Colors of Radio Galaxies at High Redshifts

Abstract

In this paper we present the first results of the Westerbork-Berkeley Deep Survey, the purpose of which is to derive the epoch dependence of the radio luminosity function (RLF) and the optical spectral energy distribution (SED) of elliptical radio galaxies. Frora calibrated photographic photometry we conclude that no spectral evolution is seen for z ≲ 0.4, but that colors of radio galaxies are 0.^m5–1.^m5 bluer for 0.4 ≲ z ≲ 0.9 than predicted from classical model spectra for ellipticals (with star formation only during the first 10⁹ yrs after formation). First-ranked cluster radio galaxies may be slightly bluer than the average radio galaxy for z ≳ 0.6. The epoch dependence of the 1.4 GHz RLF is determined in a direct way. We find almost no population evolution for z ≲ 0.3, but strong evolution for 0.3 ≲ z ≲ 0.9 for all radio powers above the break in the RLF, with enhancement factors of ∿ 100 at z ∿ 0.8.