This solicited ISO open time proposal will use the mid-infrared 'PAH' features
to distinguish between starburst and AGN nature in a sample of ultraluminous
infrared galaxies (ULIRGs)
ISO observations have confirmed previous findings that PAH features are a
starburst tracer, while they are weak or absent in classical AGNs. For a sample
of about a dozen nearby ULIRGs, PAH spectroscopy with ISOPHOT-S and
spectroscopy of high- and low-excitation fine structure lines with SWS
agree in the finding that most of them are predominantly starburst-powered.
Because of the intrinsic brightness of the PAH emission from star forming
regions, the PAH method is ideally suited for extension to more luminous
but fainter ULIRGs.
The targets for this proposal have been selected to cover the luminosity
range 10^12.25-10^12.8 solar luminosities, complementing observations from
existing GT and OT programmes and emphasizing high luminosites. Thus, a
coherent ISO dataset will be established for which AGN/starburst
discrimination is possible on the basis of the extinction-insensitive PAH
method. This dataset will allow to probe for changes in the AGN/starburst
ratio with ULIRG luminosity, and test evolutionary scenarios.