HST images and properties of the most distant radio galaxies

With the Hubble Space Telescope we have obtained images of 9 of the most distant radio galaxies. The galaxies, which have redshifts between z =2 :3 and z =3 :6, were ob- served with the WFPC2 camera in a broad band filter (F606W or F707W, roughly equivalent to V or R-band), corresponding to the near ultraviolet emission in the rest frame of the radio galaxies. The total observing time was 2 orbits per object. In this paper we present the images overlayed on VLA radio maps of comparable resolution. We also present previously unpub- lished images, taken from the HST archive, of two other high redshift radio galaxies, observed through similar broad band fil- ters. We find that on the scale of the HST observations there is a wide variety of morphological structures of the hosting galaxies: most objects have a clumpy, irregular appearance, consisting of a bright nucleus and a number of smaller components, sugges- tive of merging systems. Some observed structures could be due (at least partly) to the presence of dust distributed through the galaxies. The UV continuum emission is generally elon- gated and aligned with the axis of the radio sources, however the characteristics of the "alignment effect" differ from case to case, suggesting that the phenomenon cannot be explained by a single physical mechanism. We compare the properties of our radio galaxies with those of the UV dropout galaxies and con- clude 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 present day brightest cluster galaxies.