SIMBAD references

We have analysed galaxy properties in the environment of a sample of 70 ultrasteep spectrum (USS) radio sources selected from the Sydney University Molonglo Sky Survey and the National Radio Astronomy Observatories (NRAO) Very Large Array Sky Survey catalogues, using near-infrared data complete down to K_s= 20. We have quantified galaxy excess around USS targets using an Abell-type measurement N_0.5. We find that most of the USS fields studied are compatible with being Abell class 0 richness clusters. A statistical analysis of the distribution of companion galaxies around USS radio sources shows a pronounced tendency for such objects to be found in the direction defined by the radio axis, suggesting that they may be related to the presence of the radio sources. We have also measured the central concentration of light of the USS sample and compared these to the values obtained for field galaxies and galaxies selected using other methods. By using Spearman statistics to disentangle richnesses and concentration indices dependences, we detect a weak, but significant, positive correlation. We find that at z > 2 USS radio sources are more concentrated than field galaxies at similar redshifts, indicating that these objects trace the most massive systems at high redshift.