DUST IN THE POLAR REGION AS A MAJOR CONTRIBUTOR TO THE INFRARED EMISSION OF ACTIVE GALACTIC NUCLEI

In Figure 3, we show a broadband IR SED from about 1 μm to 90 μm combining Spitzer and AKARI data at comparably low angular resolution with high spatial resolution data obtained with 8 m telescopes at the VLT observatory in the near- and mid-IR. Although the IR emission in radio-quiet AGNs is generally dominated by dust re-emission, the optical/UV "big-blue bump" (BBB) can reach into the near-IR (Kishimoto et al. 2007, 2008). Therefore, we included the near-IR photometry presented by Prieto et al. (2010) and corrected it for BBB contamination according to the decomposition by Weigelt et al. (2012). The corrected photometry is shown in Figure 3.

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Despite the differences in angular resolution, all the data connect continuously, indicating that the nuclear IR emission originates predominantly from the AGN without contamination from star formation or other host-galactic sources. This is further supported by the absence of polycyclic aromatic hydrocarbon (PAH) features, except for a weak 7.7 μm line. Wu et al. (2009) propose a method to use the 11.2 μm PAH feature luminosity to estimate the star formation contribution and, in turn, obtain an AGN contribution to the mid-IR emission by comparing the PAH luminosity to the observed 12 μm continuum luminosity. Although not present in their sample, we can use the absence of any detectable 11.2 μm PAH feature in NGC 3783 and compare it with similar sources with comparable upper limits in Wu et al. (2009). Based on this comparison, we estimate the AGN contribution to the mid-IR emission to be >90%. Finally, the far-IR SED is monotonically decreasing starting at ∼20 μm toward long wavelengths, unlike starburst-contaminated sources that show much redder far-IR SEDs. Therefore, the presented SED can be considered to represent dust emission exclusively heated by the AGN.

The IR SED in Figure 3 shows two distinct spectral bumps. The first one peaks around 3–5 μm, the second one at ∼20 μm, with a "depression" in the 7 μm range. Similar behavior is seen in other type 1 AGNs (e.g., Edelson & Malkan 1986; Kishimoto et al. 2011b), although a lack of continuous data from the near- to mid-IR often does not reveal the two peaks as clear as in the presented case of NGC 3783. This double-peaked SED suggests that the AGN-heated IR emission does not originate from a single, continuous distribution of dust.

The visibilities and correlated fluxes of all 41 reduced data sets are shown in Figure 9. These data are being analyzed in the following.

As discussed in previous papers, sizes obtained from MIDI interferometry are physically meaningful only when compared either at the same intrinsic (e.g., half-light radius) or observed (e.g., fixed baseline length) reference to account for the inhomogeneous uv coverage and the unknown brightness distribution of the source (for more details, please follow the discussion in Kishimoto et al. 2011b; Hönig et al. 2012). Owing to the lack of suitable data, it is not possible to establish a size for the half-light radius at all P.A.s directly from the observations (in Section 4 we will determine the half-light radius with the assistance of a model). Therefore, we calculate the P.A.-dependent Gaussian half-width at half-maximum (HWHM) sizes of the mid-IR emission source in NGC 3783 for a fixed baseline length of 61 m. The results are shown in Figure 4. A two-dimensional Gaussian has been fit to all data with projected baseline length of 61 ± 5 m. If data at shorter and longer baseline lengths were available within a P.A. of ±5°, we interpolated these data to 61 m (in log-space) and used it as further input to the fit. In addition to the sizes in milliarcseconds (mas), the right ordinate axis in Figure 4 provides a scale in parsecs. The dashed colored lines are geometric fits for each wavelength bin using an elliptical Gaussian model with the semi-major axis a, the ratio a/b of major and minor axes, and the P.A. of the major axis as free parameters.

