-infrared spectrometer IRS on board the Spitzer Space Telescope. These are (to our knowledge) the first mid-IR spectra of X-ray-selected QSO2s taken. The objects have been selected according to their high intrinsic luminosities and column densities in X-rays. Their spectra strongly differ from template spectra of Type 2 active galactic nuclei (AGNs) at lower luminosities. They do not exhibit strong PAH dust emission features from circumnuclear star-forming regions, typical for lower luminosity Type 2 Seyfert galaxies or other previously used QSO2 templates, such as the (ultra)luminous infrared galaxy ([U]LIRG) NGC 6240. They also do not show the ice and silicate absorption features of highly luminous but deeply embedded compact nuclei seen in some ULIRGs. Instead, they reveal a relatively featureless, rising continuum similar to luminous Type 1 AGNs. We also find evidence for a 10 μm silicate feature in emission. Models of dusty tori in the AGN unification scenario predict this only for Type 1 AGNs. The ratio of the AGN continuum luminosity at 6 μm to the absorption-corrected 2-10 keV X-ray AGN luminosity is very similar to that found in Seyfert galaxies. X-ray-selected QSO2s are thus characterized by powerful AGNs in hosts with a luminosity due to star formation 1011 L☉. The dominance of the AGN light in the mid-IR spectra of QSO2s together with their flatter spectral energy distributions (SEDs) places important constraints on models of the cosmic infrared background and of the star formation history of the universe.

Mid‐Infrared Selection of Active Galaxies

Mid-infrared photometry provides a robust technique for identifying active galaxies. While the ultraviolet to mid-infrared (λ 5 μm) continuum of stellar populations is dominated by the composite blackbody curve and peaks at approximately 1.6 μm, the ultraviolet to mid-infrared continuum of active galactic nuclei (AGNs) is dominated by a power law. Consequently, with a sufficient wavelength baseline, one can easily distinguish AGNs from stellar populations. Mirroring the tendency of AGNs to be bluer than galaxies in the ultraviolet, where galaxies (and stars) sample the blue, rising portion of stellar spectra, AGNs tend to be redder than galaxies in the mid-infrared, where galaxies sample the red, falling portion of the stellar spectra. We report on Spitzer Space Telescope mid-infrared colors, derived from the IRAC Shallow Survey, of nearly 10,000 spectroscopically identified sources from the AGN and Galaxy Evolution Survey. On the basis of this spectroscopic sample, we find that simple mid-infrared color criteria provide remarkably robust separation of active galaxies from normal galaxies and Galactic stars, with over 80% completeness and less than 20% contamination. Considering only broad-lined AGNs, these mid-infrared color criteria identify over 90% of spectroscopically identified quasars and Seyfert 1 galaxies. Applying these color criteria to the full imaging data set, we discuss the implied surface density of AGNs and find evidence for a large population of optically obscured active galaxies.

Probing the Ionizing Continuum of Narrow‐Line Seyfert 1 Galaxies. I. Observational Results

We present optical spectra and emission-line ratios of 12 narrow-line Seyfert 1 (NLS1) galaxies that we observed to study the ionizing EUV continuum. A common feature in the EUV continuum of active galactic nuclei is the big blue bump (BBB), generally associated with thermal accretion disk emission. While Galactic absorption prevents direct access to the EUV range, it can be mapped by measuring the strength of a variety of forbidden optical emission lines that respond to different EUV continuum regions. We find that narrow emission line ratios involving [O II] λ3727, Hβ, [O III] λ5007, [O I] λ6300, Hα, [N II] λ6583, and [S II] λλ6716, 6731 indicate no significant difference between NLS1 galaxies and broad-line Seyfert 1 (BLS1) galaxies, which suggests that the spectral energy distributions of their ionizing EUV-soft X-ray continua are similar. The relative strength of important forbidden high-ionization lines like [Ne V] λ3426 compared to He II λ4686 and the relative strength of [Fe X] λ6374 appear to show the same range as in BLS1 galaxies. However, a trend of weaker F([O I] λ6300)/F(Hα) emission-line ratios is indicated for NLS1 galaxies compared to BLS1 galaxies. To recover the broad emission line profiles, we used Gaussian components. This approach indicates that the broad Hβ profile can be well described with a broad component (FWHM 3275 ± 800 km s-1) and an intermediate broad component (FWHM 1200 ± 300 km s-1). The width of the broad component is in the typical range of normal BLS1 galaxies. The emission-line flux that is associated with the broad component in these NLS1 galaxies amounts to at least 60% of the total flux. Thus, it dominates the total line flux, similar to BLS1 galaxies.

