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 em ission lines, but we estimated that this causes relatively little confusion in most cases. The high angular resolution of the HST allows a detailed st udy of the host galaxies. The images show a wide range of morphologies, inc luding (i) compact systems, (ii) systems with substructures such as multipl e emission peaks, and (iii) systems comprised of several components spread over areas of up to 100 kpc. Three galaxies appear unresolved, and in two o thers a nuclear point source dominates the emission in the central region. The morphologies of some of the lowest redshift targets are well represente d by de Vaucouleurs profiles, consistent with them being elliptical galaxie s. Their average effective radius derived is a factor of 2 smaller than tha t of z similar to 1 3CR radio galaxies at similar rest-frame wavelength. Th e near-infrared continuum light is generally well aligned with the radio ax is, 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 avail able, 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 det ermined 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 object s close to the high-redshift radio galaxies. The number density of these fa int objects is slightly higher than that observed in the deep NICMOS parall el observations of random fields : furthermore, these objects tend to be al igned with the direction of the main axis of the radio sources, suggesting that they may be related to the presence of the AGN.