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

We report the detection of absorption features in the 6-8 mum region superi mposed 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 NGC4418 to that; of embedded massive protostars and the Galactic centre, and attribute the absorption features to ice grai ns and to hydrogenated amorphous carbon grains. From the depth of the ice f eatures, we infer that the powerful central source responsible for the mid- infrared emission must be deeply enshrouded. Since this emission is warm an d originates in a compact region, an AGN must be hiding in the nucleus of N GC 4418.