SIMBAD references

The environments of a complete subsample of six of the most powerful radio-loud active galactic nuclei (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 surplus galaxy counts 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 and 1. The amplitude of the angular cross-correlation function around the AGN is shown to be 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 images cover a relatively wide field, 5x5 arcmin² , allowing the distribution of the surplus galaxy counts to be investigated. 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 fields of two AGN do show red galaxy excesses on both scales, another two fields show only a large-scale red galaxy overdensity with no pronounced central concentration, and one field shows only a sharp central peak of red galaxies with no large-scale overdensity. The final field shows little evidence for an excess on any scale; this field is associated with an unresolved radio source, perhaps indicating that only extended radio sources probe cluster environments.

Clearly, there is a large range in both the richness and the degree of concentration of any clustering environments around these distant AGN. The implications of this for both cluster formation and the nature of high-redshift AGN are discussed.