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Implications of the ISO LWS Spectrum of the Prototypical Ultraluminous Galaxy Arp 220

J. Fischer 1 - S. Satyapal 2 - G. Melnick 3 - P. Cox 4 - J. Cernicharo 5 - M.L. Luhman 1 - G.J. Stacey 6 - H.A. Smith 3 - S.D. Lord 7 - C. Skinner 8 - M.A. Greenhouse 2

1 Naval Research Laboratory, Washington, DC, USA -
2 Goddard Space Flight Center, Greenbelt, MD, USA -
3 Smithsonian Astrophysical Observatory, Cambridge, MA, USA -
4 Institut d'Astronomie Spatiale, Orsay Cedex, France -
5 CSIC/IEM, Madrid, Spain and OAN, A. de Henares, Spain -
6 Cornell University, Ithaca, NY, USA -
7 California Institute of Technology, Pasadena, CA, USA -
8 Space Telescope Science Institute, Baltimore, MD, USA

We present a low resolution (R $= \lambda /\Delta \lambda$ $\sim$ 200) far-infrared (43-197 $\mu$ m) spectrum of the ultraluminous galaxy Arp 220 obtained from deep, full range scans using the Long Wavelength Spectrometer (LWS) aboard the Infrared Space Observatory (ISO). The spectrum is vastly different from the spectra of less luminous IR bright galaxies: the fine-structure lines that are typical of IR bright galaxies are weak or absent and the spectrum is dominated by absorption lines of OH, H₂O, CH, NH₃ and [O I]. We present our developing interpretation of the far-infrared spectroscopy and the continuum distribution and discuss the implications for starburst versus AGN models.

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