galaxies within a 30arcsec aperture, centred 16arcsec (136kpc) from the radio galaxy. These galaxies appear to be highly clustered around the central RLAGN (Figure \[fig:fieldimg\]). The source density is so high in the central 0.9arcmin region that five pairs of sources were blended in the 4.5image, and their true nature was only discovered in the higher resolution ground-based data. It is possible that many of the other CARLA cluster candidates which have extremely high galaxy overdensities also suffer from blending.

Follow-up near-infrared Keck spectroscopy of C revealed five galaxies, including the RLAGN, with spectroscopic redshifts between $1.578<z<1.587$ within a projected diameter of 2Mpc (Rettura et al., in preparation). This structure therefore satisfies the criteria set out by @Eisenhardt2008 for a spectroscopically confirmed $z>1$ (proto)cluster, and so we refer to the structure as 7C from now on.

Besides the RLAGN, there are $29\pm 6$ excess galaxies within 0.9arcmin of C that are selected with the above colour and magnitude criteria[^4]. This level of clustering and overdensity is slightly greater than that of the ClG0218.3$-$0530 protocluster at $z=1.62$, which has a galaxy excess of $22\pm 6$ using the same criteria. The cluster ClG0218.3$-$0530 is a well-studied structure with a tentative 4.5$\sigma$ X-ray 