PARSEC-SCALE DUST EMISSION FROM THE POLAR REGION IN THE TYPE 2 NUCLEUS OF NGC 424

The interferometric observations have been carried out during three different campaigns in 2007, 2009, and 2010 using the mid-IR beam combiner MIDI at the VLTI in Chile. This instrument allows for spectro-interferometric observations in the mid-IR waveband from 8 to 13 μm. We used several different combinations of the four 8 m telescopes to achieve various P.A.s and projected baseline points to fill the accessible uv-plane of NGC 424. In Figure 1, we show the uv-plane coverage of our observations. The uv-plane is shown as projected on the sky to allow for an easy comparison with P.A.s. While the shorter baselines of up to 60 m cover all quadrants, the longer baselines are oriented preferentially in NE/SW direction, in line with the location of the UT telescopes at the observatory. As illustrated, our observations can be considered complete for this target considering the achievable coverage using the VLTI UTs.

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The data have been extracted by the standard MIA+EWS package developed for MIDI. Reduction and calibration were performed using the signal-to-noise ratio (S/N) maximization method as described in Kishimoto et al. (2011b). Each individual fringe-track data set and a dedicated interferometric calibrator star have been reduced together to determine the required smoothing of the calibrator data to achieve the best S/N of the correlated fluxes. During all three observing campaigns, we observed the same calibrators near NGC 424 (HD 9362 and HD 8498) for consistency, except for one set in 2007 where the only available calibrator was observed at significant spatial and temporal distance from the science target (HD 219449). Cross-calibration showed that HD 9362 and HD 8498 remained essentially constant in flux and size over the observed time span.

When two or three science fringe-track sets have been observed together (i.e., within a 15 minute interval), they were averaged after data reduction with weights according to their individual S/N. This led to three independent correlated flux measurements in 2007, 17 in 2009, and 27 in 2010 as listed in Table 1. The S/N values of all data sets range from 3.0 to 7.2 with a median of 4.3. This is slightly higher than for a typical AGN (see Kishimoto et al. 2011b) and can be explained by the mid-IR brightness of the source.

We extracted the total fluxes from all of the photometry data that have been taken together with the fringe tracks. As sky reference we used the background flux from a 5 pixel offset position to the peak of the target flux. Calibration was done using the same calibrator star as used for the interferometric data. All data sets of each year have been averaged to obtain a reference total flux of NGC 424 in the 8–13 μm range for that year. The 2007, 2009, and 2010 spectra are shown in Figure 2. While the 2009 and 2010 fluxes are consistent, the 2007 spectrum is systematically lower by a factor of 0.8 mostly independent of wavelength. Twenty to thirty percent variability over a span of two years may be possible, but given the bin-to-bin (systematic) scatter in the 2007 observations, it is more likely an observational inaccuracy. Indeed, the 2007 fluxes are based on only three photometry data sets while the 2009 and 2010 spectra are averages over 23 and 22 sets, respectively. For a good total flux reference, we calculate a "master" total flux spectrum, F_tot, from the 2009 and 2010 data by averaging both sets and spline-fitting the atmospheric ozone feature at around 9.6 μm. This spectrum will be used as the reference for all correlated fluxes F_{corr; i}. The visibilities that we will use in the data analysis are then computed as V_i = F_{corr; i}/F_tot, and visibility errors are propagated based on the total and correlated flux uncertainties. As described in Kishimoto et al. (2011b), these uncertainties contain the fluctuations of the individual correlated and total flux frames, the differences to the mean when S/N-weighting several data sets, and a 5% systematic component resulting from the analysis of low-S/N data (see Appendix Figure A.5 in Kishimoto et al. 2011b).

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In addition to our MIDI interferometry, we collected photometric data mainly from the ESO archive and literature to build a generic IR SED of the nuclear emission of NGC 424. The ESO data archive holds near-IR imaging data of NGC 424 in the Js, H, Ks, L, and M bands observed with ISAAC. We reduced the data using standard procedures. For the three near-IR bands, we first subtracted the host galaxy from the nucleus by fitting a Sersic profile to the extended (>1'') component (i.e., masking the nuclear region for the fit) before extracting nuclear fluxes. The resulting fluxes are shown in Table 2.

It is important to point out that the L- and M-band data from the 8 m telescope with a (seeing-limited) spatial resolution of about 04 are consistent, within error, with the 3.6 μm and 4.5 μm Spitzer data with a spatial resolution of about 12 reported by Gallimore et al. (2010). This can be taken as evidence that the IR emission in the central arcsecond is dominated by emission from the AGN without significant enhancement by star formation in the nuclear region. Indeed, the Spitzer Infrared Spectrograph (IRS) spectrum of the nuclear ∼4'' shown in Gallimore et al. (2010) displays polycyclic aromatic hydrocarbon (PAH) features with only small equivalent widths, the largest being the 7.7 μm feature but no line visible at 11.3 μm (Wu et al. 2010; AGN fraction >0.9 using the authors' relation between AGN contribution, IRAC and IRAS photometry).

In Figure 3, we show a broadband IR SED from about 1–90 μm combining the Spitzer data as shown in Gallimore et al. (2010) with the 8 m telescope data presented in this work. The plot illustrates the IR bump typical of dust emission and seen in all radio-quiet AGNs. According to the unification scheme, it is caused by circumnuclear dust in the equatorial region of the AGN that re-emits the absorbed "big blue bump" (BBB) UV/optical radiation (presumed to originate from the accretion disk). One of the interesting properties of this target is that we see the IR SED rising at short wavelengths, flattening in the 8–20 μm region, and then falling off toward 100 μm, despite the comparable large intrinsic apertures involved. The resolution of the MIPS data at 70 μm corresponds to ∼4.6 kpc. Yet the SED is more or less a downward continuation of the data at shorter wavelengths and much higher angular resolution (at least to first order; see below). Indeed, the whole SED from the near- to far-IR connects smoothly despite the different instruments and resolutions involved. We interpret this as evidence that the unresolved IR nucleus dominates all of the IR emission without any significant contribution from other sources, e.g., circumnuclear star-forming regions,⁵ and that the AGN is the only source of energy that produces the IR emission in the nuclear region.

