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Astron. Astrophys. 329, L45-L48 (1998)

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3. Results

The top diagram of Fig. 2 shows the azimuthally averaged visibility function of NGC 1068 together with the corresponding uniform-disk (UD) and Gaussian fits. The object visibility function was obtained by compensating both, the seeing-calibrated speckle transfer function and an additional transfer function due to the different spectra of NGC 1068 and the reference star HIC 12014 (K0). On the bottom diagram the analogous plots are shown for the reference star HIC 12014. The HIC 12014 visibility function was obtained by reducing two different data sets of HIC 12014. The reconstructed visibility function of NGC 1068 decreases to about 50 % of the zero-frequency value at the diffraction cut-off frequency [FORMULA]. The low-frequency peak is caused by the underlying galaxy. It is not yet clear whether the short horizontal part of the visibility function at [FORMULA] is caused by an additional point source or by an overestimation of the transfer function due to the different spectra of NGC 1068 and the reference star. The decrease of the NGC 1068 visibility function is much larger than the errors (see caption of Fig. 2). For example, between 10 % and 70 % of [FORMULA] the visibility function decreases by about 25 %, which is approximately three times larger than the 8 % error at 70 % of [FORMULA]. Nevertheless, future observations will be useful to confirm the result and to improve the accuracy.

[FIGURE] Fig. 2. Azimuthally averaged visibilities of NGC 1068 (top) and the unresolved reference star HIC 12014 (bottom). The diamonds indicate the observed visibilities, the solid lines the Gaussian fits, while the dashed line is a UD fit. The fit range is between 20 % and 70 % of the telescope cut-off frequency [FORMULA]. The visibility errors at 20 %, 50 %, 70 %, 80 % and 90 % of [FORMULA] are [FORMULA] 0.05, [FORMULA] 0.05, [FORMULA] 0.08, [FORMULA] 0.15, [FORMULA] 0.20, respectively.

The diameter of the resolved NGC 1068 core was derived from the reconstructed visibility function by fitting symmetric uniform disks (UD) and Gaussian models. The fits were carried out in the range between 20 % and 70 % of [FORMULA]. The fitted diameters [FORMULA] (FWHM) and [FORMULA] of the Gaussian and the UD model are [FORMULA] and [FORMULA]. These diameters correspond to 2.2 pc [FORMULA] 0.5 pc and 3.7 pc [FORMULA] 0.8 pc, respectively. An alternative model interpretation of the decreasing visibility function is, for example, an object slightly larger than the above diameters plus an unresolved central object or an object which is unresolved in one direction ([FORMULA] EW), but resolved in the direction perpendicular to it ([FORMULA] NS).

Fig. 3 shows the diffraction-limited speckle masking reconstruction of NGC 1068. The resolution of the reconstructed image is 76 mas. It shows an elongated structure in the northern direction, i.e. approximately the position of the radio jet. Fig. 4 shows the azimuthally averaged radial plots of the reconstructions.


[FIGURE] Fig. 3. Diffraction-limited speckle masking reconstruction of NGC 1068 (top) and the unresolved star HIC 12014 (bottom). The contours are plotted from 6% to 100% of peak intensity in 10 steps.

[FIGURE] Fig. 4. Azimuthally averaged radial plots of the reconstructed images of NGC 1068 (solid line) and the reference star HIC 12014 (dashed line).

Photometry was performed by comparing the integral intensities of the long-exposure images of NGC 1068 and the photometric standard star HIC 110609. It yields for the flux [FORMULA] of NGC 1068 in the K-band the value [FORMULA]. From this value we have to subtract the flux of the underlying faint extended component in order to get the flux [FORMULA] of only the 30 mas component. We determine the contribution of this extended component from the zero-frequency visibility peak discussed above to 20% [FORMULA] 10% of [FORMULA]. This results in a flux from the 30 mas component of [FORMULA].

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© European Southern Observatory (ESO) 1998

Online publication: December 16, 1997
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