Clustering and properties of K-band companion galaxies around USS radio sources

We have analyzed galaxy properties in the environment of a sample of 70 Ultra Steep Spectrum (USS) radio sources selected from the Sydney University Molonglo sky Survey and the NRAO VLA Sky Survey catalogues, using near-IR data complete down to Ks=20. We have quantified galaxy excess around USS targets using an Abell-type measurement N0.5 (Hill & Lilly 1991). 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 show 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 measure the central concentration of light of the USS sample and compare these to the values obtained for field galaxies and galaxies selected through 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.