1996A&A...307..376R

We present the results of a study of the optical morphologies of distant ultra-steep spectrum (USS) radio galaxies and their environments. The alignment between the optical and radio axes previously noted for high redshift radio galaxies is also found for the radio sources in our sample. The strength of the alignment effect is found not to depend strongly on the size of the radio source. Clustering around the radio galaxies is investigated by examining the galaxy number density as a function of projected distance from the radio galaxy. No excess of bright galaxies (R≲21.5) near the positions of the radio galaxies is found, suggesting that gravitational lensing by foreground clusters - as has been previously suggested to occur for some samples of quasars - is not as important for USS radio galaxies. Furthermore no excess of faint galaxies (R∼23) was found. However, we cannot rule out the possibility that distant radio galaxies reside in clusters, as the magnitude limit of our sample is sufficiently bright that it would be difficult to detect normal cluster galaxies at z>1 if their luminosity function and sizes are similar to those of nearby clusters. An excess of companion galaxies is detected along the axes of the radio sources. The angular separations involved range from 3.1" to 10" which corresponds to 25-80kpc at z=2. This "companion alignment" could be due to the luminosity of a merging dwarf galaxy being enhanced by scattering and/or jet-induced star formation. Alternatively, this alignment could find its origin in a galaxy distribution that is elongated in the direction of the radio axis.