Konrad R. W. Tristram

Resolving the complex structure of the dust torus in the active nucleus of the Circinus galaxy

Aims. To test the dust torus model for active galactic nuclei directly, we study the extent and morphology of the nuclear dust distribution in the Circinus galaxy using high resolution interferometric observations in the mid-infrared. Methods. Observations were obtained with the MIDI instrument at the Very Large Telescope Interferometer. The 21 visibility points recorded are dispersed with a spectral resolution of λ/δλ ≈ 30 in the wavelength range from 8 to 13 µm. To interpret the data we used a stepwise approach of modelling with increasing complexity. The final model consists of two black body Gaussian distributions with dust extinction. Results. We find that the dust distribution in the nucleus of Circinus can be explained by two components, a dense and warm disk-like component of 0.4 pc size and a slightly cooler, geometrically thick torus component with a size of 2.0 pc. The disk component is oriented perpendicular to the ionisation cone and outflow and seems to show the silicate feature at 10 µm in emission. It coincides with a nuclear maser disk in orientation and size. From the energy needed to heat the dust, we infer a luminosity of the accretion disk of Lacc = 10 10 L� , which corresponds to 20% of the Eddington luminosity of the nuclear black hole. We find that the interferometric data are inconsistent with a simple, smooth and axisymmetric dust emission. The irregular behaviour of the visibilities and the shallow decrease of the dust temperature with radius provide strong evidence for a clumpy or filamentary dust structure. We see no evidence for dust reprocessing, as the silicate absorption profile is consistent with that of standard galactic dust. We argue that the collimation of the ionising radiation must originate in the geometrically thick torus component. Conclusions. Based on a great leap forward in the quality and quantity of interferometric data, our findings confirm the presence of a geometrically thick, torus-like dust distribution in the nucleus of Circinus, as required in unified schemes of Seyfert galaxies. Several aspects of our data require that this torus is irregular, or “clumpy”.

Three-dimensional radiative transfer models of clumpy tori in Seyfert galaxies

Context. Tori of Active Galactic Nuclei (AGN) are made up of a mixture of hot and cold gas, as well as dust. In order to protect the dust grains from destruction by the surrounding hot gas as well as by the energetic (UV/optical) radiation from the accretion disk, the dust is often assumed to be distributed in clouds. Aims. A new three-dimensional model of AGN dust tori is extensively investigated. The torus is modelled as a wedge-shaped disk within which dusty clouds are randomly distributed throughout the volume, by taking the dust density distribution of the corresponding continuous model into account. We especially concentrate on the differences between clumpy and continuous models in terms of the temperature distributions, the surface brightness distributions and interferometric visibilities, as well as spectral energy distributions. Methods. Radiative transfer calculations with the help of the three-dimensional Monte Carlo radiative transfer code MC3D are used in order to simulate spectral energy distributions as well as surface brightness distributions at various wavelengths. In a second step, interferometric visibilities for various inclination as well as position angles and baselines are calculated, which can be used to directly compare our models to interferometric observations with the MIDI instrument. Results. We find that the radial temperature distributions of clumpy models possess significantly enhanced scatter compared to the continuous cases. Even at large distances, clouds can be heated directly by the central accretion disk. The existence of the silicate 10 µm-feature in absorption or in emission depends sensitively on the distribution, the size and optical depth of clouds in the innermost part of the dust distribution. With this explanation, failure and success of previous modelling efforts of clumpy tori can be understood. The main reason for this outcome are shadowing effects of clouds within the central region. We underline this result with the help of several parameter variations. After adapting the parameters of our clumpy standard model to the circumstances of the Seyfert 2 Circinus galaxy, it can qualitatively explain recent mid-infrared interferometric observations performed with MIDI, as well as high resolution spectral data.

Resolving the obscuring torus in NGC 1068 with the power of infrared interferometry: Revealing the inner funnel of dust

We present new interferometric data obtained with MIDI (MID infrared Interferometric instrument) for the Seyfert II galaxy NGC 1068, with an extensive coverage of sixteen uv points. These observations resolve the nuclear mid-infrared emission from NGC 1068 in unprecedented detail with a maximum resolution of 7 mas. For the first time, suffi cient uv points have been obtained, allowing us to generate an image of the source using maximum entropy image reconstruction. The features of the image are similar to tho se obtained by modelling. We find that the mid-infrared emission can be represented by two components, each with a Gaussian brightness distribution. The first, identified as the inner f unnel of the obscuring torus, is hot (∼800K), 1.35 parsec long, and 0.45 parsec thick in FWHM at a PA=−42 ◦ (from north to east). It has an absorption profile di fferent than standard interstellar dust and with evidence for clumpiness. The second component is 3× 4 pc in FWHM with T=∼300K, and we identify it with the cooler body of the torus. The compact component is tilted by∼ 45 ◦ with respect to the radio jet and has similar size and orientation to the obse rved water maser distribution. We show how the dust distribution relates to other observables within a few parsecs of the core of the galaxy such as the nuclear masers, the radio jet, and the i onization cone. We compare our findings to a similar study of the Circinus galaxy and other re levant studies. Our findings shed new light on the relation between the different parsec-scale components in NGC 1068 and the obscuring torus.

Resolving the innermost parsec of Centaurus A at mid-infrared wavelengths ?

