Makoto Kishimoto

The dusty heart of nearby active galaxies - II. From clumpy torus models to physical properties of dust around AGN

With the possibilities of high spatial resolution imaging and spectroscopy as well as infrared (IR) interferometry, the dusty environments (= “dusty torus”) of active galactic nuclei (AGN) are now in reach of observations. Following our Paper I on ground-based mid-IR spectro-photometry, we present an upgrade to our radiative transfer model of three-dimensional clumpy dust tori. The upgrade with respect to earlier work concerns an improved handling of the diffuse radiation field in the torus, which is approximated by a statistical approach. The models are presented as tools to translate classical and interferometric observations into characteristic properties of the dust distribution. We compare model spectral energy distributions (SEDs) for different chemical and grain-size compositions of the dust and find that clouds with standard interstellar matter (ISM) dust and optical depth τV ∼ 50 appear in overall agreement with observed IR SEDs. By studying parameter dependencies, it is shown that type 1 AGN SEDs, in particular the mid-IR spectral index, can be used to constrain the radial dust cloud distribution power law index a, while other parameters are more difficult to assess using SEDs only. Interferometry adds important additional information for modeling when it is interpreted concurrently with the SED. Although type 2 AGN can in principle be used to constrain model parameters as well, obscuration effects make the analysis more ambiguous. We propose a simple, interferometry-based method to distinguish between “compact” and “extended” radial dust distributions without detailed modeling of the data and introduce a way to easily determine individual or sample average model parameters using the observed optical depth in the silicate feature and the mid-IR spectral index.

The dusty heart of nearby active galaxies. I. High-spatial resolution mid-IR spectro-photometry of Seyfert galaxies

In a series of papers, we aim at stepping towards characterizing physical properties of the AGN dust torus by combining IR highspatial resolution observations with 3D clumpy torus models. In this first paper, we present mid-IR imaging and 8−13 μ ml owresolution spectroscopy of nine type 1 and ten type 2 AGN. The observations were carried out with the VLT/VISIR mid-IR imager and spectrograph and can be considered the largest currently available mid-infrared spectro-photometric data set of AGN at spatial resolution <100 pc. These data resolve scales at which the emission from the dust torus dominates the overall flux, and emission from the host galaxy (e.g. star-formation) is resolved out in most cases. The silicate absorption features are moderately deep and emission features, if seen at all, are shallow. The strongest silicate emission feature in our sample shows some notable shift of the central wavelength from the expected 9.7 μm (based on ISM extinction curves) to ∼10.5 μm. We compare the observed mid-IR luminosities of our objects to AGN luminosity tracers (X-ray, optical and [O iii] luminosities) and find that the mid-IR radiation is emitted quite isotropically. In two cases, IC 5063 and MCG–3–34–64, we find evidence for extended dust emission in the narrow-line region. We confirm the correlation between observed silicate feature strength and Hydrogen column density, which was recently found in Spitzer data at lower spatial resolution. In a further step, our 3D clumpy torus model has been used to interpret the data. We show that the strength of the silicate feature and the mid-IR spectral index α can be used to get reasonable constraints on the radial dust distribution of the torus and the average number of clouds N0 along an equatorial line-of-sight in clumpy torus models. The mid-IR spectral index α is almost exclusively determined by the radial dust distribution power-law index a, while the silicate feature depth mostly depends on N0 and the torus inclination. A comparison of model predictions to our type 1 and type 2 AGN reveals that average parameters of a = −1.0 ± 0. 5a ndN0 = 5 − 8 are typically seen in the presented sample, which means that the radial dust distribution is rather shallow. As a proof-of-concept of this method, we compared the model parameters derived from α and the silicate feature strength to more detailed studies of full IR SEDs and interferometry and found that the constraints on a and N0 are consistent. Finally, we may have found evidence that the radial structure of the torus changes from low to high AGN luminosities towards steeper dust distributions, and we discuss implications for the IR size-luminosity relation.

The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared

Quasars are thought to be powered by supermassive black holes accreting surrounding gas. Central to this picture is a putative accretion disk which is believed to be the source of the majority of the radiative output. It is well known, however, that the most extensively studied disk model—an optically thick disk which is heated locally by the dissipation of gravitational binding energy—is apparently contradicted by observations in a few major respects. In particular, the model predicts a specific blue spectral shape asymptotically from the visible to the near-infrared, but this is not generally seen in the visible wavelength region where the disk spectrum is observable. A crucial difficulty has been that, towards the infrared, the disk spectrum starts to be hidden under strong, hot dust emission from much larger but hitherto unresolved scales, and thus has essentially been impossible to observe. Here we report observations of polarized light interior to the dust-emitting region that enable us to uncover this near-infrared disk spectrum in several quasars. The revealed spectra show that the near-infrared disk spectrum is indeed as blue as predicted. This indicates that, at least for the outer near-infrared-emitting radii, the standard picture of the locally heated disk is approximately correct.

