An artifact of this choice of model is that the mid-far infrared regions of the SED template increase sharply for ages below the assumed duration of the starburst (1 Gyr). This causes discontinuities in the mid- and far-infrared fluxes at $z \simeq 4$ where the galaxy age first drops below this figure.

### FRIIs

The FRIIs are first split into populations of quasars and radio galaxies for which the nuclear emission is seen directly and through obscuration, respectively. Willott et al. (2000) found that for log L(151 MHz)\[W/Hz/sr\]$>26.5$ the quasar fraction is 0.4, independent of redshift and luminosity, and around 0.1 at lower radio luminosity. Taking into account the contamination by FRIs at lower luminosity, we simply assume that 40 per cent of our FRIIs are seen as quasars, i.e. those seen at viewing angle of $\leq 53$ degrees to the jet axis. To assign AGN and starburst infrared SEDs, we recall that quasars are, on average, more intrinsically luminous AGN than the radio galaxies (see e.g. Simpson 2003). To work with a parameter that is closely tied to the intrinsic strength of the nuclear emission, we convert from L(151 MHz) \[W/Hz/sr\] to L(\[OIII\]$\lambda5007$) \[W\] using a relation from Grimes, 