The density of extremely red objects around high - z radio - loud AGN ⋆

We present the results of a K-band imaging survey of 40 arcmin in fields around 14 radio-loud AGN (6 radio galaxies and 8 quasars) with z > 1.5. The survey, 80% complete to K < 19.2 and complemented by R-band imaging, aimed at investigating whether extremely red objects (EROs) 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 the differential galaxy counts in our fields suggest a systematic excess over the general field counts. At K < 19.2, we find an excess of galaxies with R−K > 6 compared to the general field. Consistently, we also find that the R − K colour distribution of all the galaxies in the AGN fields are significantly redder than the colour distribution of the field galaxies. On the other hand, the distribution of the R − K colours is undistinguishable 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 which could explain our results.