Mapping the radial structure of AGN tori

We present mid-IR interferometric observations of six type 1 AGNs at multiple baseline lengths ranging from 27 m to 130 m, reaching high angular resolutions up to λ/B ∼ 0.02 arcseconds. For two of the targets, we have simultaneous n ear-IR interferometric measurements as well, taken within a week. We find that all the objects are partially resolved at long baselines in these IR wavelengths. The multiple-baseline data directly probe the radial distr ibu ion of the material on sub-pc scales. We show that for our sample, which is small but spans over ∼2.5 orders of magnitudes in the UV /optical luminosityL of the central engine, the radial distribution clearly and systematically changes with luminosity. The brightness distribution at a given mid-IR wavelength se ems to be rather well described by a power law, which makes a si mple Gaussian or ring size estimation quite inadequate. In this c ase, a half-light radius R1/2 can be used as a representative size. We show that the higher luminosity objects become more compact in no rmalized half-light radiiR1/2/Rin in the mid-IR, whereRin is the dust sublimation radius empirically given by the L fit of the near-IR reverberation radii. This means that, cont rary to previous studies, the physical mid-IR emission size (e.g. in pc) is not proport ional to L, but increases withL much more slowly. With our current datasets, we find that R1/2 ∝ L at 8.5μm, andR1/2 nearly constant at 13 μm. The derived size information also seems to correlate with th e properties of the total flux spectrum, in particular the sma ller R1/2/Rin objects having bluer mid-IR spectral shape. We use a power-l aw temperature /density gradient model as a reference, and infer that the radial surface density distribution of the heated dust g rains at a radiusr changes from a steep ∼r−1 structure in high luminosity objects to a shallower ∼r0 structure in those of lower luminosity. The inward dust temp rature distribution does not seem to smoothly reach the sublimation temperature – on the innermost scale o f ∼Rin, a relatively low temperature core seems to co-exist with a s lightly distinct brightness concentration emitting roughly at the sublimation temperature.

The dusty torus in the Circinus galaxy: a dense disk and the torus funnel ,

Context. With infrared interferometry it is possible to resolve the nuclear dust distributions that are commonly associated with the dusty torus in active galactic nuclei (AGN). The Circinus galaxy hosts the closest Seyfert 2 nucleus and previous interferometric observations have shown that its nuclear dust emission is particularly well resolved. Aims. The aim of the present interferometric investigation is to better constrain the dust morphology in this active nucleus. Methods. To this end, extensive new observations were carried out with the MID-infrared Interferometric instrument (MIDI) at the Very Large Telescope Interferometer, leading to a total of 152 correlated flux spectra and differential phases between 8 and 13 μm. To interpret this data, we used a model consisting of black-body emitters with a Gaussian brightness distribution and with dust extinction. Results. The direct analysis of the data and the modelling confirm that the emission is distributed in two distinct components: a disklike emission component with a size (FWHM) of ∼0.2 × 1.1 pc and an extended component with a size of ∼0.8 × 1.9 pc. The disk-like component is elongated along PA ∼ 46 ◦ and oriented perpendicular to the ionisation cone and outflow. The extended component is responsible for 80% of the mid-infrared emission. It is elongated along PA ∼ 107 ◦ , which is roughly perpendicular to the disk component and thus in polar direction. It is interpreted as emission from the inner funnel of an extended dust distribution and shows a strong increase in the extinction towards the south-east. We find both emission components to be consistent with dust at T ∼ 300 K, that is we find no evidence of an increase in the temperature of the dust towards the centre. From this we infer that most of the near-infrared emission probably comes from parsec scales as well. We further argue that the disk component alone is not sufficient to provide the necessary obscuration and collimation of the ionising radiation and outflow. The material responsible for this must instead be located on scales of ∼1 pc, surrounding the disk. We associate this material with the dusty torus. Conclusions. The clear separation of the dust emission into a disk-like emitter and a polar elongated source will require an adaptation of our current understanding of the dust emission in AGN. The lack of any evidence of an increase in the dust temperature towards the centre poses a challenge for the picture of a centrally heated dust distribution.