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In Figure 5, we show confidence intervals for the semi-major axis a and the axis ratio a/b for the 61 m data. For each wavelength bin (color-coded with increasing wavelength from blue to red), we calculate a probability distribution function (PDF) based on the three-parameter geometric model. The left panel of Figure 5 compares the 1σ (solid lines) and 2σ (dashed lines) confidence intervals for a/b versus a, i.e., marginalized over the P.A. At wavelengths >8.4 μm, the elongation of the mid-IR source is significant at >3σ. Moreover, the trend that the size increases with wavelength is obvious. In the lower-right panel, we plot the marginalized PDF for the semi-major axis. The peaks of the PDF for each wavelength shift to larger sizes from short to long wavelengths. This trend is expected for emission from centrally heated dust, meaning that the hotter dust (emitting at shorter wavelengths) is closer to the AGN.

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In the upper-right panel of Figure 5, we show the marginalized PDF for the axis ratio a/b. Assuming statistical independence of each wavelength bin, we calculated a joint PDF (black solid line) and inferred an axis ratio at 61 m baseline length for the mid-IR emission source of NGC 3783 of $1.42^{+0.03}_{-0.07}$ at the 95% confidence level. We caution, however, that for a given data set the errors do have a systematic component from the uncertainty of the transfer function that leads to correlations in the errors of different wavelengths. This is not included in the error estimates. As seen in both the left and upper-right panels of Figure 5, there may be a trend of stronger elongation with increasing wavelength. Most notably, the PDFs of the shortest and longest wavelengths barely overlap. Similar trends have been found in other AGN on similar spatial scales (Raban et al. 2009, NGC 1068; Tristram et al. 2012, Circinus, Hönig et al. 2012, NGC 424) and may be explained by contribution from a hot-dust component at the shorter wavelength end that has a different orientation (see Sections 3.3.1, 4, and 5).

In Table 2, we list the fitted geometric parameters at 8.6 μm, 10.5 μm, and 12.4 μm restframe wavelength to illustrate our results. The values for each parameter were obtained as the peak of the fully marginalized PDF. The errors indicate the 1σ confidence interval. For the semi-major axis, the sizes are listed in observed units (mas), physical units (pc), and intrinsic units (r_in). The latter ones use r_in = 0.061 pc, which is the inner radius based on the optical luminosity of NGC 3783 and a fit to reverberation-measured inner radii of AGN (Suganuma et al. 2006; Kishimoto et al. 2009b), approximately consistent with the K-band reverberation mapping of Glass (1992). We also list the values of the axis ratio and P.A. based on the joint inference of all wavelength bins in the range of 8.0–12.8 μm. This establishes that the mid-IR emission region observed at 61 m baseline is elongated along the major axis at P.A. $-52^{+2}_{-3}\,^\circ$ with an axis ratio of $1.42^{+0.01}_{-0.03}$ (1σ level). Although the direction of the major axis is approximately aligned with the direction where the uv plane only has coverage at about 60 m baseline length, we note that this result is mostly unaffected by the beam shape, because we made sure that the analysis has comparable resolution at all P.A.s. Moreover, the elongation and P.A. are already apparent in the raw data (see Figure 9). Although the data at P.A. >110° comes from baseline lengths of only 60–66 m, the visibilities are of the same level or even lower than those at 128 m at about P.A. 45°. If the object were circular, we would expect the opposite, i.e., significantly higher visibilities at the shorter wavelengths.⁹

Table 2. Geometric Gaussian-fit Parameters of Our Mid-IR MIDI Data and the Near-IR AMBER Data Published in Weigelt et al. (2012)

