Dynamics of the ionization bicone of NGC 1068 probed in mid-infrared with VISIR ^*,**

¹ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, Place Jules Janssen, 92190 Meudon, France e-mail: anne.poncelet@obspm.fr
² LESIA, Observatoire de Paris, 92195 Meudon Cedex, France

Received: 20 August 2007
Accepted: 6 December 2007

Abstract

Context. We present high-spatial and medium-spectral resolution imaging and spectroscopy of the core of the Seyfert II galaxy NGC 1068, obtained in the N-band spectral range with the VLT/VISIR (the VLT Imager and Spectrometer in the InfraRed). Spectra were acquired in the ArIII 8.99 μm, SIV 10.51 μm and NeII 12.81 μm filters for two orientations of the long slit : PA = -15° and 90° oriented N to E.

Aims. The spatial evolution of the [ArIII]+[MgVII], [SIV] and [NeII] lines are investigated to constrain the gas dynamics inside the narrow line region (NLR), and to deduce the geometry and physics of the ionization cones.

Methods. Spectra are extracted in 0381 steps along the two slit positions and we obtain line parameters from fits with Gaussian profiles. The computed line ratios are compared to photoionization models.

Results. Lines are resolved and detected in emission up to ~3″ NW and ~3″ SE of the core, generally double-peaked. Features trace an outflow with radial velocities up to 10³ km s^-1, well reproduced by a hollow biconical structure associated with the ionization bicone of NGC 1068. It is inclined at ~11° and the maximum aperture is ~80°, in good agreement with optical spectroscopic studies.

Conclusions. The velocity profiles traced in the MIR are interpreted as evidence for a gas outflow along the edges of the ionization bicones. The comparison of line ratios with standard dust-free and dusty, radiation pressure-dominated photoionization models shows that the associated gas metallicity is close to the solar value and points to a relatively weak ionization parameter of a typical value U₀ = 10^-2.6 at PA = -15° along the ionization bicone.