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 $$(\sim {10}^{8}\rightarrow {10}^{9}\,{M}_{\odot}\,\text{Gyr}^{-1}).$$ Within this framework the bluest B–R colours can be easily reproduced by the further addition of small $$(\sim {10}^{9}\rightarrow {10}^{10}\,{M}_{\odot}\,\text{Gyr}^{-1})\,\text{short-lived}\,(\lt1\,\text{Gyr})$$ starbursts. However, such low-level star-formation activity cannot account for the bluest R–K colours found at z ~ 1; the bluest R–K colours imply bulk star-formation at z ≃ 2, while the reddest require most of the stars to have formed at z > 10. This may genuinely reflect a range of formation red shifts $$({z}_\text{f}\sim 2\rightarrow 20),$$ or alternatively a common z_f and very different star-formation histories at intermediate redshifts (e.g. some galaxies experiencing mergers at z ~ 2 ).