Data Type	Wavelength	Semi-major Axis a			Axis Ratio a/b	Position Angle
	(μm)	(mas)	(pc)	(rin)		(deg)
MIDI PBL 61 m	8.6	$6.3^{+0.3}_{-0.2}$	$1.34^{+0.06}_{-0.05}$	$22.0^{+1.0}_{-0.7}$	$1.20^{+0.08}_{-0.05}$	$-62^{+15}_{-13}$
	10.5	$9.0^{+0.4}_{-0.2}$	$1.91^{+0.08}_{-0.04}$	$31.3^{+1.2}_{-0.7}$	$1.38^{+0.07}_{-0.06}$	$-50^{+8}_{-7}$
	12.4	$12.0^{+0.5}_{-0.4}$	$2.55^{+0.10}_{-0.08}$	$41.8^{+1.6}_{-1.4}$	$1.48^{+0.08}_{-0.07}$	$-48^{+6}_{-6}$
	8.0–12.8	⋅⋅⋅			$1.42^{+0.01}_{-0.03}$	$-52^{+2}_{-3}$
AMBER	2.2	$0.73^{+0.35}_{-0.20}$	$0.16^{+0.07}_{-0.04}$	$2.5^{+1.2}_{-0.7}$	$1.45^{+0.60}_{-0.36}$	$48^{+36}_{-77}$

Notes. The values for the mid-IR source are based on full marginalization of the PDF, while the near-IR parameters refer to the peak in the multi-dimensional PDF. The errors indicate the 1σ confidence interval for each parameter.

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3.3.1. Near-IR Interferometry

NGC 3783 has recently been interferometrically observed in the near-IR using the VLTI/AMBER instrument. Weigelt et al. (2012) report a source size in the K band of 0.74 ± 0.23 mas or 0.16  ±  0.05 pc from the two- and three-telescope interferometry. This size is about an order of magnitude smaller than the 61 m HWHM we found in the mid-IR, qualitatively consistent with the idea that the dust is hottest closest to the AGN. The AMBER data cover a range of different P.A.s, and we aim for investigating if the object shows signs of ellipticity in the near-IR. We take the visibility measurements in the K band from Weigelt et al. (2012), convert them into Gaussian HWHM, and plot the results in Figure 6.

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The large error bars of these observations (±0.09 in visibility), combined with the high visibilities, make it difficult to interpret the near-IR emission region in terms of a non-circular model. Therefore, we use the same approach as for the MIDI data and calculate the PDF for the geometric model. The parameters for the peak value in the multi-dimensional PDF are listed in Table 2. The 1σ confidence interval is taken from the peak of the multi-dimensional PDF along the respective parameter axis. The reason we chose this approach is that when fully marginalizing the PDFs, the inference for a P.A. becomes very low and multi-peaked. This is mostly due to the lack of data between −45° and 20°. Interestingly though, when marginalizing we find that the lowest probability for the direction of the major axis is approximately at P.A. −50° to −60°, i.e., the preferred direction of the mid-IR extension. This may be considered as an indication that the near-IR and mid-IR are originating from distinctly different regions and dust distributions, and we want to express the need to fill the K-band uv plane in this object.

A rather well-constrained parameter is the semi-major axis. Full marginalization provides $a=0.73^{+0.35}_{-0.20}$ mas (1σ confidence interval). This result is consistent with the size obtained by Weigelt et al. (2012). Accordingly, the K-band interferometry size is about a factor of two larger than the inferred K-band reverberation size (based on Kishimoto et al. 2007, 2009b), which might be explained in terms of a shallow (i.e., very extended) dust distribution, as well as a high sensitivity of the reverberation cross-correlation analysis to the fastest reacting signal (Kishimoto et al. 2011a; Weigelt et al. 2012). An additional possibility we propose here is an inclination effect. The geometric fit provides marginal evidence that the K-band source is elongated. If this is indeed the case, it may be caused by inclination of the inner torus with respect to the observer. This would result in the fastest response to variability coming from the near side along the minor axis, which according to our fit is a factor of $1.45^{+0.60}_{-0.36}$ smaller than the semi-major axis. Further K-band interferometry is required, however, to formally constrain the shape of the near-IR-emitting region.

As discussed in Hönig et al. (2012), there are at least three independent ways to establish a system axis (=polar direction) of the AGN: (1) jet-like linear radio emission in the nucleus, (2) spatial extension of the narrow-line region (NLR), and (3) optical polarimetry. However, the different methods do not always result in a consistent picture as famously evidenced in NGC 1068, where large-scale and small-scale direction indicators are misaligned. In the following, we will discuss the situation in NGC 3783.