Physical Conditions in the Narrow‐Line Region of Markarian 3. I. Observational Results

We use Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) long-slit low-resolution spectroscopy from 1150 to 10300 A to study the physical conditions in the narrow-line region (NLR) of the Seyfert 2 galaxy Markarian 3. We find from the He II λ1640/λ4686 line ratio and the Balmer decrement that the extinction within Mrk 3 along the line of sight to the NLR is best characterized by an LMC-type extinction curve. We observe an extinction gradient increasing from west to east along the STIS slit (at position angle 71° measured east from north) in both line and continuum emission. We infer from this gradient that the host galaxy disk is tilted toward the observer in the east: the line of sight to the eastern emission-line cone intersects more dust in the plane of the galaxy than that to the western cone. From emission-line diagnostics we find that the NLR gas is photoionized by the hidden active galactic nucleus continuum and that its density decreases with increasing distance from the center. We model the observed continuum as a combination of reddened host galaxy light from an old stellar population, reddened H+ and He+2 recombination continua, and less reddened scattered light from the central engine with spectral index α = 1 (Lν ∝ ν-α). The host galaxy-to-scattered light ratio is estimated to be 3 : 1 at 8125 A in a 01 × 18 aperture. Using a two-component power-law model for the ionizing continuum (α = 2 for 13.6 eV < E < 0.2 keV and α = 1 for 0.2 keV < E < 50 keV), we find that the covering factor (normalized for our observation aperture) of the NLR gas is greater than 0.7%. We estimate that the amount of intrinsic nonionizing UV continuum scattered into our line of sight is 0.04%. This is consistent with our estimate of the scattering fraction for broad C IV λλ1548, 1551 emission.

Fe xxv and Fe xxvi lines from low-velocity, photoionized gas in the X-ray spectra of active galactic nuclei

We have calculated the equivalent widths of the absorption lines produced by Fexxv and Fexxvi in a Compton-thin, low-velocity photoionized material illuminated by the nuclear continuum in active galactic nuclei. The results, plotted against the ionization parameter and the column density of the gas, are a complement to those presented by Bianchi & Matt for the emission lines from the same ionic species. As an extension to the work by Bianchi & Matt, we also present a qualitative discussion on the different contributions to the He-like iron emission line complex in the regimes where recombination or resonant scattering dominates, providing a useful diagnostic tool to measure the column density of the gas. Future high-resolution missions (e.g. Astro-E2) will allow us to fully take advantage of these plasma diagnostics. In the meantime, we compare our results with an up-to-date list of Compton-thick and unobscured (at least at the iron line energy) Seyfert galaxies with emission and/or absorption lines from H- and He-like iron observed with Chandra and XMM–Newton.

Observational Requirements for High‐Fidelity Reverberation Mapping

ABSTRACTWe present a series of simulations to demonstrate that high‐fidelity velocity‐delay maps of the emission‐line regions in active galactic nuclei (AGNs) can be obtained from time‐resolved spectrophotometric data sets like those that will arise from the proposed Kronos satellite. While previous reverberation‐mapping experiments have established the size scale R of the broad emission‐line regions from the mean time delay τ = R/c between the line and continuum variations and have provided strong evidence for supermassive black holes, the detailed structure and kinematics of the broad‐line region remain ambiguous and poorly constrained. Here we outline the technical improvements that will be required to successfully map broad‐line regions by reverberation techniques. For typical AGN continuum light curves, characterized by power‐law power spectra P(f)∝f-α with α = -1.5 ± 0.5, our simulations show that a small UV/optical spectrometer like Kronos will clearly distinguish between currently viable alternative kinematic models. From spectra sampled at time intervals Δt and sustained for a total duration Tdur, we can reconstruct high‐fidelity velocity‐delay maps with velocity resolution comparable to that of the spectra, and delay resolution Δτ ≈ 2Δt, provided that Tdur exceeds the broad‐line region light crossing time by at least a factor of 3. Even very complicated kinematical models, such as a Keplerian flow with superimposed spiral wave pattern, are resolved in maps from our simulated Kronos data sets. Reverberation mapping with Kronos data is therefore likely to deliver the first clear maps of the geometry and kinematics in the broad emission‐line regions 1–100 μas from supermassive black holes.