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There are two notable exceptions from the smooth connection of all SED data. The first one is a slight offset of the MIPS data with respect to the longest wavelengths in the IRS data. This may be an indication that there exists an additional cool IR component contributing to longer wavelengths. If this were true, then we would expect that the slope changes from the downward tail of the IRS data through the MIPS data. However, this is not the case. In fact, the data seem to be just offset by a factor of 1.3, which could be a sign of an issue with flux calibration in the far-IR, but we cannot exclude the possibility of slightly extended emission between the smaller and larger apertures. Anyway, the flux is strongly and monotonically decreasing toward the far-IR.

The second discontinuity in the SED is more interesting. While the ISAAC L- and M-band measurements are well in agreement with the IRAC data and smoothly connect to the IRS short wavelengths, the MIDI total flux spectrum is significantly lower than the IRS data. The difference of a factor of 1.3–1.4 is difficult to attribute to calibration problems, since both the IRS and MIDI calibration seems to be consistent in itself (for MIDI, see Section 2.1). Under the assumption that there are no unknown systematic problems with the MIDI data, we can imagine two possibilities for this discrepancy: first, there may be extended warm emission outside of the MIDI aperture of ∼06 or 130 pc. The dust cannot be too hot (>800 K) or too cold (<200 K), however, because we do not see a disagreement between small and large apertures in the near-IR and also no bump in the IRS spectrum beyond 20 μm. Moreover, the SED in the difference spectrum is flat, so that we can estimate a temperature of the possible extended emission of ∼300–400 K. When using the r_sub–L_12 μm relation (Hönig et al. 2010) to estimate the dust sublimation radius for this type 2 source, we obtain about 0.065 pc (with significant error bars). Given the constraints from the MIDI and Spitzer aperture, the emission responsible for the mid-IR flux difference would then be located at a distance >2000 r_sub. When assuming a standard interstellar medium (ISM) dust composition, we would expect an equilibrium temperature with the AGN radiation of <100 K at that distance, which contradicts the requirements based on the SED shape as just discussed. Finally, an additional heating source that provides energy for the dust in a very narrow wavelength range is also difficult to imagine, in particular when required not to leave any other traces. We repeat that star formation is a very unlikely candidate because of the almost complete lack of PAH features (except a small feature at 7.7 μm) and the lack of a far-IR bump.

The second possibility is mid-IR variability. In Figure 3, we indicated the years in which each of the sets of observations were taken. Interestingly, the MIDI data are separated by about four years from the other observations. We recently discussed how IR variability of a dusty structure compares at different wavelengths (Hönig & Kishimoto 2011). The principle of IR variability is simple: if the incident radiation onto the dust (here, the BBB radiation) changes, the dust temperature at a given distance from the source changes as well and, consequently, the flux varies. The main difference between near- and mid-IR wavelengths is the size and distance of the emitting regions. The near-IR emission is usually confined to a very narrow region close to the sublimation radius. Therefore, the light-crossing time of this region for any variability signal is short and the initial signal is not significantly diluted (=comparable amplitudes of initial BBB signal and near-IR signal). The mid-IR emission is more complex. It is a combination of the Rayleigh–Jeans tail of hotter dust and the peak in emission from cooler dust. Moreover, the radial distribution of the dust also matters. Overall, the mid-IR emission originates from a relatively large range of distances from the AGN, so that the variability signal is smeared out and the amplitudes are smaller, unless the initial signal has a very long duration (e.g., a long-term break or jump in the AGN luminosity; see Figure 2 in Hönig & Kishimoto 2011 for an example). Therefore, mid-IR variability is a viable possibility if both (1) the amplitude of variability and (2) the size of the region where this variability originates from are physically plausible. Glass (2004) reported near-IR variability in the K band: the host-uncorrected K-band magnitude showed peak-to-valley variation of 0.34 mag over several years. Comparing the uncorrected K-band magnitude with our nuclear K-band flux reveals that about 70% of the K-band flux of Glass (2004) originates from the host. The corrected K-band variability is, therefore, a factor of ∼1.8 using our nuclear K-band measurement. This near-IR amplitude is sufficient to explain a factor of 1.4 variability in the mid-IR. The second necessary condition related to the size of the mid-IR emission region can be constrained by the elapsed time between the epochs of the mid-IR observations. Within the four years, the light propagated about 1.2 pc, so that in order to obtain the potential decrease in mid-IR flux by a factor of 1.4, the bulk of the mid-IR emission must come from within this radius. We can test this possibility with our interferometric data (see below).

The SED of NGC 424 is relatively blue when compared to other type 2 or even type 1 AGNs. In Figure 4, we show the mid-IR emission of NGC 424 (black-solid line) and two different AGN template SEDs. The red-dashed line is an AGN template from Mullaney et al. (2011) where Spitzer IRS data of 25 AGNs were corrected for galactic/star formation and averaged without distinguishing among types. We already showed that the Spitzer IRS flux of NGC 424 is consistent with the IR fluxes from ground-based 8 m data. These data have higher angular resolution than Spitzer data. At 8 μm, the point-spread function (PSF) in NGC 424 has a size of ∼55 pc. To account for possible resolution effects in the Mullaney et al. (2011) template, we used the 10 AGNs from Hönig et al. (2010, light-blue lines) with similar resolution as NGC 424 (range of 30–80 pc at 8 μm) and calculated a median template (dark-blue dash-dotted line). NGC 424 shows a bluer SED than the combined SED. The spectral index α (defined as F_ν∝ν^α) is α ∼ −2.1 for type 2s and α ∼ −1.7 for type 1s, approximately in agreement in Hönig et al. (2010) and Mullaney et al. (2011). NGC 424 has a mid-IR spectral index of α ∼ −1.1, making it the bluest type 2 compared to the other objects in Hönig et al. (2010) with only one type 1 AGN showing a bluer mid-IR spectrum. This points toward a lack of cool dust relative to the amount of hot dust in NGC 424.

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The visibilities and correlated fluxes of all reduced 37 data sets are shown in the online version of Figure 10. These data will be analyzed in the following.