Radio maps at 3.6 cm and 6 cm show an unresolved point source, so that no P.A. information can be extracted (Morganti et al. 2000; Kinney et al. 2000). Smith et al. (2002) report optical spectropolarimetry of NGC 3783. The wavelength-dependent features in the percentage polarization, polarized flux, and P.A. of the broad Hα emission line are interpreted as consistent with scattering off an equatorial disk on about the same scale or slightly larger than the broad-line region (Smith et al. 2002, 2005). As such the polarization traces the geometry of material on sub-parsec scales. The inferred polar axis from these measurements is pointed toward P.A. −45° (Smith et al. 2004).

Spatially resolved observations of NGC 3783 in several optical and near-IR emission lines (e.g., [O iii], [Si vi], H₂) have been reported and the major axis of the extended line region is found to be between P.A. −20° and 0° (e.g., Schmitt et al. 2003; Hicks et al. 2009; Müller-Sánchez et al. 2011). Several of these lines show blueshifted cores (e.g., Reunanen et al. 2003; Hicks et al. 2009) and high velocities of up to 1300 km s⁻¹ (Kraemer et al. 2001; Rodríguez-Ardila et al. 2006; Müller-Sánchez et al. 2011), meaning that gas in these extended line-emitting regions is outflowing. Integral-field spectroscopy of the [Si vi] line (Müller-Sánchez et al. 2011) shows extended line emission over 50–70 pc to the north (P.A. ∼ 3° from kinematic modeling). However, intensity maps suggest that the P.A. changes toward the north/west for smaller scales (see Figure 13 in Müller-Sánchez et al. 2011). At the 01/20 pc scale, the point-spread function (PSF; FWHM 0085) appears elongated toward P.A. ∼ −35° (see the 50% peak flux contour in the lower-right panel of their Figure 13 and the low-velocity maps in the same figure). This is also indicated in the results of the kinematic modeling where an additional small-scale component might be necessary to explain the near-nuclear [Si vi] emission (F. Müeller-Sánchez 2013, private communication).

The situation in NGC 3783 bears some resemblance to the changes of P.A. on about the same scales seen in NGC 1068 (e.g., Gallimore et al. 2004). In the case of NGC 1068, the best inference on the parsec-scale polar axis can be obtained when considering the nuclear polarization direction while the orientation of narrow-line emission on tens of parsecs scales is off by up to 45° (e.g., Miller et al. 1991; Capetti et al. 1997; Gallimore et al. 2004). In NGC 3783, we find that the polarization axis, tracing the orientation on sub-parsec scale, is only ΔP.A. = −7° off the average mid-IR axis of our MIDI observations, and about ΔP.A. = 87° off the near-IR axis, all of these directions tracing parsec scale or even smaller structures. The orientation on tens of parsecs as inferred from the narrow-line emission intensities is off the polarization P.A. by about ΔP.A. ∼ 10° (smallest scales) to 48° (larger scales; kinematic model implies opening angle of ±34°). The larger scale NLR axis is located somewhere in between the mid-IR and near-IR, while the polarization shows good alignment with the extended mid-IR parsec-scale emission and potentially the small-scale NLR, and is about perpendicular to the near-IR.

Near- and mid-IR interferometry observations trace dust on sub-parsec to parsec scales close to the optical scattering region in this type 1 AGN. Therefore, we interpret the polarization position angle in P.A. −45° as being the best reference for the direction of the polar region for our observations. Comparing this polar axis with the interferometry suggests that the mid-IR light is emitted from a region close to the polar axis while the near-IR emission originates from the AGN plane (although we note again that the inference on the P.A. of the near-IR emission is not good at the moment). On larger scales, this preferential axis is probably misaligned with the small-scale polar axis (see also Reunanen et al. 2010), indicating some kind of realignment of the material during the inflow/outflow process.

