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Astron. Astrophys. 353, 465-472 (2000)

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4. Data analysis

We present in Fig. 1 the L and M band images of NGC 1068 . We used a magnitude (log) scale because of the high dynamics of the images provided by the AO. The images have been deconvolved using a Lucy-Richardson algorithm (MIDAS package). We have also observed NGC 1068 in the L' band: this image is very similar to the one obtained in the L band. Thus, our current data analysis and subsequent discussion will be based on the two L and M bands, only.

[FIGURE] Fig. 1. Left: L band image of NGC 1068 . Right: M band image of NGC 1068 . North is up, East is to the left.

The L and M band images show:

i) an unresolved core down to the resolution (FWHM) of 0.24" (16 pc) and 0.33" (22 pc), respectively at 3.5 and 4.8µm. This core has already been observed at 3.6µm by Chelli et al. (1987) and at 2.2µm by Marco et al. (1997), Thatte et al. (1997) and Rouan et al. (1998). The latter give an upper limit of the core size (FWHM) of 0.12" (less than 8 pc).

ii) an elongated structure at P.A. [FORMULA] 100o particularly prominent in the M band, but also quite well outlined in the L band. This structure is obviously coincident with the structure seen in the K band by Rouan et al. (1998) and is roughly perpendicular to the axis of the inner ionizing cone (P.A.=15o, Evans et al. 1991). It extends in total over [FORMULA] 80 pc, with a bright spot at each of the [FORMULA] E and [FORMULA] W edges at a radius of [FORMULA] 25 pc from the central engine.

iii) an extended emission along the NS direction, almost aligned with the radio axis and the ionizing cone axis. At low level isophotes (in particular in the L band), a change in the direction of the axis of this emission can be noticed, reminiscent of a similar change of direction of the radio jet (Gallimore et al. 1996a).

Down to faint levels, the 4.8µm thermal infrared emission appears to be extended over [FORMULA] 3" in diameter ([FORMULA] 210 pc).

It is striking that the two different AO systems used, ADONIS for the L and M bands and PUEO for the K band, reveal a similar structure of the AGN environment. These AO systems are using WFS of different types, Shack-Hartmann for ADONIS and curvature for PUEO, as well as deformable mirrors of different types, piezo-stack for ADONIS and bimorph for PUEO. The Lucy-Richardson deconvolution applied on both data sets uses PSFs obtained in two different ways: we used an observed stellar PSF in the case of the ADONIS data set - as the L and M band data are less sensitive to rapid PSF fluctuations - and we used the PSF recovered from the AO loop parameters in the case of the PUEO data set. The two AO experiments differ in many aspects, while leading to a similar result for the structure of the AGN dusty environment. Therefore we are quite confident that this structure is real and not hampered by significant AO artifacts (Chapman et al. 1999). Finally it should be noticed that the high resolution image of the AGN in NGC 1068 obtained in the K band with the AO system at the Keck telescope (www2.keck.hawaii.edu/realpublic/ao/ngc1068.html) reveals a comparable structure, pending that a precise orientation and a scale be provided for the Keck AO data set.

4.1. Location of the emission peaks at 3.5 and 4.8µm and nature of the unresolved core

As it was not possible to observe simultaneously in the visible and in the infrared, we took advantage of a characteristic feature of AO systems which is to preserve the optical center for all objects: the infrared camera field has a position fixed in regard to the centroid of the visible counterpart of the object observed. Indeed, by observing a star (PSF or photometric standard), we determine a reference position in the infrared image to within the precision we are aiming at in this study (better than one infrared camera pixel, 0.1"). Any offset of the galaxy infrared peak relatively to the star infrared peak would then reveal an intrinsic offset between the galaxy infrared light peak and the galaxy visible light peak. This is a method for positioning infrared versus visible sources in the AGN.

To improve the precision, we have fitted the PSF and the NGC 1068 emission peaks by Gaussian profiles. The L and M band peaks in NGC 1068 are coincident within the positional precision given above. We have also derived the position of this L and M peak in NGC 1068 with respect to the visible peak, following the procedure described in Sect. 3: it is offset by [FORMULA]" S and [FORMULA]" W of the visible continuum peak and therefore is found to be coincident with the K band emission peak (Marco et al. 1997), within the error bars.

Therefore, the compact core at 3.5 and 4.8µm can be identified with the unresolved core detected at 2.2µm (Marco et al. 1997; Thatte et al. 1997; Rouan et al. 1998), itself found to be coincident with the mid-infrared emission peak at 12.4µm (Braatz et al. 1993), the radio source S1 (Gallimore et al. 1996a, b) and the center of symmetry of the UV polarization map (Capetti et al. 1995). This strengthens considerably the interpretation of the core infrared emission originating from hot/warm dust in the immediate surroundings of the central engine.

4.2. The torus-like structure at 3.5 and 4.8µm

The location, position angle and extension of the P.A. [FORMULA] 100o structure are strongly suggestive of a dusty/molecular torus. The two bright spots on the edges of the structure outline the "disky" nature of the torus, up to a radius of [FORMULA] 40 pc from the central engine. This dusty/molecular torus would be responsible for the collimation of UV radiation from the central engine, leading to the ionizing cone (Pogge 1988; Evans et al. 1991). The overall spatial extension of the torus is found to be [FORMULA] 80 pc at 3.5 and 4.8µm, while it appears to be slightly smaller at 2.2µm, [FORMULA] 50 pc. Under the very simple assumption of optically thick grey-body dust radiation (Barvainis 1987), it is well understood that the emission at 2.2µm traces hotter dust (T[FORMULA] 1300 K) than the emission at 4.8µm (T[FORMULA] 600 K). The observed difference in size would then signal the existence of a temperature gradient of the grains across the torus.

4.3. The North South extended emission

The NS extended emission (overall extent [FORMULA] 3" down to faint emission levels) is also detected at 2.2µm on a similar scale (Rouan et al. 1998) and at 10 and 20µm on a larger scale, although along a similar P.A. (Alloin et al. 1999). This structure can be related to the emission of hot/warm dust associated with NLR clouds identified in the northern side of the ionization cone from HST data (Evans et al. 1991) and hidden behind the disc of the galaxy in its southern side. Additional local heating processes, e.g. related to shocks induced by the jet propagation, might be at work as well along the NS extension. The latter suggestion stems from the conspicuous change of direction of the emission at 3.5µm, following that of the radio jet. Indeed, Kriss et al. (1992) have shown through the analysis of line emission that in NGC 1068 the emitting gas in the NLR is partly excited through shocks triggered by the radio jet.

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© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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