3.2.1. The Size of the Mid-IR Source in NGC 424

A note on interferometric sizes. The correlated fluxes can be compared to the total fluxes to obtain visibilities that, in turn, can be converted into source sizes using a suitable model. AGN sources have often been modeled by means of a Gaussian flux distribution resulting in sizes based on the Gaussian FWHM or half-width at half-maximum (HWHM). However, it was shown recently that sizes inferred from single-Gaussian models can be quite misleading since the underlying brightness distribution barely resembles a Gaussian (Kishimoto et al. 2011b). In fact, these sizes can be considered rather a measure of the array resolution than the source size. This becomes even more of an issue when two-dimensional sizes are determined from data where the baseline lengths change with P.A.—meaning the source shape depends on the uv-coverage, a typical problem in contemporary IR interferometry owing to the limiting number of telescopes in the arrays. There are two ways to approximately overcome this problem. First, one chooses a fixed visibility, infer the baseline lengths/spatial wavelengths for all P.A.s at this visibility, and calculates a corresponding size. This has been done in Kishimoto et al. (2011b) in terms of the half-visibility radius R_V0.5 for a fixed visibility of 0.5 or related half-light radius R_1/2. Second, it is also possible to take a fixed projected baseline length (PBL) and infer the visibilities for all P.A.s at this PBL. The first method is more related to the intrinsic structure of the object while the second one reflects the shape that a telescope with an aperture the size of the PBL would see. Both methods, however, overcome the problem of inhomogeneous uv-coverage to some extent.

NGC 424 emission source. In the left panel of Figure 5, we show the P.A.-dependent Gaussian HWHM sizes of the mid-IR emission source in NGC 424 at a fixed PBL of 58 m as obtained from our data. Where no data were available in the range of 55–61 m, we used pairs of data at similar P.A.s and interpolated or extrapolated to 58 m (including errors). Kishimoto et al. (2011b) showed that the brightness distributions in AGNs follow simple power laws, so that we linearly inter/extrapolated in log-space. Because of the resulting well-sampled P.A. coverage, some of the source characteristics can be easily inferred. First, the object is smallest at the shortest wavelength bin and becomes larger with wavelength (see also Figure 6). This is qualitatively consistent with the idea that hotter dust (primarily emitting shorter wavelengths) is located closer to the AGN. Second, the mid-IR source is non-circular. While this is not too obvious in the 58 m data, we also show Gaussian HWHM sizes at 100 m in the middle panel of Figure 5. The sizes in east/west direction are clearly smaller than those close to north/south for all wavelengths. It should be noted, however, that there are larger gaps in the P.A. coverage at 100 m because of the limited uv-coverage by the UT telescopes (see Figure 1). In the right panel of Figure 5, we show the P.A. dependence of the half-light radius in NGC 424. As previously noted, this measure is independent of the actual baseline length. However, data interpolation is reversed with respect to the fixed-baseline method and results in larger uncertainties of the spatial wavelength because the change in visibilities with baselines is small. In the right panel of Figure 5, we show all data pairs that allow us to establish a clear half-light radius. The problem is particularly relevant to the shorter wavelength data as can be seen by the sometimes erratic size changes at 9.1 μm (yellow circles). Taking all three panels together makes it clearer that the objects displays some degree of elongation.

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To be more quantitative about the object's size and shape, we fitted ellipses to the different measurements. In the left and middle panel of Figure 5 we show fits to each wavelength bin of the Gaussian sizes at 58 m and 100 m. Because of the higher uncertainties in the determination of the half-light radius, only the median visibility of all wavelengths was used for the fit in the right panel of Figure 5. In Table 3, some reference fitting results are listed. The size is typically between 0.9 pc and 2.2 pc, depending on baseline length, wavelength, and orientation. The half-light size of (2.0 ± 0.3) pc × (1.5 ± 0.3) pc would make a variability origin of the difference between the MIDI and Spitzer+ISAAC fluxes physically possible (see Section 3.1), but we note again that the difference spectrum is cool. Therefore, a final conclusion about the origin of the flux difference cannot be made. For the sizes please note that due to the uv-coverage, the largest (relative) uncertainties in the sizes are toward the semi-minor axis for the 58 m fit and toward the semi-major axis for the 100 m fit.

Table 3. Geometric Fits to the Visibilities of NGC 424

Fit Type	Wavelength	Semi-major Axis a				Semi-minor Axis b			Position Angle
	(μm)	(mas)	(pc)	(rin)		(mas)	(pc)	(rin)	(deg)
Gaussian	9.1	8.0 ± 1.0	1.7 ± 0.2	26 ± 3		6.5 ± 0.7	1.4 ± 0.2	21 ± 2	−40 ± 23
(PBL 58 m)	11.0	8.6 ± 1.2	1.9 ± 0.3	28 ± 3		7.9 ± 1.0	1.7 ± 0.2	25 ± 3	...
	12.5	10.2 ± 1.7	2.2 ± 0.4	33 ± 5		8.9 ± 1.8	1.9 ± 0.4	29 ± 5	...
	8.4–12.5	8.7 ± 0.9	1.9 ± 0.2	30 ± 3		7.6 ± 1.0	1.7 ± 0.2	26 ± 3	−40 ± 8
Gaussian	9.1	5.7 ± 1.5	1.2 ± 0.3	18 ± 4		4.0 ± 0.3	0.9 ± 0.1	13 ± 1	−12 ± 26
(PBL 100 m)	11.0	6.5 ± 1.6	1.4 ± 0.4	21 ± 5		4.9 ± 0.6	1.1 ± 0.1	16 ± 1	...
	12.5	6.8 ± 1.1	1.5 ± 0.2	22 ± 3		5.7 ± 1.3	1.2 ± 0.3	18 ± 4	...
	8.4–12.5	6.2 ± 0.8	1.4 ± 0.2	21 ± 2		4.7 ± 0.7	1.0 ± 0.2	16 ± 2	−5 ± 11
Half-light radius	9.1–12.5	9.1 ± 0.9	2.0 ± 0.2	31 ± 3		7.0 ± 1.1	1.5 ± 0.3	24 ± 4	−21 ± 3

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We present the sizes in observed (milliarcseconds) and physical (pc) units, as well as in intrinsic units, i.e., in units of the sublimation radius. The latter one is very useful when we want to compare properties in different objects with different luminosities (Kishimoto et al. 2009a, 2011b). For that we have to establish an inner radius, r_in of the dust distribution (also referred to as sublimation radius). Usually, this is inferred either from reverberation mapping or near-IR interferometry (e.g., Suganuma et al. 2006; Kishimoto et al. 2009b, 2011a). Since we do not have either information in this type 2 AGN, we use an estimate based on the 12 μm luminosity as discussed in Hönig et al. (2010). Taking the mean between Spitzer and MIDI total flux, we obtain r_in ∼ 0.065 pc for a luminosity of νL_ν(12 μm) = 5.4 × 10⁴³ erg s⁻¹.

