Near-infrared adaptive optics dissection of the core of NGC 1068 with NAOS-CONICA

¹ LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France e-mail: dgratadour@gemini.edu
² DOTA-ONERA, Av. de la division Leclerc, Châtillon, France

Received: 15 October 2004
Accepted: 1 September 2005

Abstract

We present an update analysis of recent near-infrared adaptive optics observations of NGC 1068 obtained with NAOS-CONICA at VLT/UT4. Ks, and bands images were deconvolved using MISTRAL, a regularized algorithm based on a maximum a posteriori estimation of the object. Two regularization methods, one including a new maximum likelihood estimation of the object Power Spectral Density, and an edge preserving one, have been tested and converge to consistent results. The deconvolved images show a coherent evolution of the IR emission from 2.2 to 4.8 . Deconvolution brings new elements: a) it strengthens the very peculiar nature of the four parallel elongated nodules previously discovered along the jet, which appear unresolved perpendicular to their long axis; b) it underlines the strong correlation between UV clouds and IR features, and c) it provides a more accurate multi-wavelength registration of the actual active nucleus. The overall aspect of the central IR emission seems to point to the jet as a major mechanism to shape the NLR. For each identified structure, we derive a color temperature now based on three bands (M, L and K), before and after deconvolution, confirming the need for clumps of dust at unexpectedly high and almost constant temperature (about 500 K) up to 70 pc north of the nucleus. We explore several mechanisms to explain the color temperature and show that shocks, induced for instance by the interaction of the jet with a giant cloud, is unlikely to be the dominating mechanism to heat the dust. We detail our model of transient heating of Very Small Grains and show that it can provide a consistent explanation of the K, L, M colors and their lack of variation with distance when 0.6 nm diamond-like grains are heated by 4 to 8 eV UV photons. However, we do not exclude the possibility that part of the excitation could come from shocks. At Ks, deconvolution reinforces the previous claim that the central core is partially resolved along the N-S direction: the best fit to our data is an elliptical Gaussian extended along with a 2.1 pc FWHM along this direction. This result agrees with the predictions of the radiative transfer model we previously developed to interpret the spectroscopic behavior at K, and is consistent with VLTI/VINCI measurements. Several questions are raised by this study: a) is the jet dominant in shaping the NLR of this AGN? b) what is the real state of the dust in the environment of the core; c) is the simple doughnut torus model able to explain IR emission of the central source with a morphology that appears increasingly complex at small scale?