ISO FAR-IR spectroscopy of IR-bright galaxies and ULIRGs

Based on far-infrared spectroscopy of a small sample of nearby infrared-bri ght and ultraluminous infrared galaxies (ULIRGs) with the ISO Long Waveleng th Spectrometer(1) we find a dramatic progression in ionic/atomic fine-stru cture emission line and molecular/atomic absorption line characteristics in these galaxies extending from strong [O III]52,88 mu m and [N III]57 mu m line emission to detection of only faint [C II]158 mu m line emission from gas in photodissociation regions in the ULIRGs. The molecular absorption sp ectra show varying excitation as well, extending from galaxies in which the molecular population mainly occupies the ground state to galaxies in which there is significant population in higher levels. In the case of the proto typical ULIRG, the merger galaxy Arp 220, the spectrum is dominated by abso rption lines of OH, H2O, CH, and [O I]. Low [O III]88 mu m line flux relati ve to the integrated far-infrared flux correlates with low excitation and d oes not appear to be due to far-infrared extinction or to density effects. A progression toward soft radiation fields or very dusty H II regions may e xplain these effects.