EDP Sciences
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Volume 367, Number 2, February IV 2001
Page(s) 487 - 497
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20000569

A&A 367, 487-497 (2001)
DOI: 10.1051/0004-6361:20000569

Mid-Infrared observations of NGC 1068 with the Infrared Space Observatory

E. Le Floc'h1, 2, I. F. Mirabel1, 3, O. Laurent1, 4, V. Charmandaris5, P. Gallais1, M. Sauvage1, L. Vigroux1 and C. Cesarsky6

1  CEA/DSM/DAPNIA, Service d'Astrophysique, 91191 Gif-sur-Yvette, France
2  European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
3  Instituto de Astronomía y Física del Espacio, cc 67, suc 28., 1428 Buenos Aires, Argentina
4  Max Planck Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany
5  Cornell University, Astronomy Department, Ithaca, NY 14853, USA
6  European Southern Observatory, Karl-Schwarzschild-Str, 85748 Garching bei München, Germany

(Received 31 October 2000 / Accepted 22 December 2000)

We report on Mid-Infrared (MIR) observations of the Seyfert 2 galaxy NGC 1068, obtained with ISOCAM in low-resolution spectro-imaging mode. The spatial resolution ($\sim$5´´) allows us to disentangle the circumnuclear starburst regions from the emission of the active galactic nucleus (AGN). The global spatial distribution of the Unidentified Infrared Bands (UIBs) is similar to the cold dust component, traced by the 450 $\mu$m emission and the gaseous component obtained from the 12CO(1-0) map. However, a shift between the maximum of the UIB and 450 $\mu$m emission is clearly seen in our maps. The UIBs in the MIR (5-16 $\mu$m) originate almost exclusively from the starburst regions in the galactic disk with an emission peaking at the extremity of the stellar/gaseous bar at a distance of 1 kpc from the AGN. The spectrum of the nucleus is characterized over the whole 5-16 $\mu$m range by a strong continuum which can be fitted with a power law of index $\alpha=-1.7$. Moreover, the high [NeIII]/[NeII] ratio ( $\lesssim \!2.5$) in the nuclear region argues for a hard radiation field from the AGN. Observations indicate that the AGN in NGC 1068 contributes less than $\sim$5% to the total integrated UIB emission even though its hot dust continuum contributes as much as 75% to the total MIR flux. On the contrary, the nuclear contribution to the cold dust emission decreases considerably at submillimeter wavelengths and does not represent more than 25% of the total integrated emission at 450 $\mu$m.

Key words: galaxies: active -- galaxies: individual: NGC 1068 -- galaxies: seyfert -- galaxies: ISM -- infrared: galaxies

Offprint request: E. Le Floc'h, elefloch@cea.fr

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© ESO 2001