ISO-SWS spectroscopy of Arp 220: A highly obscured starburst galaxy

We report the first 2.4 - 45μm spectroscopic study of Arp 220 obtained with the Short Wavelength Spectrometer onboard ISO. Observations of mid infrared lines penetrate deep into the highly obscured regions where the luminosity originates and give direct clues to their sources of excitation. From the observed Brackett line ratios and from the [S III] 18.7μm/33.5μm line ratio we derive a mean extinction of A_V ≈ 50 mag, significantly higher than most earlier values. No AGN indicators such as the high excitation [Ne V] or [O IV] lines are detected to an upper limit significantly less than in bona fide AGN galaxies, indicating that a central AGN could only be a minor contributor to the total luminosity. The intensities of the low excitation [Ne II] and [S III] lines, and the extinction corrected ratio of the Lyman continuum and bolometric luminosity imply that massive star-formation is the dominant source of luminosity in Arp 220. The spectra include bright emission of the pure rotational S(1) and S(5) lines of molecular hydrogen. If the extinction to the emitting region is the same as to the ionized medium, as much as 10% of the ISM of Arp 220 could be in the warm phase sampled by these lines.