HST images and properties of the most distant radio galaxies

With the Hubble Space Telescope we have obtained images of 9 of the most di stant radio galaxies. The galaxies, which have redshifts between z = 2.3 an d z = 3.6, were observed with the WFPC2 camera in a broad band filter (F606 W or F707W, roughly equivalent to V or R-band), corresponding to the near u ltraviolet emission in the rest frame of the radio galaxies. The total obse rving time was 2 orbits per object. In this paper we present the images ove rlayed on VLA radio maps of comparable resolution. We also present previous ly unpublished images, taken from the HST archive, of two other high redshi ft radio galaxies, observed through similar broad band filters. We find tha t on the scale of the HST observations there is a wide variety of morpholog ical structures of the hosting galaxies: most objects have a clumpy, irregu lar appearance, consisting of a bright nucleus and a number of smaller comp onents, suggestive of merging systems. Some observed structures could be du e (at least partly) to the presence of dust distributed through the galaxie s. The UV continuum emission is generally elongated and aligned with the ax is of the radio sources, however the characteristics of the "alignment effe ct" differ from case to case, suggesting that the phenomenon cannot be expl ained by a single physical mechanism. We compare the properties of our radi o galaxies with those of the UV dropout galaxies and conclude that (i) the most massive radio galaxies may well evolve from an aggregate of UV dropout galaxies and (ii) high redshift radio galaxies probably evolve into presen t day brightest cluster galaxies.