  The stellar initial mas function (IMF) has been described as being invariant,
bottom heavy or top-heavy in extremely dense star burst conditions. To provide
usable observable diagnostic we calculate redshift dependent spectral energy
distributions of stellar populations in extreme star burst clusters which are
likely to have been the precursors of present day massive globular clusters
(GCs) and of ultra compact dwarf galaxies (UCDs). The retention fraction of
stellar remnants is taken into account to asses the mass to light ratios of the
ageing star-burst. Their redshift dependent photometric properties are
calculated as predictions for James Webb Space Telescope (JWST) observations.
While the present day GCs and UCDs are largely degenerate concerning
bottom-heavy or top-heavy IMFs, a metallicity- and density-dependent top-heavy
IMF implies the most massive UCDs, at ages <100 Myr, to appear as objects with
quasar-like luminosities with a 0.1-10% variability on a monthly time scale due
to core collapse supernovae.
