THE PHYSICAL CAUSES OF QUASAR EVOLUTION

at optical wavelengths we know that quasars evolve such that at a given luminosity the space density increases as redshift increases up to a redshift of 23, but we do not know whether this change is due to density or luminosity evolution. we intend to do photometry on a sample of bright quasars at a variety of wavelengths, at 7.3, 11.5, 25, 60, and 100 microns. with these data we will determine the spectral energy distributions. this sample of quasars has been carefully constructed such that it spans a wide range in redshift and a narrow range in optical luminosity, in order to eliminate the usual luminosityredshift degeneracy inherent in most quasar samples. with this sample of quasars we will be able to see if the ir s.e.d.s of the quasars also change with redshift and we can test the hypothesis that the observed evolution in the optical is due to the evolution in the covering factor of dust. the quasars have been selected from hewitt and burbidge, and the recently completed edinburgh multicolour quasar survey. none of the sources are in the central program. iso data is essential for this experiment as we need to measure the s.e.d.s of these sources in the ir and over as wide a wave length range as possible. note two diagrams are included in the paper version of this proposal but are excluded from the electronic version. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]