Near-IR images of the torus and micro-spiral structure in NGC 1068 using adaptive optics

Abstract

We present diffraction-limited near-IR images in J, Hand K of the nucleus of NGC 1068, obtained with the Adaptive Optics system Pueo at CFHT. The achieved resolution (0″.12) reveals several components, particularly prominent on the [J-K] image: a) an unresolved, conspicuous core (size < 9 pc); b) an elongated structure at P.A. ≈ 102°, beginning to show up at radius ≈ 15 pc; c) a S-shaped structure with radial extent « 20 pc, including a bar-like central elongation at P.A. ≈ 15° and two short spiral arms. A precise registration of the IR peak was carried out relative to the HST I-band peak. The K unresolved core is found to be close to the location of the putative central engine (radio source S1). Consistent with the Unified Model of AGN, the near-IR core is likely the emission from the hot inner walls of the dust/molecular torus. The extremely red colors of the 0″.2 diameter core, [J-K]=7.0, [H-K]=3.8, lead to an intrinsic extinction AV >25, assuming classical dust grains at 1500 K. The elongated structure at P.A. ≈ 102° may trace the presence of cooler dust within and around the torus. This interpretation is supported by two facts at least: a) the elongated structure is perpendicular to the local radio jet originating at S1; b) its direction follows exactly that of the disk of ionized gas recently found with the VLBA. Regarding the S-shaped feature, the near-IR flux of the bar-like central elongation at P.A.= ≈ 4° , if interpreted in terms of free-free emission from ionized gas, is roughly consistent with the level of 5 GHz emission. However, the radio spectrum behaviour is indicative of synchrotron emission and we rather interpret the 2.2 μm emission as originating from warm dust in the shaded part of NLR clouds or in stellar photospheres. The shape itself suggests an extremely compact barred spiral structure, that would be the innermost of a series of nested spiral structures, as predicted by models and simulations. This is supported by the inner stellar distribution - deduced from the J image - which clearly follows an exponential disk with a 19 pc scale-length, precisely that expected from the rotation of a bar twice this size. Show more