``ISOCAM Observations of Arp 220''
by
Charmandaris, V., Mirabel, I.F., Tran, D., Laurent, O, Cesarsky, C.J., Gallais, P., Sauvage, M. & Vigroux, L.
Service d'Astrophysique/DAPNIA/DSM, CEA Saclay
F-91191, Gif-sur-Yvette, France

Rencontres de Moriond, ``Extragalactic Astronomy in the Infrared'', 1997, p.283

We report on observations of the nearest ultraluminous infrared galaxy Arp 220 (IC 4553) made by the ISOCAM instrument of the Infrared Space Observatory.We performed imaging at 6, 6.75, 9.72 and 15$\mu m$ (through the LW4, LW2, LW7 and LW3 filters respectively) of a 50x50 arcsec field centered on the galaxy. More than 90\% of the emission in all filters originates from an unresolved central source. CVF spectra of the same field covering the mid-infrared range (5--17$\mu m$) were also obtained. Unlike observations of active regions in other galaxies of our sample, the PAH bands as well as the [NeII] and [NeIII] emission lines are surprisingly weak. The most striking feature though, is the presence of a strong silicate absorption band centered at 9.7$\mu m$, which is unlike any other active galaxy of our sample. A detailed comparison with other far-IR observations (Fischer 1997) reveals that even at mid-IR we probe reprocessed photons and not direct emission from the central source. As a result our estimate of Av~45 mag, based on a ``cold screen'' model for the detected 9.7$\mu m$ absorption, should only be considered as a lower limit to the overall absorption along the line of sight to the central region.

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