Context. To reveal the origin of mid-infrared radiation from the core of Centaurus A, we carried out interferometric observations with the MID-infrared Interferometer (MIDI) at ESO’s VLTI telescope array. Aims. Observations were obtained with four baselines between unit telescopes of the VLTI, two of them roughly along the radio axis and two orthogonal to it. The interferometric measurements are spectrally resolved with λ/∆λ = 30 in the wavelength range 8 to 13 µm. Their resolution reaches 15 mas at the shortest wavelengths. Supplementary observations were obtained in the near-infrared with the adaptive optics instrument NACO, and at mm wavelengths with SEST and JCMT. Methods. The mid-infrared emission from the core of Centaurus A is dominated by an unresolved point source (<10 mas). Observations with baselines orientated perpendicular to the radio jet reveal an extended component which can be interpreted as a geometrically thin, dusty disk, the axis of which is aligned with the radio jet. Its diameter is about 0.6 pc. It contributes between 20% (at λ � 8 µm) and 40% (at λ � 13 µm) to the nuclear flux from Centaurus A and contains dust at about 240 K. We argue, that the unresolved emission is dominated by a synchrotron source. Its overall spectrum is characterized by an Fν ∼ ν −0.36 power-law which cuts off exponentially towards high frequencies at νc = 8 × 10 13 Hz and becomes optically thick at ν<ν 1 � 45 GHz. Results. Based on a Synchrotron Self Compton (SSC) interpretation for the γ-ray emission, we find a magnetic field strength of 26 µT and a maximum energy of relativistic electrons of γc = Ec/mec 2 = 8500. Near γc, the acceleration time scale is τacc = 4 days, in good agreement with the fastest flux variations, observed at X-ray frequencies. Our SSC model argues for a Doppler factor δ � 1w hich – together with the jet-counter jet ratio of the radio jets on parsec scale – results in an upper limit for the bulk Lorentz factor Γjet < 2.5, at variance with the concept of a “mis-directed BL Lac object”. Conclusions. We estimate a thermal luminosity of the core, Pth � 1.3 × 10 34 W = 1.5 × 10 −4 × LEdd, intermediate between the values for highly efficiently accreting AGN (e.g. Seyfert galaxies) and those of typical FR I radio galaxies. This luminosity, which is predominantly released in X-rays, is most likely generated in an Advection Dominated Accretion Flow (ADAF) and seems just sufficient to heat the dusty disk.

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.

Parsec-scale dust distributions in Seyfert galaxies - Results of the MIDI AGN snapshot survey

Aims. The emission of warm dust dominates the mid-infrared spectra of active galactic nuclei (AGN). Only interferometric observations provide the necessary angular resolution to resolve the nuclear dust and to study its distribution and properties. The investigation of dust in AGN cores is hence one of the main science goals for the MID-infrared Interferometric instrument MIDI at the VLTI. As the first step, the feasibility of AGN observations was verified and the most promising sources for detailed studies were identified. Methods. This was carried out in a “snapshot survey” with MIDI using Guaranteed Time Observations. In the survey, observations were attempted for 13 of the brightest AGN in the mid-infrared which are visible from Paranal. Results. The results of the three brightest, best studied sources have been published in separate papers. Here we present the interferometric observations for the remaining 10, fainter AGN. For 8 of these, interferometric measurements could be carried out. Size estimates or limits on the spatial extent of the AGN-heated dust were derived from the interferometric data of 7 AGN. These indicate that the dust distributions are compact, with sizes on the order of a few parsec. The derived sizes roughly scale with the square root of the luminosity in the mid-infrared, s ∝ √ LMIR, with no clear distinction between type 1 and type 2 objects. This is in agreement with a model of nearly optically thick dust structures heated to T ∼ 300 K. For three sources, the 10 μm feature due to silicates is tentatively detected either in emission or in absorption. Conclusions. The faint AGN of the snapshot survey are at the sensitivity limit of observations with MIDI. Thus, the data set presented here provides a good insight into the observational difficulties and their implications for the observing strategy and data analysis. Based on the results for all AGN studied with MIDI so far, we conclude that in the mid-infrared the differences between individual galactic nuclei are greater than the generic differences between type 1 and type 2 objects.

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.

DUST EMISSION FROM A PARSEC-SCALE STRUCTURE IN THE SEYFERT 1 NUCLEUS OF NGC 4151

We report mid-infrared interferometric measurements (based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, programme number 081.B-0092(A)) with approx10 mas resolution, which resolve the warm (T = 285{sup +25} {sub -50} K) thermal emission at the center of NGC 4151. Using pairs of Very Large Telescope 8.2 m telescopes with the Mid-infrared interferometric instrument and by comparing the data to a Gaussian model, we determined the diameter of the dust emission region, albeit only along one position angle, to be (2.0 +- 0.4) pc (FWHM). This is the first size and temperature estimate for the nuclear warm dust distribution in a Seyfert 1 galaxy. The parameters found are comparable to those in Seyfert 2 galaxies, thus providing direct support for the unified model. Using simple analytic temperature distributions, we find that the mid-infrared emission is probably not the smooth continuation of the hot nuclear source that is marginally resolved with K-band interferometry. We also detected weak excess emission around 10.5 mum in our shorter baseline observation, possibly indicating that silicate emission is extended to the parsec scale.

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.