The infrared continuum of active galactic nuclei

We discuss the different physical processes contributing to the infrared continuum of active galactic nuclei (AGNs), assuming that both photoionization from the active centre and shocks ionize and heat the gas and dust contained in an ensemble of clouds surrounding the nucleus. In our model, radiation transfer of primary and secondary radiation throughout a cloud is calculated consistently with collisional processes due to the shock. We consider that the observed continuum corresponds to reprocessed radiation from both dust and gas in the clouds. Collisional processes are important in the presence of shocks. The grains are sputtered crossing the shock front. The models are constrained by sputtering as well as by the far-infrared data. The model is applied to the continuum of Seyfert galaxies from which the best estimate of the nuclear, stellar subtracted, emission is available. The results show that radiation-dominated high-velocity clouds are more numerous in Seyfert 1-1.5 whereas shock-dominated low-velocity clouds are dominant in Seyfert type 2. This result is in full agreement with the unified model for AGNs, by which high-velocity clouds, placed deeper into the central region and therefore reached by a more intense radiation, should play a more significant role in the spectra of broad-line objects. We could therefore conclude that in type 2 objects, radiation is partly suppressed by a central dusty medium with a high dust-to-gas ratio. Once the model approach is tested, a grid of models is used to provide a phenomenological analysis of the observed infrared spectral energy distribution. This empirical method is a useful tool to rapidly access the physical conditions of the AGN emitting clouds. For this, analytical forms are derived for the two processes contributing to the infrared emission: dust emission and thermal bremsstrahlung produced by the narrow-line region clouds. Their relative contribution provides a measurement of the dust-to-gas ratio.

Optical Photometry and the Continuum of Active Galactic Nuclei

In this paper, the continuum emission of active galactic nuclei is studied using broadband B, V, R, and I photometry. The nuclear contribution is estimated from the observations using two different approaches. In the first one, the images are deconvolved by the seeing profile and the corrected images are used to derive the nuclear contribution. In the second method, in order to extract the stellar contribution, a de Vaucouleurs brightness profile is assumed for the bulge. After subtraction of this component, the total nuclear emission is obtained from the corrected image. Both methods indicate that the stellar contribution is dominant. The average contribution of the featureless continuum to the total observed continuum is 30% in the four bands. We show that over 2 orders of magnitude the Hα emission-line luminosity correlates with the continuum emission in all the observed bands. As a consequence, the galaxies in the sample must have similar color index. This result provides a method to estimate the reddening correction for the nuclear continuum, which we found to be lower than the emission-line extinction correction estimated from the observed Hα/Hβ observed line ratio.

Ultraviolet Hubble Space Telescope Snapshot Survey of 3CR Radio Source Counterparts at Low Redshift

We present UV images of 27 3CR radio galaxies with redshifts z < 0.1 that have been imaged with the Space Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope (HST). The observations employed the NUV MAMA and broadband filters with peak sensitivity at 2200 A. We find that the UV luminosities show approximately a factor of 10-100 higher dispersion than the optical. We compare the UV morphologies with optical V- and R-band WFPC2 snapshot survey images. We have found dramatic, complex, and extended UV emission from radio galaxies even at zero redshift. We find a diverse range of UV morphologies, some completely divergent from their visual morphology, which are reminiscent of the chaotic high-z radio galaxy structures seen in rest-frame UV. The UV morphologies show regions of star formation, jets, and possible scattered active galactic nucleus continuum. The UV emission is generally not aligned with the radio structure. We also detect the diffuse UV emission of the host galaxy. We propose that these are the same physical phenomena as observed at high redshift, but on a smaller spatial scale.

The optical/near-infrared broad-line emission and hard X-ray continuum of active galactic nuclei and ultraluminous infrared galaxies

We compare the luminosities of the hard X-ray and optical/near-infrared broad-line emission from activegalactic nuclei (AGNs) and ultraluminous infrared galaxies (ULIRGs). Both types of emission are thought to be good indicators of activity driven by supermassive black holes. We selected objects for which both X-ray and broad-line data are available, and, additionally, for which we can obtain information about the far-infrared and/or morphological properties. We ®nd that, although the hard X-ray to broad Ha luminosity ratio LX=L…broad Ha† is relatively constant in nearly all of the objects examined, there exist some objects showing a distinctly small LX=L…broad Ha† ratio. These small LX=L…broad Ha† objects are thought to be undergoing a

Infrared Spectroscopy of NGC 4151: Probing the Obscured Ionizing Active Galactic Nucleus Continuum

