High angular resolution K-band spectroscopy of the nucleus of NGC 1068 with PUEO-GRIF (CFHT)

Free access article

Issue		A&A Volume 411, Number 3, December I 2003


Page(s)		335 - 342
Section		Galactic structure and dynamics
DOI		http://dx.doi.org/10.1051/0004-6361:20031376

A&A 411, 335-342 (2003)
DOI: 10.1051/0004-6361:20031376

High angular resolution K-band spectroscopy of the nucleus of NGC 1068 with PUEO-GRIF (CFHT)

D. Gratadour¹, Y. Clénet¹, D. Rouan¹, O. Lai² and T. Forveille²

¹ LESIA, Observatoire de Paris, 5 place Jules Janssen, 92195 Meudon, France
e-mail: firstname.lastname@obspm.fr
² CFHT Corporation, 68-1238 Mamalahoa Highway, Kamulea HI 96743, USA

(Received 21 January 2003 / Accepted 7 July 2003)

Abstract
We provide new high angular resolution K-band spectroscopic observations of the nucleus of NGC 1068 obtained with the new 3-D spectrograph GriF coupled to PUEO, the CFHT adaptive optics system. Results match well with the general framework of the unified model of active galactic nuclei (AGN) and bring more precise evaluation of the size and structure of the close AGN environment. The K-band emission is dominated by the thermal continuum of the compact central source (albeit resolved: $FWHM\sim0.12$ ´´) with a color temperature around 950 K (reddened sublimation temperature of the dust, corresponding to 1200 K when deredenned), interpreted as the border of a sublimation cavity within the dust torus, surrounding the central core. Coronal emission lines ([ $\ion{Si}{vi}$ ], [ $\ion{Si}{vii}$ ] and [ $\ion{Ca}{viii}$ ]) are detected from a few tenths of arcseconds around this compact source and a new [ $\ion{Si}{vi}$ ] emission spot is found at 0.7´´ north of the central source. The Brackett $\gamma$ emission features a very broad component ( FWHM=2500 km s ^-1) in the central 200 mas and we claim that it correponds to the broad line region seen through the dust. A narrower component ( FWHM=1600 km s ^-1) is seen at larger angular distance and is likely the trace of the narrow line region. Molecular hydrogen emission lines are detected beyond 0.6´´ of the central source. We interpret the lack of emission in the center as a lack of excited molecular H ₂ along the line of sight, because of the putitative dust torus shading gas from UV photons. No evidence of stellar activity in a 1´´ radius around the central source can be found. We also compare these observations to the results of a numerical model of a torus harboring an active galactic nucleus. The main characteristics of the observed spectra (slope, flux) match well with the unified scheme of AGN, in the case of a dense ( $\tau_V=40$ ) obscuring torus containing silicate grains with an inclination angle of around 22° with regards to the line of sight.

Key words: galaxies: Seyfert -- galaxies: individual: NGC 1068 -- infrared: galaxies -- instrumentation: adaptive optics -- instrumentation: spectrographs -- radiative transfer

Offprint request: D. Gratadour, damien.gratadour@obspm.fr

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