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Issue A&A
Volume 365, Number 3, January III 2001
Page(s) L353 - L356
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:20000557



A&A 365, L353-L356 (2001)
DOI: 10.1051/0004-6361:20000557

The obscured mid-infrared continuum of NGC 4418: A dust- and ice-enshrouded AGN

H. W. W. Spoon1, J. V. Keane2, A. G. G. M. Tielens2, D. Lutz3 and A. F. M. Moorwood1

1  European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
e-mail: hspoon@eso.org, amoor@eso.org
2  Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
e-mail: j.keane@astro.rug.nl, tielens@astro.rug.nl
3  Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
e-mail: lutz@mpe.mpg.de

(Received 16 November 2000 / Accepted 1 December 2000 )

Abstract
We report the detection of absorption features in the 6-8 $\mu$m region superimposed on a featureless mid-infrared continuum in NGC 4418. For several of these features this is the first detection in an external galaxy. We compare the absorption spectrum of NGC 4418 to that of embedded massive protostars and the Galactic centre, and attribute the absorption features to ice grains and to hydrogenated amorphous carbon grains. From the depth of the ice features, we infer that the powerful central source responsible for the mid-infrared emission must be deeply enshrouded. Since this emission is warm and originates in a compact region, an AGN must be hiding in the nucleus of NGC 4418.


Key words: galaxies: individual: NGC 4418 -- galaxies: ISM -- galaxies: Seyfert -- infrared: ISM: lines and bands

Offprint request: H. W. W. Spoon, hspoon@eso.org

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© ESO 2001


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