The Infrared Space Observatory (ISO) Short-Wavelength Spectrometer (SWS) infrared spectroscopic observations of the nucleus of Seyfert galaxy NGC 4151, which are described in a companion paper, are used in conjunction with a compilation of UV to IR narrow emission line data to determine the spectral shape of the obscured extreme-UV continuum that photoionizes the narrow-line-emitting gas in the active galactic nucleus (AGN). We present a new method for determining the best-fitting photoionizing continuum and emission-line cloud model from a heterogeneous set of emission-line data. For NGC 4151, we find a best-fit model that reproduces the observed line fluxes to within a factor of 2 on average, and that is consistent with the observed geometry of the optical narrow-line region (NLR). Our model consists of a clumpy, optically thick (ionization-bounded) gas distribution, with a hydrogen gas density of ~1000 cm-3 and a volume-filling factor of 6.510-4. Our best-fitting spectral energy distribution (SED) falls sharply beyond the Lyman limit and then rises sharply again toward 100 eV. In particular, it does not display the blue signature of a hot accretion disk. We find that this SED, which best reproduces the NLR line emission on the 100-500 pc scale, does not have enough UV photons to produce the observed broad-line region (BLR) recombination emission from the inner 1 pc. This suggests that the BLR is photoionized by the intrinsic continuum source, which does have a strong UV component (perhaps a big blue bump), but that this UV component is absorbed by material located between the NLR and BLR. Our analysis suggests that the absorber consists of ~51019 cm-2 of neutral hydrogen. Such an absorber has been independently detected by UV absorption lines. Using our new method, we confirm our previous conclusion that a big blue bump is present in the SED of the Seyfert 2 galaxy Circinus.

Chemical Abundance Calibrations for the Narrow-Line Region of Active Galaxies

We investigate two chemical abundance calibrations for the narrow-line region (NLR) of active galaxies in terms of three easily observable optical emission-line ratios, namely, [O III] λλ4959, 5007/Hβ; [N II] λλ6548, 6584/Hα; and [O II] λ3727/[O III] λλ4959, 5007. The calibrations are obtained from a grid of models on the assumption that the main process responsible for the production of these lines is photoionization by a typical active galactic nucleus continuum. The elemental abundances vary with oxygen, except nitrogen, which is assumed to be a product of secondary nucleosynthesis. The calibrations are calculated for the range 8.4 ≤ 12 + log (O/H) ≤ 9.4 and tested using NLR data for a sample of Seyfert galaxies and LINERs that have H II regions in the vicinity of the nucleus. The gaseous abundances of these H II regions have been determined in previous works, and the NLR abundances are obtained on the assumption that they can be extrapolated from those of the H II regions. The calibrations work very well for the Seyfert galaxies, yielding abundance values that agree with those obtained from the H II regions, and can thus be used for quick estimates of the chemical abundances of the NLRs. For the LINERs, the calibrations yield lower values than those derived from the corresponding H II regions, suggesting that the assumptions of the models do not apply for them and that there are different physical processes at work in the NLR of the LINERs.

Deep [ITAL]Hubble Space Telescope[/ITAL]/Planetary Camera Imaging of a Young Compact Radio Galaxy at [CLC][ITAL]z[/ITAL][/CLC] = 2.390

We present deep 63-orbit Hubble Space Telescope/Planetary Camera images at ~006 FWHM resolution in the filters B450, V606, and I814—as well as in redshifted Lyα—of the radio source 53W002, a compact narrow-line galaxy at z = 2.390 from the Leiden-Berkeley Deep Survey. These images allow us to distinguish several morphological components: (1) an unresolved nuclear point source (500 pc at z=2.390 for H0=75, q0 = 0), likely the central active galactic nucleus (AGN) that contains 20%-25% of the total light in BVI; (2) a compact continuum core (re 005); (3) a more extended envelope with an r1/4-like light profile and re 025 (~2 kpc); (4) two blue roughly colinear across the nucleus, aligned with the radio source axis and contained well within the size of the radio source. The (B-I) color maps may suggest a narrow dust lane crossing between the nucleus and the smaller blue cloud. The radio source is not smaller than the distance between the blue continuum clouds and coincides with a bright Lyα arc in the western cloud, suggesting that jet-induced star formation could cause both blue clouds, except the outer parts of the western cloud. The shape of this larger blue cloud suggests reflected AGN continuum light shining through a cone (plus reradiated Lyα in emission). The Owens Valley Radio Observatory interferometric CO detection (Scoville et al.) on both sides of 53W002—and in the same direction as the continuum clouds and the radio jet—also suggests a star-bursting region induced by its radio jet, at least in the inner parts. Hence, both mechanisms likely play a role in the alignment effect. Even at radio powers ~1.5 dex fainter than the 3CR sources, we thus find many of the same aligned features and complex morphology, although at much smaller angular scales and lower optical-UV luminosities. We discuss the consequences for 53W002's formation in the context of the 16 subgalactic objects at z2.40 around 53W002 (Pascarelle et al.).

