HST, radio and infrared observations of 28 3CR radio galaxies at redshift z ∼ 1 - II. Old stellar populations in central cluster galaxies

Abstract

Hubble Space Telescope images of 3CR radio galaxies at redshifts 0.6 < z < 1.8 have shown a remarkable variety of structures, generally aligned along the radio axis, indicating that the radio source strongly influences the optical appearance of these galaxies. In this paper we investigate the host galaxies underlying this aligned emission, combining the HST data with ground-based infrared images. An investigation of the spectral energy distributions of the galaxies shows that the contribution of the aligned blue component to the K-band light is generally small (∼10 per cent). The radial intensity profiles of the galaxies are well matched at radii ≲35 kpc by de Vaucouleurs' law, demonstrating that the K-band light is dominated by that of an elliptical galaxy. There is no evidence for a nuclear point source, in addition to the de Vaucouleurs profile, with a contribution ≳ 15 per cent of the total K-band flux density, except in two cases, 3C 22 and 3C 41. We conclude that the K-band emission of the distant 3CR galaxies is dominated by starlight. The magnitudes, colours and location of the distant 3CR galaxies on the projected fundamental plane indicate that their stellar populations formed at high redshift and have since been evolving passively. Large characteristic radii are derived for the 3CR galaxies, indicating that they must be highly evolved dynamically, even at a redshift of 1. At radii larger than ∼35 kpc, a combined galaxy profile clearly shows an excess of emission as compared with de Vaucouleurs' law, indicating that at least some of the galaxies possess cD-type haloes. This supports other independent evidence for the hypothesis that the distant 3CR galaxies lie in moderately rich (proto-)clusters. Since the nearby FR II galaxies in the 3CR catalogue lie in more diffuse environments and do not possess cD haloes, the galactic environments of the 3CR galaxies must change with redshift. The K-z relation of the 3CR galaxies cannot, therefore, be interpreted using a standard 'closed-box, passively evolving stellar population' model, whereby the galaxies that host distant 3CR sources will evolve into the galaxies that host nearby 3CR FR II sources. At redshifts z ∼ 1, the absolute K magnitudes of the stellar populations of the 3CR galaxies are brighter than those of the lower radio power 6C galaxies, indicating that the 3CR galaxies contain a greater mass of stars; this is consistent with them lying towards the centres of clusters. Powerful high-redshift radio galaxies possess radio beams the kinetic power of which is close to the Eddington limiting luminosity of a central supermassive black hole. Since the mass of the black hole is likely to scale in proportion to the mass of the host galaxy, the 3CR galaxies will contain more massive central engines than the 6C galaxies, which accounts for their more powerful radio emission. At redshifts z ≲ 0.6, the beam power of the radio sources is limited by the availability of fuel for the central engine rather than by the black hole mass, and so no correlation is expected between the radio power and the mass of the host galaxy.