The fitting result described in the previous section makes use of all 41 data sets and is not restricted to a certain baseline length as in the geometrical model for the 61 m baseline. Hence, the object geometry is less dependent on the probed spatial scales and the model than the analysis in Section 3.2. Therefore, it is important to realize that the inferred axis ratio s = a/b = 3.0 reflects the intrinsic geometry more realistically than the axes ratio of the ∼1.4 derived from the geometric modeling of the 61 m data (see Section 3.2 and Figures 4 and 5). This is well illustrated in Figure 7. Here, we show the observed 12.5 μm visibilities for all 41 data sets and compare them to the best-fitting model (see Table 3). For the minor axis, the 12.5 μm visibility drops to 0.5 at about a baseline length of ∼84 m (red data points). Along the major axis (blue data points), even the shortest baselines at about 50 m show visibility levels lower than 0.5, meaning that the intrinsic axis ratio must be >2. The model predicts that the half-visibility level is reached already at 28.1 m, leading to the axis ratio as listed in Table 3. Using the model we can infer a half-light radius at 12.5 μm, despite the low visibility levels along the major axis, of (20.0  ±  3.0) mas × (6.7  ±  1.0) mas, corresponding to (4.23  ±  0.63) pc × (1.42  ±  0.21) pc or (69.4  ±  10.8) r_in × (23.3 ± 3.5) r_in.

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We also tested what kind of effect we may expect on single-telescope mid-IR images of NGC 3783. For that, we synthesized a telescope PSF using a two-dimensional Airy function with a core width of 035 as typically measured for calibrator stars in VLT/VISIR images at 12.5 μm (Hönig et al. 2010; Reunanen et al. 2010). This PSF is used as a convolution core for a 12.5 μm model image from our best-fit model parameters. We simulated "observed" model images, binned to the 0075 pixel⁻¹ detector resolution of VISIR, and assumed a detection with S/N = 100 of the source and S/N = 1000 of a bright PSF reference star. Under these conditions, we subtracted the scaled reference star from the "observed" model image and measured how much flux is left over from the extended structure. We found that there should not be any detectable extended flux for NGC 3783 based on our best-fit model down to <1%. This is consistent with an analysis of several ground-based mid-IR images by D. Asmus et al. (2013, in preparation), which did not find any extended structure around the nucleus of NGC 3783. However, if we put NGC 3783 in place of the object with the highest intrinsic spatial resolution in the mid-IR, NGC 1068 (D = 14.4 Mpc, r_in ≈ 0.1 pc), we would expect that about 10%–12% of the observed flux could be detected as extended emission in the polar region, with the direction of elongation well recovered. A comparison to Circinus (D = 4.2 Mpc, r_in ≈ 0.025 pc) yielded similar results (6%–8% extended flux). Interestingly, both Circinus and NGC 1068 show much stronger contribution of mid-IR emission from the polar region in single-telescope images than expected from the NGC 3783 model (e.g., Packham et al. 2005; Mason et al. 2006), which may indicate a different intrinsic distribution of the polar dust and/or a viewing-angle effect since both objects are type 2s while NGC 3783 is a type 1 AGN.

The two-component model spatially and spectrally disentangles the hot and warm components. As evident from the SED, the hot component occupies primarily the near-IR wavelength region and is responsible for the 3–5 μm bump while the warm component causes the mid-IR bump. Using the model for decomposing the emission into hot and warm components, we can determine the observed covering factors of both components as the ratio of the total luminosity in each component to the total luminosity of the UV/optical BBB emission L_BBB. We want to note that these observed covering factors are based on observations along a single line of sight and, therefore, assume isotropy of both the BBB and IR emission. They do not take into account possible anisotropy/optical depth effects of the IR emission, which may be preferentially emitted vertically (type 1 AGN). As a consequence, the intrinsic covering factors in this type 1 AGN are probably smaller than the observed values. The actual degree of anisotropy in the mid-IR is still a matter of debate (e.g., Buchanan et al. 2006; Gandhi et al. 2009; Hönig et al. 2011; Asmus et al. 2011).

