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

Abstract

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. The galaxies were observed in H-band which, for 18 of the 19 targets, samples the rest-frame optical emission longwards of the 4000 A break. The images show a wide range of morphologies, from unresolved to compact systems, to systems with substructures such as multiple emission peaks, to systems comprised of several components spread over large areas. The morphologies of 5 of the lowest redshift targets are well represented by a de Vaucouleurs profile, consistent with them being elliptical galaxies. Their average effective radius is a factor of 2 smaller than that of z=1 3CR radio galaxies at similar restframe wavelengths. The near infrared continuum light is generally well aligned with the radio axis and the aligned light has typical V-H colors of around 3.5-4. For several galaxies where WFPC2 images were available, we computed a high resolution map of the optical-to-infrared spectral indices: all multi-component systems present net color gradients between the various clumps. We argue that in these galaxies the continuum emission has probably a stellar origin. Finally in many cases we observe nearby faint objects close to the radio sources. The number density of these faint objects is slightly higher than what is found in the deep NICMOS parallel observations of random fields. These objects also tend to be aligned with the radio sources axis, suggesting that they may be related to the presence of the AGN. (shortened astro-ph version)