We present the results of a large program conducted with the Very Large Telescope and augmented by observations with the Keck telescope to search for forming clusters of galaxies near powerful radio galaxies at 2.0 < z < 5.2 .
Besides MRC 1138–262 at z = 2.16 , the radio galaxy observed in our pilot program , we obtained narrow- and broad-band images of eight radio galaxies and their surroundings .
The imaging was used to select candidate Ly \alpha emitting galaxies in \sim 3 \times 3 Mpc ^ { 2 } areas near the radio galaxies .
A total of 300 candidate emitters were found with a rest-frame Ly \alpha equivalent width of EW _ { 0 } > 15 Å and significance \Sigma \equiv EW _ { 0 } / \Delta EW _ { 0 } > 3 .
Follow-up spectroscopy was performed on 152 candidates in seven of the radio galaxy fields .
Of these , 139 were confirmed to be Ly \alpha emitters , four were low redshift interlopers and nine were non-detections .
With the adopted criteria the success rate is 139 / 152 = 91 % .
In addition , 14 objects with EW _ { 0 } < 15 and/or \Sigma < 3 were confirmed to be Ly \alpha emitters .
Combined with the 15 Ly \alpha emitters near MRC 1138–262 , we have determined Ly \alpha redshifts for 168 objects near eight radio galaxies .

At least six of our eight fields are overdense in Ly \alpha emitters by a factor 3–5 as compared to the field density of Ly \alpha emitters at similar redshifts , although the statistics in our highest redshift field ( z = 5.2 ) are poor .
Also , the emitters show significant clustering in velocity space .
In the overdense fields , the width of the velocity distributions of the emitters is a factor 2–5 smaller than the width of the narrow-band filters .
Taken together , we conclude that we have discovered six forming clusters of galaxies ( protoclusters ) .
We estimate that roughly 75 % of powerful ( L _ { \mathrm { 2.7 GHz } } > 10 ^ { 33 } erg s ^ { -1 } Hz ^ { -1 } sr ^ { -1 } ) high redshift radio galaxies reside in a protocluster .
The protoclusters have sizes of at least 1.75 Mpc , which is consistent with the structure sizes found by other groups .
By using the volume occupied by the overdensities and assuming a bias parameter of b = 3 - 6 , we estimate that the protoclusters have masses in the range 2 - 9 \times 10 ^ { 14 } M _ { \sun } .
These protoclusters are likely to be progenitors of present-day ( massive ) clusters of galaxies .
For the first time , we have been able to estimate the velocity dispersion of cluster progenitors from z \sim 5 to \sim 2 .
The velocity dispersion of the emitters increases with cosmic time , in agreement with the dark matter velocity dispersion in numerical simulations of forming massive clusters .

— cosmology : large scale structure of Universe