Far-infrared spectroscopy obtained with the ISO LWS has shown that there is strong variation (more than 2 orders of magnitude) in the [CII] / FIR ratios in galaxies extending from blue compact dwarfs, to normal and starburst galaxies, down to elliptical and ultraluminous galaxies. The variation in the relative [CII] line strength has been attributed to low metallicity (blue compact dwarfs), high <Go> / n for normal galaxies and ULIGs, soft radiation fields in ellipticals, and extinction or enhanced abundance of dust in ionized regions in ULIGs.
Full ISO/LWS far-infrared (43 - 197 µm) spectra of six nearby IR-bright galaxies reveal a dramatic progression of decreasing fine-structure line emission from ionized species to molecular absorption line dominated. The archetypical ULIG, Arp 220 lies at the absorption line dominated end of this sequence. For Arp 220, radiative transfer models indicate that it is optically thick in the FIR and that the water molecules observed in absorption are radiatively excited. If extinction plays a role in the sequence it appears from this analysis that the affected regions are heavily obscured even in the far-infrared, while the detected line emission is not more obscured in ULIGs than in starbursts. Linear correlation between the PAH 6.2 µm feature strength and the [CII] 158 µm line strength in starbursts and ULIGs suggests a similar effect for these emitting species, and that the detected PAH emission is not more obscured in ULIGs than in lower luminosity starbursts.
Ultimately many of these effects are related to geometry and the physical location of dust, for which more detailed models and SIRTF spectroscopy will be invaluable.
I would like to thank my collaborators on the LWS infrared-bright galaxies science team, M. Luhman, S. Satyapal, C. Dudley, E. Gonzalez, P. Cernicharo, M. Wolfire, P. Clegg, J. Colbert, P. Cox, M. A. Greenhouse, V. Harvey, C. M. Bradford, S. Lord, M. Malkan, G. Melnick, G. Stacey, H. A. Smith, L. Spinoglio, G. Stacey, and S. Unger, for their contributions to the work reviewed here, some still in press or in preparation, and to S. Malhotra and S. Madden for access to preprints before publication. I would also like to acknowledge the skill and dedication of the LWS instrument and data analysis teams at Vilspa, RAL, and IPAC. This work was supported by the Office of Naval Research and the NASA ISO grant program.