The colour evolution of high-redshift radio galaxies

This paper considers the interpretation of the optical-to-infrared continuum radiation from radio galaxies at large redshift. Using a new sample of ∼ 70 radio galaxies with B, R, and K photometry, we carry out a comparison of observed colours with the predictions of new evolutionary synthesis models. While the bulk of objects at Z >∼1 are very red in R-K, with most galaxies apparently consistent with 'passive' evolution corresponding to formation redshifts >∼ 5, all high-redshift objects are 'active' in B-R. The simplest model to account for these observations is one in which the old and young stellar populations are decoupled: the majority of stars formed at Z >> 2, but even the most passive galaxies experience continuing 'low'-level star formation thereafter. (author)