The ellipse fits illustrate some key characteristics of the mid-IR emission source. They confirm the obvious trend that the source size increases with wavelength, as illustrated in Figure 6. Although the error bars are large (dominated by systematic errors in the visibility), a systematic trend of larger mid-IR emission sources for longer wavelengths is seen.

3.2.2. Position-angle Dependencies of the Mid-IR Source

The elliptical fits also provide information of the direction of elongation. In Table 3, we list the P.A. of the half-light radius for those fits that allow for such a constraint. While the formal fitting errors are considerable, all P.A.s systematically indicate an elongation toward the NW/SE direction. The fits that average over all wavelengths give approximately −5° < P.A. < −40°. There may be a trend that the P.A. changes slightly from the lower-resolution data at 58 m to the 100 m data. While such a trend would be reminiscent of interferometry data of the Circinus (Tristram et al. 2012), the uncertainty in the geometric determination of the P.A. does not allow us to formally establish such a skew (see also the P.A. for the half-light radius), in particular because the 100 m data do not have any coverage in the direction of the major axis. Therefore, any P.A. determined for the major axis in the 100 m data must be considered uncertain.

We note that this elongation is not affected by the inhomogeneous uv-coverage, since this has been taken care of as discussed above. However, when deriving sizes for each wavelength, only the spatial information of the interferometric data is used. We will now include the spectral information in our analysis to test the result from the geometric model fitting. For that we use a simple model of dust emission to characterize the mid-IR emission at as many P.A.s as possible that are covered by our observations. For each P.A., we simulate a brightness distribution of the form

$$\begin{equation} S_\nu (r) = B_\nu (T(r)) \cdot f_0\left(\frac{r}{r_\mathrm{in}}\right)^\alpha , \end{equation} \tag{ 1 }$$

where r is the distance from the AGN, r_in is the inner radius of the dust distribution, f₀ is the normalized surface density at the inner radius, α is the surface density power-law index, and B_ν the blackbody radiation with temperature T(r) at distance r. In the following analysis, we will fix f₀ = 1 for simplicity but use it in Section 4 as a free parameter. For the temperature we assume that the dust is in local thermal equilibrium and parameterize the temperature gradient by T(r) = T_sub · (r/r_in)^β. We use a sublimation temperature T_sub = 1 500 K and adopt β = −0.36 from the near-IR dust opacity model by Barvainis (1987). The only remaining parameter is the power-law index α given the r_rin value estimated in Section 3.1. This optically thin model describes the directly AGN-heated dust as we see it in the mid-IR, while any possible cooler dust is not covered. However, the far-IR SED does not suggest the presence of such a cool component (see Figure 3 and Section 3.1).

In Figure 7, we show the P.A. dependence of the surface density distribution power-law index α. Where applicable we fitted the model to two or three data sets with different baseline length (filled circles). If no second set was available within 10° single baseline data were used (open circles). The data clearly show a change of α with P.A. From about P.A. −90° to −45°, the power-law index becomes less negative. Toward larger positive P.A.s, α turns more negative with a peak in the range of 50°–80°. This change can be either interpreted as a P.A. dependence of the brightness distribution (i.e., traced back to a change in α, β, or a combination of both), or a P.A. dependence of r_in, which is used for normalization. In either case it illustrates that, indeed, the mid-IR source changes structurally with P.A., and the shallowest α values (=least negative) in P.A. ∼ − 30° corresponds to about the same direction as the major axis in the geometric fits.

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The analysis of the NGC 424 interferometric data presented so far was based on the observations with as little model dependence as possible in order to extract basic properties of the mid-IR emission. However, only specific aspects of the data have been investigated. Now, we want to model the full data set of mid-IR interferometric observations together with the SED. This can be done, in principle, with advanced models of clumpy tori (for such examples, see Hönig et al. 2006, 2010; Tristram & Schartmann 2011). On the other hand, the visibilities in NGC 424 do not go below 0.4 and models suggest that effects that clearly distinguish homogeneous models from clumpy ones will only appear when an object is better resolved (Schartmann et al. 2008; Hönig & Kishimoto 2010). This means the present observations are not sensitive to substructure but only to the overall distribution of the brightness.

We therefore decided to use a rather simple physical model to reproduce the interferometric and photometric data simultaneously to avoid overinterpretation. The model follows Kishimoto et al. (2011b) and was successfully applied to type 1 AGNs. The brightness distribution is defined by the dust surface density power-law index α for the emitting dust and the temperature gradient β of the dust (see Equation (1)), plus an additional hot ring component (see below). Here, β is left as a free parameter and T(r) starts at a maximum temperature T_max that we also fit for. The surface density f₀ at the inner radius will be used as a free parameter now (see Section 3.2.2). We note that we do not need an ad hoc assumption of r_in but can leave it as a free parameter as well. It sets the normalization of both the fluxes and the visibilities. For a given set of parameters, a larger r_in will result in higher fluxes (larger emitting surface) and lower visibilities (larger sizes). As already discussed, the mid-IR emission is elongated. Therefore, we describe the source shape, and as a consequence the brightness profile, as an ellipse with the ratio s = a/b between the semi-major and semi-minor axes and orientation ϕ of the major axis as parameters.⁶

The hottest emission in this model originally designed for type 1 AGNs is coming from a ring (in addition to the power law) at about the inner radius with temperature T_in = 1 400 K and with a contribution factor f_hot. This hot ring has been held responsible for the observed near-IR bump in the SEDs of many type 1 AGNs (e.g., Edelson & Malkan 1986; Kishimoto et al. 2011b; Mor & Netzer 2011). Since NGC 424 is a type 2 AGN, we will show model fits with and without the hot ring. The origin of this hot emission component is not fully understood. It may either be the result of a different dust size/composition in the hot region of the torus (silicates sublimate at cooler temperatures than graphite grains; small grains heat up to the sublimation temperature at larger distances), or coming from a graphite-dust component in the innermost part of the polar outflow region. We will discuss an additional possibility in Section 5.4. In any case, the near-IR bump is typically not seen in type 2 AGNs, indicating significant extinction toward this component. From the presence of the silicate features in emission, we conclude that the dust column toward a hot-dust component may be optically thin at >8 μm. From about 4 to 8 μm typical ISM dust extinction curves do not change significantly (but note that the dust may have a non-ISM composition; see Section 5.4). However, toward the near-IR J, H, and K bands—the peak regions of the hot-dust component—the extinction increases dramatically by a factor of 5–10. To avoid these subtle problems, we ignore the shortest wavelengths in the fitting process.

