Infrared magnitude-redshift relations for luminous radio galaxies

Infrared magnitude-redshift relations (K-z relations) for the 3CR and 6C samples of radio galaxies are presented for a wide range of plausible cosmological models, including those with non-zero cosmological constant Omega(Lambda). Variations in the galaxy formation redshift, metallicity and star formation history are also considered. The results of the modelling are displayed in terms,of magnitude differences between the models and no-evolution tracks, illustrating the amount of K-band evolution necessary to account for the observational data. Given a number of plausible assumptions, the results of these analyses suggest that: (i) cosmologies which predict T0H0 greater than or similar to 1 (where T-0 denotes the current age of the Universe) can be excluded; (ii) the star formation redshift should lie in the redshift interval 5 less than or equal to z(f) less than or equal to 20, values towards the lower end of the range being preferred in cosmologies with larger values of T0H0; (iii) the Einstein-de Sitter model (Omega(0) = 1, Omega(Lambda) = 0) provides a reasonable fit to the data; and (iv) models with finite values of Omega(Lambda) can provide good agreement with the observations only if appropriate adjustments of other parameters such as the galaxy metallicities and star formation histories are made. Without such modifications, even after accounting for stellar evolution, the high-redshift radio galaxies are more luminous (that is, more massive) than those nearby in models with finite Omega(Lambda), including the favoured model with Omega(0) = 0.3, Omega(Lambda) = 0.7. For cosmological models with larger values of T0H0, the conclusions are the same regardless of whether any adjustments are made or not. The implications of these results for cosmology and models of galaxy formation are discussed.