An LWS spectroscopic survey of infrared-bright galaxies

We present a comparative observational overview of the results of a far-infrared (43 - 197 mu m) spectroscopic survey of a small sample of relatively nearby infrared-bright galaxies using the Infrared Space Observatory (ISO) Long Wavelength Spectrometer (LWS). The survey reveals a dramatic progression in emission and absorption line characteristics in these galaxies extending from strong [O III] 52, 88 mu m and [N III] 57 mm fine-structure line emission to detection of only faint [C II] 158 mu m line emission from gas in photodissociation regions. The several ultraluminous galaxies (ULIGs) included in the full spectral sample have the weakest emission line characteristics and show strong molecular absorption lines. We suggest that the variation in apparent excitation is not due to variation in density or far-infrared extinction, but rather may be due to differences in radiation hardness. The molecular absorption spectra 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. The origin of the [C II] 158 mu m deficit in ultraluminous galaxies remains uncertain.