Jackie Fischer 1 M. L. Luhman 1 S. Satyapal 2 J. R. Brauher 3 P. E. Clegg 4 J. W. Colbert 5 P. Cox 6 M. A. Greenhouse 2 S. D. Lord 3 M. A. Malkan 5 G. Melnick 7 H. A. Smith 7 G. J. Stacey 8& S. J. Unger 4
1 Naval Research Laboratory, Washington, DC USA
2 NASA Goddard Space Flight Center, Greenbelt, MD USA
3 IPAC, California Institute of Technology, Pasadena, CA USA
4 Queen Mary and Westfield College, London, UK
5 University of California, Los Angeles, CA USA
6 Institut d'Astrophysicque Spatiale, Orsay, France
7 Smithsonian Astrophysical Observatory, Cambridge, MA USA
8 Cornell University, Ithaca, NY USA
A far-infrared (43 - 197 
)
spectroscopic survey of a small sample
of local infrared-bright galaxies using the Infrared Space Observatory
Long Wavelength Spectrometer reveals a dramatic progression in emission
and absorption line characteristics in these galaxies extending from
strong [O III] 52,88
and [N III] 57
fine-structure line
emission to
detection of only faint [C II] 158
line emission from gas in
photo-dissociation regions. The several ultraluminous galaxies
included in the sample have the weakest emission line characteristics
and show strong absorption from molecular and atomic species such as
OH, H20, and [O I]. The progression in emission line characteristics
is most easily explained by differences in the radiation hardness of
the starburst thought to be responsible for the far infrared emission,
due either to the age of the starburst or to the upper mass limit of
the IMF. We discuss the effects of density and dust embedded in HII
regions and in clouds along the line of sight. We discuss these
effects in light of other infrared and radio observations and the
implications for our understanding of the nature of the ultraluminous
galaxies.