Cluster of galaxies around seven radio-loud QSOs at

II. K-band images

¹ Astrophysikalisches Institut Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Instituto de Física de Cantabria, Universidad de Cantabria-CSIC, Avd. de Los Castros S/N, 35005-Santander, Spain e-mail: gserrano@ifca.unican.es

Corresponding author: S. F. Sánchez, ssanchez@aip.de

Received: 25 July 2002
Accepted: 19 September 2002

Abstract

We have conducted a NIR study of the environments of seven radio-loud quasars at redshifts . In present paper we describe deep K band images obtained for the fields of  arcmin around the quasars with 3σ limiting magnitudes of . These fields were previously studied using deep B and R 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 vs. K 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.