We estimate the luminosity of the BBB in NGC 3783 by combining the UV/optical observations compiled in Prieto et al. (2010) and a ground-based nuclear 4000–8000 Å spectrum (Jones et al. 2009, scaled to the photometric data), and scaling an AGN BBB template to the extinction-corrected data (multi-power-law template as shown in the Appendix of Hönig & Kishimoto 2010). Assuming that the optical/UV emission in NGC 3783 follows such a template, we integrate the scaled template and obtain $L_\mathrm{BBB} = 1.0 \times 10^{44}\,\mathrm{erg\,{\rm s}^{-1}}$. This is notably different from the integration of the data itself in Prieto et al. (2010; 3.9 × 10⁴³ erg s⁻¹) where the intention was to quantify the directly observed BBB luminosity. Indeed, NGC 3783 shows signs of extinction toward the shortest wavelengths, and the template fitting implies a line-of-sight optical depth of τ_V = 0.3. We estimate the systematic uncertainty of our L_BBB determination as about a factor of two, primarily due to AGN variability in this source (Onken & Peterson 2002).

Decomposing the IR SED, we obtain L_hot = 4.2 × 10⁴³ erg s⁻¹ and L_warm = 9.2 × 10⁴³ erg s⁻¹ for the hot and warm components, respectively. The total IR luminosity is, therefore, L_IR = 1.3 × 10⁴⁴ erg s⁻¹, leading to a total IR covering factor of $C_\mathrm{IR} = L_\mathrm{IR}/L_\mathrm{BBB} = 1.3^{+1.3}_{-0.7}$ (errors based on the optical variability). For the hot and warm components, we determine covering factors of $C_\mathrm{hot} = 0.42^{+0.42}_{-0.21}$ and $C_\mathrm{warm} = 0.92^{+0.92}_{-0.46}$ and a ratio C_warm/C_hot = 2.2, meaning that the warm dust covers about twice as much solid angle around the AGN as the hot dust. Yet, the hot-dust covering factor is still significant and larger than the typical value of ∼7% derived from single-wavelength K-band data of a sample of type 1 AGN (Landt et al. 2011).

The parsec-scale polar-oriented axis ratio of the mid-IR emission of about 3:1 is the strongest such elongation detected so far. It has to be pointed out again that NGC 3783 is a type 1 AGN with multiple clearly detected broad lines in the optical as well as low polarization consistent with an equatorial scatterer (Smith et al. 2004). Additionally, the [O iii] emission appears as only marginally extended (Schmitt et al. 2003). Together with the IR SED and the low X-ray column density, this suggests that the type 1 classification is due to a relatively low inclination of the equatorial obscurer (i.e., torus) in NGC 3783 as opposed to a "lucky strike" through a hole in a clumpy torus that is seen more edge-on. Therefore, the observed strong elongation points toward dust that is significantly elevated from the equatorial plane of the AGN.

We have argued in Hönig et al. (2012) for the type 2 AGN NGC 424 that the intrinsic polar elongation with axis ratio of 2.2:1 poses a challenge to the standard interpretation of the mid-IR emission with torus models. Indeed, the only models that showed a qualitatively consistent geometry with the observations were the hydrodynamic simulations of Schartmann et al. (2009, their Figure 8). However, these models failed to reproduce the observed SED. NGC 3783 has a relatively red mid-IR SED (see spectral slopes in Hönig et al. 2010; Kishimoto et al. 2011b for a comparison to a small sample of other type 1s) that might be qualitatively consistent with the model shown in Schartmann et al. (2009, their Figure 9). On the other hand, for more face-on inclinations as in NGC 3783, the model does not reproduce the strong polar elongation, aside from the lack of a near-IR emission bump at around 3–5 μm.¹⁰ Therefore, we conclude that the strong parsec-scale polar elongation seen in NGC 3783 (as well as some other AGNs) is not accounted for in current torus models and challenges the paradigm that the IR emission is dominated by the torus.

