The inner hot dust in the torus of NGC 1068

Abstract

Context. The central region of NGC 1068 is one of the closest and most studied active galactic nuclei. It is known to be type 2, meaning that its accretion disk is obscured by a large amount of dust and gas. The main properties of the obscuring structure are still to be determined. Aims. We aim to model the inner edge of this structure, where the hot dust responsible for the near-infrared emission reaches its sublimation temperature. Methods. We used several methods to interpret the K-band interferometric observables from a GRAVITY/VLTI observation of the object. At first, we used simple geometrical models in image reconstructions to determine the main 2D geometrical properties of the source. In a second step, we tried to reproduce the observables with K-band images produced by 3D radiative transfer simulations of a heated dusty disk. We explore various parameters to find an optimal solution and a model consistent with all the observables. Results. The three methods are consistent in their description of the image of the source, an elongated structure with ∼4 × 6 mas dimensions and its major axis along the northwest–southeast direction. The results from all three methods suggest that the object resembles an elongated ring rather than an elongated thin disk, with the northeast edge appearing less luminous than the southwest one. The best 3D model is a thick disk with an inner radius r = 0.21−0.03+0.02 pc and a half-opening angle α1/2 = 21 ± 8° observed with an inclination i = 44−610° and PA = 150−138°. A high density of dust n = 5−2.5+5 M⊙ pc−3 is required to explain the contrast between the two edges by self-absorption from the closer one. The overall structure is itself obscured by a large foreground obscuration AV ∼ 75. Conclusions. The hot dust is not responsible for the obscuration of the central engine. The geometry and the orientation of the structure are different from those of the previously observed maser and molecular disks. We conclude that a single disk is unable to account for these differences, and favor a description of the source where multiple rings originating from different clouds are entangled around the central mass.