As discussed in the previous section, there may be several problems involved when trying to fit the near-IR photometric data, most notable the possible extinction. We therefore restrict our modeling efforts to the data >3 μm. In order to capture the downturn of the SED longward of 20 μm, but not overweighting the spectroscopic data over the interferometric, we extract fluxes at 22 μm and 32 μm from the Spitzer data and treat them as photometric data points. In addition, to account for the offset of the MIDI data with respect to the rest of the SED (see Section 3.1) the photometry has been corrected by a factor of 1.3, so that the Spitzer IRS 8–13 μm matches the MIDI spectrum.

To speed up the fitting process and gain S/N, we resampled and binned the interferometric data. The 8–13 μm range will be represented by nine wavelength bins per data set in the fitting process (see Figure 10 in the Appendix). While the 2009 and 2010 data are in general agreement for all baseline configurations that match between both epochs, two of the 2007 data sets show significant disagreement. In particular, the two U3–U4 sets with only moderately different PBL and P.A. (see Table 1) are inconsistent with each other. There are several technical difficulties involved when dealing with the 2007 data (e.g., the IRIS beam-tracking system of the VLTI did not work properly at that time on targets with K-magnitude ∼10, resulting in poor/unstable beam overlap) in addition to the low-quality total flux reference of 2007 (see Section 2.1), so that we will not include these data in the fitting process. However, the fits will be overplotted onto the 2007 data in Figure 10 as a comparison.

Last, the visibilities and correlated fluxes show a distinct change of slope at the shortest wavelength bin at about 8.4 μm with respect to the rest of the spectrum. There are two possibilities that might explain this feature in the data. First, the change of slope could be real. As a consequence, the wavelength-dependent sizes would not become smaller with decreasing wavelength anymore. Indeed the 8.4 μm sizes may be even systematically larger than for the neighboring 9.1 μm bin (see Figure 6). While this is difficult to explain within simple radiative equilibrium, it could be an effect of extinction/obscuration at shorter wavelengths. However, as discussed in the previous section, the overall increase of extinction toward shorter wavelengths in this range is rather small for any kind of dust (ISM, large grains, silicates, graphites). Another possibility for a real decrease in visibility is contamination by the 7.7 μm PAH feature. Indeed, this feature is weakly present in the Spitzer IRS spectrum, and the wings may still be seen on the short-wavelength side of our MIDI total flux spectrum. If these wings are not present in the correlated fluxes, but only in the total flux, the visibility would be expected to be lower, as seen here. However, in this case the lower visibilities are not physically related to the parsec-scale dust distribution. Second, the lower 8.4 μm visibilities/correlated fluxes may be an instrumental/calibration effect. Indeed, the sensitivity of the MIDI detector is sharply changing at these short wavelengths, in addition to generally lower flux levels than at longer wavelengths and shorter coherence times. To avoid problems with these lower visibility at 8.4 μm that are most likely not related to the parsec-scale dust, we will exclude it from the fitting process (i.e., using exclusively 9–13 μm data).

In Table 4, we list the results for a number of different modeling approaches (models as described in Section 4.1). The first model uses all the data as described in Section 4.2 and fits the temperature/density-gradient model plus the hot ring. The second fit excludes the L- and M-band photometry as well as the hot ring that is essentially only necessary to account for these wavelengths (see Section 4.1). The resulting reduced χ²_r of both models are equal, underlining this point. Finally, we use only the MIDI data in the 9–13 μm range and model it with the temperature- and density-gradient (or T/d-gradient) model without hot ring, again with a χ²_r ∼ 0.8 as the previous fits.

The resulting parameters point toward a surface density distribution with power-law index α ∼ −0.9... − 1.0. This result is consistent with the more model-independent determination of α illustrated in Figure 7, taking into account that the more physical model accounts for the P.A. dependence by ellipticity. The value of α is rather on the steeper side when compared to type 1 AGNs (Kishimoto et al. 2011b), which is particularly interesting since torus models would predict generally redder colors for type 2s than type 1s because of obscuration effects.

The orientation of the mid-IR emission is quite similar to what we got from the geometric fits (consistent within error bars). A P.A. of −27°  ±  2° is within the range of P.A.s as listed in Table 3. The a/b ratio of 2.2 in the model illustrates the strong ellipticity of the mid-IR source. This ratio may seem larger than visible in Figure 5, but these perceived differences come, at least in part, from resolution effects (see also the stronger ellipticity for the 100 m data than for the 58 m data).⁷

For illustration, we also fitted the model to the photometry-only data. The model using the hot ring + temperature gradient is basically unconstrained with the range of parameters of 2 × χ²_{r; min} given in Table 4. The same happens when using the T/d-gradient model on the MIDI-only photometry (i.e., the 8–13 μm total flux spectrum). Both limits are, at best, marginally consistent with the fits involving interferometry. Better constraints are achieved for the T/d-gradient model fitted to the mid-IR photometry without the L- and M-band fluxes. Although the range of parameters allowed is still considerable, it is in overall agreement with the fits including interferometry. This illustrates that the spatial information is important to properly constrain such models. Using complex models on SED-only data may therefore lead to results contradictory to the actual spatial distribution of the dust, unless the wavelength range and/or parameters are carefully chosen (for a more detailed discussion with respect to clumpy torus models, see Hönig & Kishimoto 2010).

Finally, we show the 11 μm visibilities versus baseline in Figure 8 for all baseline configurations, except UT1–UT3. The data sets for this configuration have been taken under the worst observing conditions, which resulted in difficult calibration, large error bars, and the lowest S/N of all configurations (see Table 1). Therefore, we exclude these sets from the plot for clarity. The P.A. for each data point is color coded to be able to distinguish the baseline configurations closer to the major axis (toward dark blue) and to the minor axis (toward red) as set by the model orientation. As discussed before, the inhomogeneous distribution of telescope P.A.s and projected baselines make a direct assessment of any kind of elongation difficult. However when concentrating on the shorter baseline lengths (50–60 m), a trend is seen that the blue data points show systematically lower visibilities than the green points. At the longest baselines (>100 m), the green data points, about in between major and minor axes, are also systematically lower than the red data that are pointing approximately in the direction of the minor axis in the model. As a comparison and further way to prove the elongation and direction of the mid-IR source, we overplotted the visibility curves for different P.A.s based on the T/d-gradient model without L/M-band photometry. This illustrates that, in spite of the scatter and the inhomogeneity of the data, the mid-IR source shape and radial brightness distribution are well reproduced by the elliptical T/d-gradient model.

