ISO-SWS SPECTROSCOPY OF ARP-220 - A HIGHLY OBSCURED STARBURST GALAXY

We report the first 2.4 - 45 mu m spectroscopic study of Arp 220 obtai ned with the Short Wavelength Spectrometer onboard ISO. Observations o f mid infrared lines penetrate deep into the highly obscured regions w here the luminosity originates and give direct clues to their sources of excitation. From the observed Brackett line ratios and from the [S III] 18.7 mu m/33.5 mu m line ratio we derive a mean extinction of A(V ) approximate to 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 A GN galaxies, indicating that a central AGN could only be a minor contr ibutor to the total luminosity. The intensities of the low excitation [Ne II] and [S III] lines, and the extinction corrected ratio of the L yman continuum and bolometric luminosity imply that massive star-forma tion is the dominant source of luminosity in Arp 220. The spectra incl ude bright emission of the pure rotational S(1) and S(5) lines of mole cular hydrogen. If the extinction to the emitting region is the same a s to the ionized medium, as much as 10% Of the ISM of Arp 220 could be in the warm phase sampled by these lines.