  The environments of a complete subsample of 6 of the most powerful radio-loud
AGN at redshifts z~1.6 are investigated, using deep RJK imaging to depths of
R~26, J~22.4 and K~20.6. An excess of galaxy counts in the K-band is seen
across these fields; these are predominantly associated with red galaxies
(R-K>4) of magnitudes 17.5<K<20.5 found within radial distances of ~1 Mpc of
the AGN host. These are exactly (though not uniquely) the magnitudes, colours
and locations that would be expected of old passive elliptical galaxies in
cluster environments at the redshifts of these AGN. Using both an Abell-style
classification scheme and investigations of the angular and spatial
cross-correlation functions of the galaxies, the average environment of the
fields around these AGN is found to be consistent with Abell cluster richness
classes 0 to 1. The amplitude of the angular cross-correlation function around
the AGN is a strong function of galaxy colour, and is highest when only those
galaxies with the colours expected of old elliptical galaxies at these
redshifts are considered.
  The galaxy overdensities are found on two scales around the AGN: (i)
pronounced central concentrations on radial scales within 150 kpc; where
present, these are composed almost entirely of red (R-K>4) galaxies, suggesting
that the morphology-density relation is imprinted into the centres of clusters
at a high redshift. (ii) weaker large-scale excesses extending out to between 1
and 1.5 Mpc radius. The presence or absence of galaxy excesses on these two
scales, however, differs greatly between the six different fields. The
implications of these results for both cluster formation and the nature of high
redshift AGN are discussed (abridged).

Astrophysics