Cluster of galaxies around seven radio-loud QSOs at <formula notation="TeX">$\mathsf 1<z<\mathsf{1.6}$</formula>

We have conducted a NIR study of the environments of seven radio-loud quasars at redshifts <formula notation="TeX">$1<z<1.6$</formula>. In present paper we describe deep <formula notation="TeX">$K$</formula> band images obtained for the fields of <formula notation="TeX">$\sim6\times6$</formula> arcmin around the quasars with 3<formula notation="TeX">$\sigma$</formula> limiting magnitudes of <formula notation="TeX">$K\sim20.5$</formula>. These fields were previously studied using deep <formula notation="TeX">$B$</formula> and <formula notation="TeX">$R$</formula> band images (Sánchez & González-Serrano 1999). Using optical and NIR data together, a significant excess of galaxies has been found whose optical-NIR colours, luminosity, spatial scale, and number of galaxies are compatible with clusters at the redshift of the quasar. We have selected a sample of cluster candidates analyzing the <formula notation="TeX">$R-K$</formula> vs. <formula notation="TeX">$K$</formula> diagram. A ~25% of the candidates present red optical-NIR colours and an ultraviolet excess. This population has been also found in clusters around quasars at the same redshifts (Tanaka et al. 2000; Haines et al. 2001). These galaxies seem to follow a mixed evolution: a main passive evolution plus late star formation processes. The quasars do not inhabit the core of the clusters, being found in the outer regions. This result agrees with the hypothesis that the origin/feeding mechanism of the nuclear activity is merging processes. The quasars inhabit the region where a collision is most likely to produce a merger.