Observations we have obtained during the first half year of ISO have clearly
demonstrated that ISO spectroscopy can provide key information to determine
the nature of ultraluminous IRAS galaxies:
1) SWS spectroscopy of fine structure lines penetrates the high extinction
and provides a quantitative diagnostic of the ionizing continuum (Starburst
or AGN). This powerful technique is however limited to the brightest dozen
of ULIRGs for sensitivity reasons; these observations form a key part of our
guaranteed time observations.
2) Low resolution PHT-S spectra we have obtained on a small number of ULIRGS
in guaranteed time (fig.1) clearly observe a significant spread
in the absolute and relative strengths of PAH emission features. It is
expected from pre-ISO data and confirmed by ISO spectroscopy that the
behaviour of the PAH features reflects the strength and properties of the
radiation field in which they are excited and may distinguish between AGNs and
starbursts. Observations of PAH features thus provide a second route to
obtain quantitative information on the nature of luminous IRAS galaxies,
having less direct diagnostic power but allowing to observe a larger and
complete sample allowing detailed statistical analysis.
We hence propose to carry out a survey of low resolution 2.5-12um spectroscopy
of emission features in a complete sample of ultraluminous IRAS galaxies. We
point out that ISO will provide a wide database of comparison objects of
various types (Seyferts, Starbursts, normal galaxies, galactic sources...),
both from our own observations and those of a variety of other groups.