In order to be more quantitative about how much of the mid-IR flux might still be emitted by the torus, we use our best-fit model from Section 4 and compare the polar-extended flux to the total mid-IR emission. The remaining flux may be associated with the torus. In Figure 8, we show the relative fraction of polar mid-IR emission and torus emission in the 8–13 μm range. About 60%–90% of the emission arises from the polar region with a strong dependence on wavelength. This is tied to the overall increase in size toward the polar region with wavelength.

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In the previous section, we discussed the covering factors of the hot and warm components in NGC 3783 and compared them to the low values in the K band. One possibility to unite the low K-band covering factors, the ∼30% C_hot for NGC 3783, and the factor of two larger C_warm is significant flaring in the dust distribution around the AGN. In this scenario, the total surface of dust clouds directly exposed to the AGN grows rapidly with distance (see also Figure 9 in Hönig et al. 2012 and Figure 4 in Kishimoto et al. 2013). Accordingly, the dust has to be elevated significantly. Turbulence does not seem to be a very good driver since most hydrodynamic simulations show comparably flat distributions (e.g., Wada et al. 2009). In fact, the main reasons why the models by Schartmann et al. (2009) show some polar elongation are the initial conditions which assume that the dust starts out in a more or less spherical distribution on scales of tens of parsecs (M. Schartmann 2013, private communication). Recently, we argued that radiation pressure on dust both from UV/optical and IR photons may drive an optically thin dusty wind from the inner region of the torus (Hönig et al. 2012; Kishimoto et al. 2013). This wind is not currently accounted for in radiative transfer models that are used to interpret the IR emission of AGN. A notable feature of the torus+wind model are two regions that differ not only in their locations (equatorial torus versus polar wind) but also in their optical depths. This is reflected by the two distinct spectral bumps in the IR SED where the torus would be responsible for the emission peaking in the 3–5 μm bump, while the wind mainly contributes to the mid-IR. The spatial distinction is suggested by comparison of the near-IR and mid-IR interferometry as discussed in Section 3.3.1.

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We want to emphasize that the observations do not require that the polar cone is filled with dust. The elongation that we see is a projection of the three-dimensional geometry onto the plane of the sky. This means that it is well possible that the polar mid-IR emission is "hollow," corresponding to typical wind scenarios (e.g., Elvis 2000; Keating et al. 2012; Roth et al. 2012). In fact, the polar-elongated mid-IR emission in Circinus is brightest close to the edge of the ionization cone (Tristram et al. 2012). This would help to reconcile polar-extended dust with type 1 AGN being observed without significant optical extinction.

In the context of the wind scenario, it may be interesting to note that NGC 3783 is one of only three type 1 AGNs with a (weak) water maser detection (J. Braatz 2013, private communication¹¹). Although the data are not sufficient to determine the kinematic structure of the maser-emitting clouds in NGC 3783, the other two type 1 AGNs with water masers (NGC 4051 and NGC 4151; Hagiwara et al. 2003; Braatz et al. 2004) show outflow characteristics. If the maser emission in NGC 3783 also originated from the outflow region, it supports the idea of outflowing clouds with significant column, potentially bearing dust, in about the polar direction that can emit significant IR emission.

Finally, we want to add that the sub-unity emissivities that favor graphite grains (see Section 4) are qualitatively consistent with our expectations for the dusty wind. In Hönig et al. (2012), we discuss that the most likely wind-launching region is the innermost, hottest region of the torus where both the radiation from the AGN and from the IR emission of the dust itself has the potential to uplift dust grains via radiation pressure (Krolik 2007). It is, therefore, conceivable that the dust composition in the wind reflects the dust composition in the hottest region of the torus. Observational evidence strongly suggests that large graphite grains dominate this hot sublimation zone (e.g., Kishimoto et al. 2007, 2009a; Mor et al. 2009; Kishimoto et al. 2011a; Mor & Netzer 2012).