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The orientation of the major axis can be put in the context of the AGN system axis in NGC 424. Although Nagar et al. (1999) report that their 20 cm/1.5 GHz radio observations may indicate a marginal extension in about the same direction as their beam size, this could not be confirmed with their higher-resolution 3.6 cm/8.4 GHz data. Mundell et al. (2000) consider the source to be unresolved at both frequencies in their data with better S/N, in line with the analysis by Thean et al. (2000). Consequently, a possible linear radio structure that could indicate the system axis is not detected.

Durret & Bergeron (1987) report [O iii] imaging observations of the nucleus. They note an elongation of the [O iii] in P.A.  ∼  70°. The extension is seen best in the faintest two contours of 1.25% and 2.5% relative to their peak contour. The authors note that the determination of the continuum (dominated by the host galaxy) is uncertain, which may result in problems subtracting the continuum emission from the line image. They give an elongation of the [O iii] emission in P.A. 70°, with the galaxy major axes in P.A. 59° (i.e., ΔP.A. = 11°). Unfortunately, no Hubble Space Telescope image at higher quality is available.

Moran et al. (2000) report results from optical spectro-polarimetry of the nuclear region of NGC 424. While the unpolarized spectrum shows signs of a strong host component of 45% (quite typical for Seyfert 2 AGNs), the polarized flux displays broad emission lines, which indicates a hidden type 1 nucleus. From their polarization data, the authors give a polarization angle of 43°. In type 2 AGNs, the polarization angle has been found to be dominated by scattering (either electrons or dust) in the polar region of the AGN and be approximately orthogonal to the linear radio axis where such a feature was detected (e.g., Antonucci 1983, 1993, 2001; Smith et al. 2004). It is, therefore, also orthogonal to the symmetry axis of the torus. In order to confirm that the scatterer is located in the polar region, we estimated the intrinsic polarization of the scattered light. Murayami et al. (1998) report deep spectral observations of the Hα line and detect shallow broad wings to the otherwise narrow emission line. We extracted the flux from their spectrum and compared it to the polarized (Stokes) flux from Moran et al. (2000). The broad component of the Hα line shows about 15% polarization, which is typical for scattering over a sizable solid angle as in the polar region of the AGN (e.g., Antonucci & Miller 1985).

Based on the polarization measurements, the system axis for NGC 424 would be established in approximately P.A. 43° − 90° = −47°. This is not consistent with the [O iii] P.A. Both methods are not accurate, though, and offsets have to be expected (e.g., Antonucci 2001; Schmitt et al. 2003; Smith et al. 2004). In the particular case of NGC 424—based on our analysis of the polarization data and given the mentioned uncertainties with the [O iii] data—we consider the polarization data as the more reliable method to estimate the system axis. As a consequence, we expect that the system axis is close to P.A. −47°, which is very close to the direction of the major axis of the mid-IR emission (P.A. −27°), meaning that the parsec-scale mid-IR emission shows elongation in the polar direction.

The elongation of the mid-IR emission in the polar direction, instead of the "disk/torus plane," has been observed recently in some objects (e.g., Bock et al. 2000; Packham et al. 2005).

Circinus galaxy. Packham et al. (2005) show mid-IR images of the AGN in the Circinus galaxy at 8.8 μm and 18.3 μm with some extended emission ∼40 pc to the east and west of the nucleus that coincides spatially with the rim of the known [O iii] outflow cone. Reunanen et al. (2010) compare their VLT/VISIR 11.8 and 18.7 μm images to near-IR [S vii] coronal line images and demonstrate that the mid-IR extended emission traces the narrow emission line region (NLR). Packham et al. (2005) further note that this extended emission is responsible for about 30% of the total emission in the central 35 while the other ∼70% originate from the unresolved nucleus (<20 pc in their images). Both the color temperature and intensity of the extended emission are in good agreement with optically thin dust in local thermal equilibrium and heated by the AGN.

NGC 1068. The outflow region of NGC 1068 displays significant mid-IR emission that extends out to more than 50 pc from the nucleus (e.g., Bock et al. 2000; Tomono et al. 2001) and associated with emission from dust clouds (e.g., Tomono et al. 2006). The extended emission contributes about 70% to the 11.6 μm flux in 12 or 85 pc aperture around the nucleus (Mason et al. 2006). Yet, on parsec scales the emission is dominated by the circumnuclear dust and extends about perpendicular to the large-scale outflow cavity (Jaffe et al. 2004; Raban et al. 2009). It is noteworthy, though, that the elongation in outflow direction has also been observed by Speckle imaging in the near-IR (H and K bands) with a K-band size of 1.3 pc × 2.8 pc (error ±0.3 pc; Wittkowski et al. 1998; Weigelt et al. 2004). While one may argue about peculiarities of NGC 1068, NGC 424 and Circinus indicate that a polar-extended mid-IR emission is not untypical in AGNs.

Other objects. Hönig et al. (2010) presented mid-IR 8 m telescope images of IC 5063 and MCG–3–34–64 that also showed mid-IR emission in polar direction. In these cases, however, the extension is observed on scales of >100 pc, while in NGC 424 we are dealing with a factor of 100 smaller distance. In NGC 1386, Reunanen et al. (2010) report extended mid-IR emission in approximately north/south direction. This is at about the same direction as the [O iii] outflow⁸ (Ferruit et al. 2000) and radio-emission P.A. (Nagar et al. 1999) in this object. Finally, we note that the type 1 AGN NGC 3783 is another candidate with even stronger elongation in polar direction on parsec scales (Hönig et al. 2010; Kishimoto et al. 2011b; S. F. Hönig et al. 2012, in preparation).

