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The obscured mid-infrared continuum of NGC 4418

Summary: 5
The obscured mid-infrared
continuum of NGC 4418:
a dust- and ice-enshrouded AGN
Originally published as a Letter to the Editor:
H.W.W. Spoon, J.V. Keane, A.G.G.M. Tielens, D. Lutz & A.F.M. Moorwood
WE report the detection of absorption features in the 6­8 µm 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 center, 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.
82 CHAPTER 5: The obscured mid-infrared continuum of NGC 4418
5.1 Introduction
The ISO mission has considerably enhanced our knowledge of the mid-IR properties of nor-
mal, starburst, Seyfert and Ultra-luminous Infrared Galaxies (ULIRGs). The spectra of most
sources are dominated by ISM emission features, the most prominent of which are the well-


Source: Attal, Stéphane - Institut Camille Jordan, Université Claude Bernard Lyon-I
Spoon, Henrik - Department of Astronomy, Cornell University


Collections: Mathematics; Physics