Letter to the Editor

Hot Very Small dust Grains in NGC 1068 seen in jet induced structures thanks to VLT/NACO adaptive optics ^*

¹ LESIA – Observatoire de Paris-Meudon, UMR 8109 CNRS, 92195 Meudon, France
² ONERA – DOTA, 92322 Châtillon, France
³ LAOG – Observatoire de Grenoble, UMR 5571 CNRS, 38041 Grenoble, France
⁴ LUTH – Observatoire de Paris-Meudon, UMR 8102 CNRS, 92195 Meudon, France
⁵ MPE, Postfach 1312, 85741 Garching bei München, Germany
⁶ ESO, Karl-Schwarzschild -Str. 2, 85748 Garching bei München, Germany

Corresponding author: D. Rouan, daniel.rouan@obspm.fr

Received: 18 March 2003
Accepted: 15 May 2003

Abstract

We present K, L and M diffraction-limited images of NGC 1068, obtained with NAOS+CONICA at VLT/YEPUN over a region around the central engine. Hot dust ( K) is found to be distributed in three main structurally different regions: (a) in the true nucleus, seen as a quasi-spherical, however slightly NS elongated, core of extremely hot dust, resolved in K and L with respective diameters of ≈5 pc and 8.5 pc; (b) along the North–South direction, according to a spiral arm like structure and a southern tongue; (c) as a set of parallel elongated nodules (wave-like) on each side, albeit mainly at north, of the jet, at a distance of 50 to 70 pc from the central engine. The IR images reveal several structures also clearly observed on either radio maps, mid-IR or HST UV-visible maps, so that a very precise registration of the respective emissions can be done for the first time from UV to 6 cm. These results do support the current interpretion that source (a) corresponds to emission from dust near sublimation temperature delimiting the walls of the cavity in the central obscuring torus. Structure (b) is thought to be a mixture of hot dust and active star forming regions along a micro spiral structure that could trace the tidal mechanism bringing matter to the central engine. Structure c) which was not known, exhibits too high a temperature for “classical” grains; it is most probably the signature of transiently heated very small dust grains (VSG): nano-diamonds, which are resistant and can form in strong UV field or in shocks, are very attractive candidates. The “waves” can be condensations triggered by jet induced shocks, as predicted by recent models. First estimates, based on a simple VSG model and on a detailed radiative transfer model, do agree with those interpretations, both qualitatively and quantitatively.