NICMOS observations of high-redshift radio galaxies:: Witnessing the formation of bright elliptical galaxies?

We present the results of a near-infrared imaging program of a sample of 19 radio galaxies with redshift between 1.7 and 3.2, using the NICMOS Camera 2 on the Hubble Space Telescope (HST). The galaxies were observed in H band, which, for 18 of the 19 targets, samples the rest-frame optical emission longward of the 4000 Angstrom break. For many sources this band contains emission lines, but we estimated that this causes relatively little confusion in most cases. The high angular resolution of the HST allows a detailed study of the host galaxies. The images show a wide range of morphologies, including (i) compact systems, (ii) systems with substructures such as multiple emission peaks, and (iii) systems comprised of several components spread over areas of up to 100 kpc. Three galaxies appear unresolved, and in two others a nuclear point source dominates the emission in the central region. The morphologies of some of the lowest redshift targets are well represented by de Vaucouleurs profiles, consistent with them being elliptical galaxies. Their average effective radius derived is a factor of 2 smaller than that of z similar to 1 3CR radio galaxies at similar rest-frame wavelength. The near-infrared continuum light is generally well aligned with the radio axis, and the aligned light is very red, with typical V - H colors of similar to3.5-4. For several galaxies, where WFPC2 V - or R-band images were available, we computed a high-resolution map of the optical-to-infrared spectral indices : all multicomponent systems present net color differences between the various clumps, and we argue that most probably the continuum emission has a stellar origin. Indicative ages of these stellar populations, as determined by the amplitude of the 4000 break AZ vary between 0.5 and 1.3 Gyr. Finally, in many of the small NICMOS frames we observe nearby faint objects close to the high-redshift radio galaxies. The number density of these faint objects is slightly higher than that observed in the deep NICMOS parallel observations of random fields : furthermore, these objects tend to be aligned with the direction of the main axis of the radio sources, suggesting that they may be related to the presence of the AGN.