Emission Line and Ultraviolet to X‐Ray Continuum Correlations: Constraints on the Anisotropy of the Ionizing Continuum in Active Galactic Nuclei

Anisotropic emission of the ionizing continuum is a general prediction of accretion disk models. In this paper, we present the results of correlation analysis of the UV emission line and UV to X-ray continuum properties for a large sample of broad emission line AGNs observed with ROSAT, IUE, and HST. We find strong correlations between the C IV/Lyα ratio, the equivalent width of C IV, and the UV to soft X-ray spectral slope. The results are in good agreement with the photoionization calculation, suggesting that the overall ionizing continuum can well match the observed UV to soft X-ray spectrum. These results are consistent with the assumption of an isotropic ionizing continuum shape. Our analysis suggests a small range for the big blue bump cutoff energy for the objects in this sample, consistent with the similar results of Laor et al. and Walter & Fink based on continuum properties. The mean UV to X-ray spectral slope is similar to the soft X-ray spectral slope. This similarity also holds for radio-loud and radio-quiet objects separately. This suggests that the two might be drawn from the same distribution. The two spectral slopes are only weakly correlated. The UV to X-ray spectral index is correlated with absolute optical magnitude. This result confirms the earlier suggestion that the ionizing continua are softer for higher luminosity objects.

The vertical structure and ultraviolet spectrum of X-ray irradiated accretion disks in active galactic nuclei

Motivated by recent work indicating that the UV continuum in active galactic nuclei may be produced by reradiation of energy absorbed from X-rays irradiating an accretion disk, we present a calculation of the vertical structures and ultraviolet spectra of X-ray-irradiated accretion disks around massive nonrotating black holes. After finding the radial dependence of vertically integrated quantities for these disks, we solve the equations of hydrostatic equilibrium, energy balance, and frequency-dependent radiation transfer as functions of altitude. To solve the last set of equations, we use a variable Eddington factor method. We include electron scattering, free-free and H I, He I, and He II bound-free opacities, and the corresponding continuum cooling processes. The incident X-ray flux heats a thin layer of material 3-4 scale heights above the midplane of the disk. This X-ray-heated skin has two layers: (1) a radiation pressure-supported region in which the UV flux is created, and, immediately above this layer, (2) a warmer zone, optically thin to UV radiation, formed where the X-ray ionization parameter is large. In the lower layer, the gas pressure is nearly independent of altitude, but the temperature increases upward. The fraction of the incident hard X-ray flux that emerges in the UV falls with increasing ${u{m}{705F}}$ --> (the accretion rate in Eddington units). At frequencies below the Lyman edge, the slope of the continuum (d ln L?/d ln ?) varies from -1.6 to 0.8 as ${u{m}{705F}}$ -->/m -->8 increases from 0.001 to 1. Here m8 is the mass of the central black hole in units of 108 M?. In all cases examined (0.003?${u{m}{705F}}$ -->?0.3 and 0.27 ? m8 ? 27), the Lyman edge appears in emission. The amplitude of the Lyman edge feature increases with m8 but is relatively independent of ${u{m}{705F}}$ -->. The amplitude of the Lyman edge emission feature increases with disk inclination. Compton scattering in disk coronae can smooth the Lyman edge feature only if ?c 0.5, where ?c is the Thomson depth of the coronae. While the overall spectral shape predicted by X-ray irradiation may be compatible with observations, the Lyman edge emission feature it predicts is not. This finding raises questions for many otherwise plausible models in which X-ray irradiation plays a major role.

