A&A 414, 873-883 (2004)
DOI: 10.1051/0004-6361:20031656

Mid-infrared spectral evidence for a luminous dust enshrouded source in Arp 220

H. W. W. Spoon¹, A. F. M. Moorwood², D. Lutz³, A. G. G. M. Tielens^{1, 4}, R. Siebenmorgen² and J. V. Keane<sup>5

1</sup>  Kapteyn Institute, PO Box 800, 9700 AV Groningen, The Netherlands
²  European Southern Observatory, Karl-Schwarzschild Strasse 2, 85748 Garching, Germany
³  Max-Planck-Institut für Extraterrestrische Physik (MPE), Postfach 1312, 85741 Garching, Germany
⁴  SRON, PO Box 800, 9700 AV Groningen, The Netherlands
⁵  NASA-Ames Research Center, Mail Stop 245-6, Moffett Field, CA 94035, USA

(Received 2 July 2003 / Accepted 24 October 2003)

Abstract
We have re-analyzed the 6-12  m ISO spectrum of the ultra-luminous infrared galaxy Arp 220 with the conclusion that it is not consistent with that of a scaled up version of a typical starburst. Instead, both template fitting with spectra of the galaxies NGC 4418 and M 83 and with dust models suggest that it is best represented by combinations of a typical starburst component, exhibiting PAH emission features, and a heavily absorbed dust continuum which contributes ~40% of the 6-12  m flux and likely dominates the luminosity. Of particular significance relative to previous studies of Arp 220 is the fact that the emission feature at 7.7  m comprises both PAH emission and a broader component resulting from ice and silicate absorption against a heavily absorbed continuum. Extinction to the PAH emitting source, however, appears to be relatively low. We tentatively associate the PAH emitting and heavily dust/ice absorbed components with the diffuse emission region and the two compact nuclei respectively identified by Soifer et al. (2002) in their higher spatial resolution 10  m study. Both the similarity of the absorbed continuum with that of the embedded Galactic protostars and results of the dust models imply that the embedded source(s) in Arp 220 could be powered by, albeit extremely dense, starburst activity. Due to the high extinction, it is not possible with the available data to exclude that AGN(s) also contribute some or all of the observed luminosity. In this case, however, the upper limit measured for its hard X-ray emission would require Arp 220 to be the most highly obscured AGN known.

Key words: galaxies: individual: Arp220 -- galaxies: ISM -- galaxies: nuclei -- galaxies: starburst -- infrared: galaxies

Offprint request: H. W. W. Spoon, spoon@isc.astro.cornell.edu

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