Near-IR 2D-spectroscopy of the 4″ × 4″ region around the active galactic nucleus of NGC 1068 with ISAAC/VLT

Abstract

New near-IR long slit spectroscopic data obtained with ISAAC on VLT/ANTU (ESO/Paranal) complement and extend our previously published near-IR data (Alloin et al. 2001) to produce Brγ and H2 emission line maps and line profile grids of the central 4″ × 4″ region surrounding the central engine of NGC 1068. The seeing quality together with the use of an 0.3″ wide slit and 0.3″ slit position offsets allow one to perform 2D-spectroscopy at a spatial resolution ≈ 0.5″. Slit orientations (PA = 102° and PA = 12°) were chosen so as to match respectively the equatorial plane and the axis of the suspected molecular/dusty torus in NGC 1068. The selected wavelength range from 2.1 to 2.2 μm is suitable to detect and analyze the Brγ and H2 emission lines at a spectral resolution corresponding to 35 km s-1. An asymmetric distribution of H2 emission around the continuum peak is observed. No H2 emission is detected at the location of the strong 2.2 μm continuum core (coincident within error-bars with the central engine location), while two conspicuous knots of H2 emission are detected at about 1 inch on each side of the central engine along PA = 90°, with a projected velocity difference of 140 km s-1: this velocity jump has been interpreted in Alloin et al. (2001) as the signature of a rotating disk of molecular material. From this new data set, we find that only very low intensity Brγ emission is detected at the location of the two main knots of H2 emission. Another knot with both H2 and Brγ emission is detected to the North of the central engine, close to the radio source C where the small scale radio jet is redirected and close to the brightest [OIII] cloud NLR-B. It has a counterpart to the South, placed almost symmetrically with respect to the central engine, although mainly visible in the Brγ emission. The northern and southern knots appear to be related to the ionization cone. At the achieved spectral resolution, the H2 emission line profiles appear highly asymmetric with their low velocity wing being systematically more extended than their high velocity wing. A simple way to account for the changes of the H2 line profiles (peak-shift with respect to the systemic velocity, width, asymmetry) over the entire 4″ × ″ region, is to consider that a radial outflow is superimposed over the emission of the rotating molecular disk. We present a model of such a kinematical configuration and compare our predicted H2 emission profiles to the observed ones. Excitation of the H2 line is briefly discussed: X-ray irradiation from the central engine is found to be the most likely source of excitation. Given the fact that the material obscuring our direct view toward the central engine is Compton thick (NH ≥ 1024 cm-2), the observed location of the main H2 knots at a distance of 70 pc from the central engine suggests that the rotating molecular disk is warped.