The role of an extended corona in the formation of emission lines and continuum in active galactic nuclei

We study a model of an accretion disc surrounded by an extended corona of the temperature 10^7 - 10^8 K. This corona modifies the disk spectrum since it redirects a significant fraction of the emission from the central parts towards the more distant parts of the disk. The same corona is indirectly the source of the broad emission lines because we expect the formation of cool clouds at the basis of the corona due to thermal instabilities. We model the number of the clouds and their motion through the corona adopting a few different physically sound assumptions. Comparing the predicted optical/UV continua and emission line ratios and profiles with the observed values we favor a particular model of a typical quasar. It radiates at ~0.5 of the Eddington luminosity, the corona surrounding the disk is additionally heated in excess of the Inverse Compton heating, and the broad line clouds are most probably destroyed when accelerated vertically above the sound speed within the corona.

X-ray and optical continua of active galactic nuclei with extreme Fe II emission

We present the results of ROSAT PSPC observations of three active galactic nuclei (AGN) with extremely strong Fe IT emission (PHL 1092, lRAS 07598 + 6508 and I Zw 1) and two AGN with very weak Fe IT emission (Mrk 10 and 110). The weak Fe II emitters have X-ray spectra typical of Type 1 AGN (OC = 1.35 and 1.41, where oc is the spectral energy index). Of the strong Fe II emitters, two have steep spectra (PHL 1092 has oc = 3.5, and I Zw 1 has = 2.0) and the third, lRAS 07598 + 6508, is barely detected and so is extremely X-ray-quiet (ocox =2.45). During our observations, PHL 1092 varied by a factor of 4, unusually fast for such a high­ luminosity object, and requiring an efficiency of matter-to-energy conversion of 2 per cent or more. Compiling recently published data on other strong Fe II emitters, we find that they are always X-ray-quiet, and usually X-ray-steep. Adding these data to the complete uvx: -selected quasar sample of Laor et aI., we find a statistical connection of Fe IIIHP with ex.: but not a simple relationship: weak Fe II emitters always have flat spectra, but strong Fe II emitters can be either flat or steep. A much cleaner relationship exists between Fe II strength and X-ray loudness, as quantified by OCix' the spectral index between 11lII1 and 2 keY. We also confirm that Fe IIIHP anticorrelates with Balmer line velocity width, which in tum correlates well with both ex.: and OCix in the sense that AGN with narrow lines are X-ray-quiet. There is also marginal evidence that Fe IIIHP correlates with both optical continuum slope and the curvature of the optical-UV-X-ray continuum: strong Fe IT objects tend to have steeper continua and weaker 'blue bumps'. The amount of extinction required to explain the optical steepening compared to normal quasars [E (B - V) in the range 0.2 to 0.6] suggests absorbing columns in the range (1- 3) x 1021 cm -2, just about the right amount to reduce the ROSAT-band X-ray flux by enough to explain

The Observational Appearance of Slim Accretion Disks

We reexamine the hypothesis that the optical/UV/soft X-ray continuum of Active Galactic Nuclei is thermal emission from an accretion disk. Previous studies have shown that fitting the spectra with the standard, optically thick and geometrically thin accretion disk models often led to luminosities which contradict the basic assumptions adopted in the standard model. There is no known reason why the accretion rates in AGN should not be larger than the thin disk limit. In fact, more general, slim accretion disk models are self-consistent even for moderately super-Eddington luminosities. We calculate here spectra from a set of thin and slim, optically thick accretion disks. We discuss the differences between the thin and slim disk models, stressing the implications of these differences for the interpretation of the observed properties of AGN. We found that the spectra can be fitted not only by models with a high mass and a low accretion rate (as in the case of thin disk fitting) but also by models with a low mass and a high accretion rate. In the first case fitting the observed spectra in various redshift categories gives black hole masses around 10^9 solar masses for a wide range of redshifts, and for accretion rates ranging from 0.4 to 8 solar masses/year. In the second case the accretion rate is around 10^2 solar masses/year for all AGN and the mass ranges from 3*10^6 to 10^8 solar masses. Unlike the disks with a low accretion rate, the spectra of the high-accretion-rate disks extend into the soft X-rays. A comparison with observations shows that such disks could produce the soft X-ray excesses claimed in some AGNs. We show also that the sequence of our models with fixed mass and different accretion rates can explain the time evolution of the observed spectra in Fairall 9.

The broad continuum of active galactic nuclei and their emission mechanism. II. The infrared spectrum: Wu Jian-xin & Zhou You=yuanPhysics Department, South China Normal University, Guangzhou 510631

The broad continuum of active galactic nuclei and their emission mechanism. II. The infrared spectrum: Wu Jian-xin & Zhou You=yuanPhysics Department, South China Normal University, Guangzhou 510631

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.