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

9. Conclusion

The observation, for the first time at high angular resolution, of NGC 1068 at 3.5 and 4.8µm provides new information to build a more realistic model of this AGN under the current unification scheme.

With regard to the AGN structure, we do observe:

(i) an unresolved core, already known at 2.2µm to have a size (FWHM) less than 8 pc, and interpreted as the inner region of a dusty/molecular torus in which the central engine of NGC 1068 is embedded,

(ii) along P.A. 100^o, an extended emission up to 40 pc on either side of the core, particularly prominent at 4.8µm. This structure, also detected at 2.2µm up to 20 pc on either side of the core, is coincident in P.A. with the parsec-scale disc of ionized gas detected with VLBI (P.A. 110^o), and is found to be roughly perpendicular to the axis of the ionization cone in NGC 1068 . We interpret it as the trace, up to a 40 pc radius, of this dusty/molecular torus seen edge-on,

(iii) an extended emission along the NS direction up to 50 pc from the core and with rather symmetrical properties on either side, both at 3.5 and 4.8µm. Again, this extended emission is detected, both at 2.2µm on a similar scale, and at 10 and 20µm on a slightly larger scale. It reveals the presence of dust in the NLR, heated both by hard radiation within the ionizing cone and by shocks associated with the AGN radio jet.

With regard to the dust temperature and dust distribution in the central arcsec of the nucleus, we get a final picture as follows. As close as r 1 pc from the central engine, the dust is heated up to its evaporation temperature, 1500 K. Then the dust temperature declines very rapidly and reaches T 500 K at 28 pc. The total mass of warm dust within a 22 pc radius region is found to be around 0.5 .

It is observed as well that the near infrared flux of NGC 1068 , in the 2.2 to 4.8µm range, has increased by a factor of two over some 20 years, while in the 10µm window the flux increase is only by a factor of 1.2.

Our results bring observational evidence of a dusty torus in the AGN of NGC 1068 . They further support AGN modelling in the framework of the unification scheme: a thick torus surrounding a central engine (black hole and accretion disc). Although several models of the AGN in NGC 1068 are available, none matches in detail all the aspects of the current near-infrared results obtained at a subarsec scale. Such new observational constraints make it both timely and exciting to run updated models. Yet, we are aware that the most convincing and undisputable argument for the presence of the torus-like structure will come from a study of the kinematics of the gas within the 100 central parsec of NGC 1068 . We expect such information to be soon obtained from ISAAC/ANTU observations on Paranal.

© European Southern Observatory (ESO) 2000

Online publication: December 17, 1999
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