The extended mid-IR emission in polar region in NGC 424 gives rise to the question if it is consistent with the paradigm that emission from the dusty torus dominates the mid-IR emission. This paradigm is well summarized in Mor et al. (2009, their Figure 4) where the authors show results from decomposition of Spitzer SEDs of type 1 AGNs into the torus, a dust NLR, and an additional hot-dust components. According to the median of all decompositions, the torus contributes ⩾80% of the flux between 7 and 15 μm and dominates (>50%) in the range from 4 to 25 μm (the long-wavelength side being limited by wavelength coverage of the data). However, our data suggest that in NGC 424 dust in the polar region dominates the mid-IR emission. Indeed, we do see elongation in polar direction at all observed baseline lengths, i.e., a consistency in elongation toward polar direction at these spatial scales. At the longest baseline lengths, the visibilities have dropped to ∼0.4, meaning that at least 60% of the mid-IR single-aperture/total flux show this elongation direction. Thus, it is not too far reaching to interpret this as evidence that ≳ 60% of the mid-IR total flux emission originates from the polar region.

Interestingly, Tristram et al. (2012) report on new mid-IR interferometric observations of the Circinus galaxy on short baseline lengths, complementing previous results on longer baselines (Tristram et al. 2007). These new data indicate that the mid-IR emission is extended in outflow direction not only in the single-telescope aperture as mentioned above, but also on the new baselines. Only at baseline lengths longer than ∼50 m does the orientation of the mid-IR emission (described as a 0.4 pc "disk") change direction toward perpendicular to the outflow region and, therefore, in the direction expected for the classical torus. However, we note that at these baselines the visibilities decreased to ∼0.15, meaning that 85% of the single-aperture mid-IR flux originates from the polar region on scales as small as few parsecs.

Whether our data for NGC 424 can be considered consistent with the paradigm of mid-IR emission primarily originating from the torus depends on the question if torus models can reproduce the elongation in polar direction in an obscured AGN. It might be possible that self-shielding effects in the mid-plane of the torus cause the torus to appear elongated in polar direction while the dust is still strongly concentrated to the mid-plane. An additional factor could be anisotropic illumination from the BBB/accretion disk onto the torus, i.e., radiation escaping preferentially in polar direction. Detailed radiative transfer models that also include visibility studies have been presented by Schartmann et al. (2008, smooth and clumpy) and Hönig & Kishimoto (2010, clumpy), the former also invoking anisotropy of the accretion disk. Both studies agree that for sets of models that do represent well the IR SEDs as observed for typical AGNs, the visibilities should be higher in polar direction (i.e., smaller size) than along the mid-plane of the torus in type-2-like edge-on viewing angles, and more circular toward face-on orientations. Strong polar extension is not to be expected. This also means that although there is self-shielding in the equatorial plane of the torus, the dust mass in typical radiative transfer models is not sufficient to cause strong-enough obscuration that the mid-IR emission is seen elongated in polar direction.

The radiative transfer models use ad hoc parameterizations of the dust distribution, while hydrodynamical simulations attempt at getting the mass distribution self-consistently from global parameters of the AGN and its environment. Wada et al. (2009) present high-resolution numerical simulations of ISM accretion in the inner 32 pc of an AGN. Although the authors did not perform radiative transfer simulations of the hydrodynamic models, the mass density distribution in the inner parsecs is quite flat (disk-like). At larger radii (>10 pc), the disk flares resulting in a scale height of the order unity, which is also typically used in the previously mentioned radiative transfer models. Therefore, it is not expected that these models result in strong polar-extended mid-IR emission. Schartmann et al. (2009) present hydrodynamic simulations of the central 100 pc around an AGN, assuming that the dust is produced in situ by a stellar cluster. The authors show (log-scale) mid-IR radiative transfer images and SEDs of their standard model. Indeed, these models do show elongation in polar direction and might be considered a better match to our observations. As the authors note, the polar elongation is the result of heavy self-absorbing in a dense torus-plane region, so that only the funnels of their mass distribution are illuminated. As a consequence, the overall IR emission is dominated by cool dust resulting in very red SEDs, incompatible with NGC 424 (note that the SED of the type 2 AGN NGC 424 is significantly bluer in the mid-IR than those of type 1 AGNs NGC 4151 and Mark 841 that have been modeled using these simulations; see Figure 10 in Schartmann et al. 2009). In particular, the observed silicate emission feature in NGC 424 would require an orientation close to face-on in these models, which, in turn, would make the mid-IR emission appear rather circular (i.e., dominated by the inner disky structure as noted by the authors) and probably inconsistent with the a/b = 2.2 axis ratio in NGC 424. Additionally, the observed high hydrogen column density would be difficult to reproduce.

As mentioned above, self-absorption of mid-IR radiation in the plane of the torus might be a possibility for the polar extension of the mid-IR emission. If the polar-extended mid-IR emission was caused by a high optical depth in the mid-IR (i.e., the mid-IR emission of the dust in the torus gets reabsorbed), then we would have to expect that the "missing" mid-IR emission from the plane is re-emitted at longer wavelengths. Consequently, we would see either a small bump in the nuclear SED at longer wavelengths (where the optical depth becomes smaller than unity), or, at least, that the nuclear SED appears very red (as illustrated by the hydrodynamic simulations of Schartmann et al. 2009). In NGC 424, however, neither is seen. On the contrary, longward of the MIDI-covered 8–13 μm range and the 18 μm silicate feature, the SED turns down (see Figure 3),⁹ meaning that there is probably no significant amount of radiating dust at longer wavelengths outside of what we see with MIDI.

Concluding our analysis from the previous section, it is very unlikely that in NGC 424 the elongation in polar direction is (directly) associated with the dusty torus. It seems much more likely that ≳60% of the mid-IR emission arises in the polar region. When also considering the results by Tristram et al. (2012) on Circinus, we may argue that the torus becomes energetically insignificant in the mid-IR in these two sources (and probably NGC 3783). Considering that a few other sources also show extension in polar direction in the mid-IR on larger scales (and, in contrast, there are no clear cases with dominant disk-like mid-IR emission), it is not reaching too far to discuss possible implications for AGNs in general.

First, we will have to address the nature of the emission in the polar region in NGC 424. As indicated by our model results shown above, the emission is consistent with dust in thermal equilibrium with the AGN. It is therefore reasonable to assume that the mid-IR emission is indeed originating from dust. Since we know that ≳60% of the mid-IR single-aperture emission originates from the polar region, we can use the IR SED in this wavelength region as a further constraint. The IRS and MIDI spectra show weak silicate emission features at 10 and 18 μm, which by its own points toward dust as the origin. Moreover, it indicates that the emitting dusty medium is optically thin to its own thermal radiation (see similarities and small anisotropy of mid-IR emission of type 1 and type 2 AGNs; Hönig et al. 2011). Therefore, if we would look through the polar region onto the AGN (type 1 case), the overall column would still be optically thin. This would be consistent with the unification scheme that requires mostly unobscured views of the central engine along face-on line of sights (at least in a statistical sense). However, if the dust is optically thin in the mid-IR, the weakness of the silicate features would argue for a deficiency of silicate grains with respect to graphite grains in ISM dust mixtures, as expected when dust is exposed to the BBB radiation field (e.g., Phinney 1989).

