The hot dust in the heart of NGC 1068's torus: A 3D radiative model constrained with GRAVITY/VLTi

Abstract

The central region of NGC 1068 is one of the closest and most studied Active Galactic Nucleus. It is known to be of 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. We model the inner edge of this structure, where the hot dust responsible for the near-infrared emission reaches its sublimation temperature. At first, we use simple geometrical models in image reconstructions to determine the main 2D geometrical properties of the source. In a second step, we try 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. The three methods indicate an elongated structure with 4 x 6 mas dimensions and its major axis along the NW/SE direction. All three of them suggest that the object looks like an elongated ring rather than an elongated thin disk, with its northeast edge less luminous than the South-West one. The best 3D model is a thick disk with an inner radius r=0.21 +/- 0.03 pc and a half-opening angle $\alpha_{1/2}=21\pm8\degree$ observed with an inclination $i=44_{-6}^{10}$\,$\degree$ and $PA=150_{-13}^{8}$\,$\degree$. A high density of dust $n=5_{-2.5}^{+5}\ M_{\odot}.pc^{-3}$ is required to explain the contrast between the two edges by self-absorption from the closer one. The overall structure is obscured by a large foreground obscuration $A_V \sim 75$. 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. A single disk is unable to account for these differences, and we favor a description of the source where multiple rings are entangled around the central mass.