The density of extremely red objects around high-z radio-loud active galactic nuclei

We present the results of a K -band imaging survey of 40 arcmin² in fields around 14 radio-loud active galactic nuclei (AGN), comprising six radio galaxies and eight quasars, with z >1.5. The survey, which is 80 per cent complete to K <19.2 mag and complemented by R -band imaging, aimed at investigating whether extremely red objects are present in excess around high- z AGN, and to study the environment of z >1.5 radio galaxies and radio-loud quasars. At 18< K <19 mag, the differential galaxy counts in our fields suggest a systematic excess over the general field counts. At K <19.2 mag we find an excess of galaxies with R ? K >6 in comparison with the general field. Consistently, we also find that the R ? K colour distribution of all the galaxies in the AGN fields is significantly redder than the colour distribution of the field galaxies. On the other hand, the distribution of the R ? K colours is indistinguishable from that of galaxies taken from literature fields around radio-loud quasars at 1< z <2. We discuss the main implications of our findings and we compare the possible scenarios that could explain our results.