NGC 424 is a type 2 AGN and it certainly has an obscuring medium along the line of sight to the accretion disk and BLR given the observed high X-ray column density in spite of displaying a silicate emission feature (a mismatch in column density versus optical/IR obscuration is common in AGNs; Maiolino et al. 2001). This means that it offers support for the validity of the unification scheme in this source. The unification scheme also requires a geometrically thick torus—for which we did not find evidence in the mid-IR here. However, angle-dependent obscuration does not require that the scale height is large at all distances from the AGN, although this is what most torus models assume. It is well possible that the geometrically thick part that gives rise to the required torus covering factor in the unification scheme is limited to the innermost dusty region. Indeed, as mentioned before, type 1 AGNs show a distinct near-IR "3–5 μm bump" in the SED from hot dust, and this component has different emission characteristics than the mid-IR emission (Mor et al. 2009; Kishimoto et al. 2011b). The size of this component is consistent with AGN-heated, large graphite grains near their sublimation temperature (Kishimoto et al. 2011a). Moreover, IR interferometry suggests that this hot-dust component has a surface filling factor at least comparable to or even larger than the cool dust (see Figure 11 of Kishimoto et al. 2011b). One interpretation of this result is a considerable covering factor of the hot-dust component (see also Mor et al. 2009 for an SED-based argument on the covering factor of the hot component). We may, therefore, speculate that the classical, obscuring torus is, in fact, related to the hot-dust emitting dust component rather than the mid-IR-emitting component.

In Figure 9, we show a sketch of the dust distribution around NGC 424 as this scenario would suggest. The torus flares strongly in the innermost part, but becomes geometrically thin in the outer regions. In this picture, the near-IR emission originates from the obscuring part of the torus while the mid-IR emission comes primarily from the polar region. The IR-emitting sources in the polar regions have been drawn as clouds, but they could also be filamentary. This inhomogeneous structure of the dust is motivated by the mid-IR interferometry of Circinus (Tristram et al. 2007), but also mid-IR imaging of NGC 1068 (e.g., Galliano et al. 2005). Moreover, in Circinus (but not necessarily in NGC 1068), the extended mid-IR emission in the single-telescope images seems to be associated with the edge of the outflow region (Tristram et al. 2007, 2012). We also see an offset of 20° between mid-IR P.A. and the polarization angle in NGC 424. This could be a sign of an edge-brightening effect as expected in a cone-like region filled with optically thin dust. Therefore, we placed the polar-region clouds preferentially around the edges of the opening cone in the sketch. Note that this is not a full picture of the distribution of the dust but only a sketch for the IR-emitting material and the polar region may well be filled more homogeneously with dust clouds.

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This picture also has several implications related to the physical mechanisms around the AGN. The new observations presented here can be interpreted as strong support for a dusty wind (as used in models for the X-ray emission of some AGNs; e.g., Komossa & Fink 1997; Komossa & Bade 1998; Turner et al. 2003). This wind could be driven by radiation pressure on dust grains in the AGN environment. For an AGN in the Seyfert regime radiating at about a tenth of its Eddington luminosity L_Edd (defined by electron scattering), radiation pressure is sufficient to unbind dust from the gravitational potential (the corresponding dust Eddington luminosity is L_{Edd; dust} ∼ 10^{−4... − 5} L_Edd; Pier & Krolik 1992; Hönig & Beckert 2007). The mid-IR spectra also hint toward the source of the dust that is driven outward in polar direction: as discussed in Section 5.4 the dust seems to be deficient of silicate grains. Silicates have a lower sublimation temperature than graphite grains (Phinney 1989), making them disappear from a dusty medium at generally larger distances from the heating source than graphite grains (for a given grain size). This is consistent with the near-IR interferometry and reverberation mapping sizes in AGNs that favor large graphite grains for the hottest dust at the sublimation temperature (Kishimoto et al. 2007, 2009b), and similar requirements from optical extinction curves in type 1 AGNs (Gaskell et al. 2004). This inner region is also the one closest to the AGN where the dust is directly exposed (i.e., without significant self-obscuration) to the strongest radiation field. It is therefore plausible to expect that the dust is driven away from this inner part of the torus. This scenario seems qualitatively in agreement with recent radiation feedback simulations in hydrodynamic simulations (Roth et al. 2012). Keating et al. (2012) recently presented IR SEDs of wind models that could eventually serve as a starting point for a more quantitative test of dusty winds.

Radiatively driven winds or radiation pressure in both the optical and IR have also been suggested as a mechanism to support the apparent geometrical thickness of the torus (e.g., Königl & Kartje 1994; Elvis 2000; Krolik 2007). Of course, this mechanism works best in the inner part of the torus close to the sublimation radius because of self-shielding effects further out (Krolik 2007). As a consequence, the geometrical thickness would be limited to just the inner few sublimation radii distances from the AGN where, incidentally, the dust is hot enough to cause the near-IR bump seen in type 1 AGNs. As pointed out in Kishimoto et al. (2011b), an analogous near-IR bump in the dusty disks of young stellar objects (YSOs) has been described as a "puffed-up inner rim" (Natta et al. 2001; Dullemond et al. 2001). In this scenario, we would see the near-IR bump pronounced or weak/absorbed depending on our line of sight to the hotter or cooler side of this compact structure. Unlike in AGNs, however, a dusty outflow associated with the puffed-up rim has not been observed. This difference is not unexpected given the much softer radiation field of a star compared to an AGN and the associated lower strength of UV/optical radiation pressure to launch such a wind.

The question remains if objects like NGC 424 and Circinus (and possibly NGC 1068) are a special class of objects or if the dusty polar regions are more generic to the AGN population (see also Schweitzer et al. 2008). As discussed above, there are signs of polar extensions in the mid-IR on scales of tens of parsecs in other Seyfert galaxies as well, but we do not have the interferometry data to test if this polar component is energetically dominant in these sources. If it is generic, then it will require adjustments to current models of the dust emission of AGNs.