Amongst the main issues in extra-galactic astrophysics that can be addressed by
ISO are: (1) the relationship between starburst galaxies and AGN; and (2) the
ability of AGN unification schemes to explain the FIR emission of AGN. To do
this requires the photometric study of a large, statistically complete sample.
We propose measuring the FIR SEDs of a complete, cleanly-defined sample of IRAS
galaxies selected on the basis of their 25 micron flux. The FIR is surpassed
only by the X-ray region in its ability to select unbiased samples of AGN, and,
by selecting galaxies about a 25 micron flux cut, we shall obtain a much higher
fraction of starbursts and AGN than is found in the many other samples of IRAS
galaxies for which photometric follow-up projects are being undertaken with
ISO. Our sample will include some normal galaxies, which will be used as a
control sample for understanding the far-infrared emission processes of active
galaxies: their presence is unavoidable, since IRAS colour information is
insufficient by itself for selecting only starbursts and AGN, which is the
reason that we require the greatly increased quality of photometric information
obtainable from ISO.
The library of far-infrared SEDs produced by this project will complement those
obtained by other ISO galaxy samples, which typically select a much higher
fraction of normal galaxies, to build up a picture of the full range of
far-infrared emission from galaxies. These data will fuel ongoing research at
Imperial College into techniques for modelling far-infrared emission.  The
improved far-infrared models and observed SEDs that will result from this
project, will enable us greatly to advance the study of the AGN and starburst
phenonmenae in galaxies, in particular testing AGN unification schemes.