every protocluster undergoes such an episode, strengthening the link postulated by @McLure01.

This relationship between galaxy overdensities and AGN suggests a method for locating high-$z$ clusters: we can use quasars as convenient ‘anchors’ for our search. This technique has already been exploited by others with notable success: for example, @Stiavelli05 tentatively report the detection of clustering around a radio-quiet quasar at $z = 6.28$.

To date most galaxy clusters detected around AGN have been identified based on statistical overdensities of objects observed in their vicinity. A better strategy for overcoming foreground contamination is to identify individual star forming galaxies in the AGN field by their characteristic redshift dependent features. In particular, Lyman $\alpha$ emission has been used to identify high redshift galaxies for some time. Among the first high redshift objects identified by emission lines were the $z = 4.55$ Ly $\alpha$ emitters observed in the field of the quasar BR B2237-0607 by @Hu96. Since then, a series of highly profitable observations of Ly $\alpha$ emitters in AGN fields have been carried out. @Kurk00 and @Pentericci00 used a combination of narrow- and broad-band imaging with follow-up spectroscopy to identify a galaxy overdensity within 1.5 Mpc of the $z = 2.156$ 