The innermost dusty structure in active galactic nuclei as probed by the Keck interferometer

We are now exploring the inner region of type 1 active galactic nuclei (AGNs) with the Keck interferometer in the near-infrared. Adding to the four targets previously studied, we report measurements of the K-band (2.2 μm) visibilities for four more targets, namely AKN120, IC 4329A, Mrk6, and the radio-loud QSO 3C 273 at z = 0.158. The observed visibilities are quite high for all the targets, which we interpret as an indication of the partial resolution of the dust sublimation region. The effective ring radii derived from the observed visibilities scale approximately with L 1/2 , where L is the AGN luminosity. Comparing the radii with those from independent optical-infrared reverberation measurements, these data support our previous claim that the interferometric ring radius is either roughly equal to or slightly larger than the reverberation radius. We interpret the ratio of these two radii for a given L as an approximate probe of the radial distribution of the inner accreting material. We show tentative evidence that this inner radial structure might be closely related to the radio-loudness of the central engine. Finally, we re-observed the brightest Seyfert 1 galaxy NGC 4151. Its marginally higher visibility at a shorter projected baseline, compared to our previous measurements obtained one year before, further supports the partial resolution of the inner structure. We did not detect any significant change in the implied emission size when the K-band flux was brightened by a factor of 1.5 over a time interval of one year.

Possible evidence for a common radial structure in nearby AGN tori

We present a quantitative and relatively model-independent way to assess the radial structure of nearby AGN tori. These putative tori have been studied with long-baseline infrared (IR) interferometry, but the spatial scales probed are different for different objects. They are at various distances and also have different physical sizes that apparently scale with the luminosity of the central engine. Here we look at interferometric size information, or visibilities, as a function of spatial scales normalized by the size of the inner torus radius Rin. This approximately eliminates luminosity and distance dependence and, thus, provides a way to uniformly view the visibilities observed for various objects and at different wavelengths. We can construct a composite visibility curve over a wide range of spatial scales if different tori share a common radial structure. The currently available observations do suggest, independent of models, a common radial surface brightness distribution in the mid-IR that is roughly of a power-law form r −2 as a function of radius r and extends to ∼100 times Rin. Taking into account the temperature decrease toward outer radii with a simple torus model, this corresponds to the radial surface density distribution of dusty material directly illuminated by the central engine roughly in the range between r 0 and r −1 . This should be tested with further data.

EVIDENCE FOR A RECEDING DUST SUBLIMATION REGION AROUND A SUPERMASSIVE BLACK HOLE

The near-IR emission in Type 1 active galactic nuclei (AGNs) is thought to be dominated by the thermal radiation from dust grains that are heated by the central engine in the UV/optical and are almost at the sublimation temperature. A brightening of the central source can thus further sublimate the innermost dust, leading to an increase in the radius of the near-IR emitting region. Such changes in radius have been indirectly probed by the measurements of the changes in the time lag between the near-IR and UV/optical light variation. Here we report direct evidence for such a receding sublimation region through the near-IR interferometry of the brightest Type 1 AGN in NGC 4151. The increase in radius follows a significant brightening of the central engine with a delay of at least a few years, which is thus the implied destruction timescale of the innermost dust distribution. Compiling historic flux variations and radius measurements, we also infer the reformation timescale for the inner dust distribution to be several years in this galactic nucleus. More specifically and quantitatively, we find that the radius at a given time seems to be correlated with a long-term average of the flux over the previous several (~6) years, instead of the instantaneous flux. Finally, we also report measurements of three more Type 1 AGNs newly observed with the Keck interferometer, as well as the second epoch measurements for three other AGNs.

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 spectral energy distribution (SED) with a temperature/density-gradient model including an additional inner hot 1400 K ring component. Based on observations made with ESO telescopes at Paranal Observatory under programme IDs 083.B-0212(A) and 087.B-0578(A).

Constraining properties of dusty environments by infrared variability

We present model simulations of time-variable infrared (IR) emission from dust as a consequence of variability of the incident radiation. For that we introduce a generalized treatment for temperature variations in a dusty environment, which is not limited to any specific astronomical source. The treatment has been incorporated into a simplified clumpy torus model, with the radial brightness distribution as the main parameter, to study the IR emission of type 1 active galactic nuclei (AGN). We show that any variability signal in the optical is smoothened stronger if the brightness distribution is very extended, and this smoothing strongly depends on wavelength. This also affects time lags between the optical and near-/mid-IR emission, which can be up to 10s of sublimation radii for long wavelengths and extended brightness distributions. The dependence of time lag on wavelength and distribution can be used to quantify the brightness distribution in an AGN torus, either by comparing optical light curves to near-IR and mid-IR light curves, or by directly comparing near-IR to mid-IR light curves. Moreover, our model has been applied to near-IR data of the nearby Seyfert 1 galaxy NGC 4151. We show that the simple model can reproduce the overall observed variability signal and found that about 40% of the energy in the variability signal in the V-band has been converted into K-band variability. This low value may be explained by a “snowball” model of gradually-sublimating clouds at the inner edge of the torus. We also note that our modeling does not support a change of time lag/sublimation radius over the observed light curve epoch in spite of a significant change in V-band emission.