Deviations from passive evolution - Star formation and the UV excess in z ∼ 1 radio galaxies

Galaxy colours are determined for two samples of 6C and 3CR radio sources at z ∼ 1, differing by a factor of ∼ 6 in radio power. Corrections are made for emission line contamination and the presence of any nuclear point source, and the data analysed as a function of both redshift and the radio source properties. The galaxy colours are remarkably similar for the two populations, and the UV excess evolves with radio source size similarly in both samples, depsite the fact that the alignment effect is more extensive for the more powerful 3CR radio galaxies. These results seem to suggest that the alignment effect at these redshifts does not scale strongly with radio power, and is instead more closely dependent on galaxy mass (which is statistically comparable for the two samples). However, it is likely that the presence of relatively young ( ∼ < several 10 years old) stellar populations has considerably contaminated the K−band flux of these systems, particularly in the case of the more powerful 3CR sources, which are ∼ 0.5mag more luminous than the predictions of passive evolution models at z ∼ 1. The higher luminosity of the 3CR alignment effect is balanced by emission at longer wavelengths, thereby leading to comparable colours for the two samples.