Recently, we conducted a Very Large Telescope (VLT) large program to search for forming clusters by looking for overdensities of Ly-alpha emitters around high redshift radio galaxies. In total seven proto-clusters were discovered, including a proto-cluster around the radio galaxy MRC 0316--257 at z ~ 3.13. This structure has an excess of Ly-alpha emitters by a factor of 3 as compared to the field, and the derived mass is 2-5 x 10^14 M_sun. The Ly-alpha emitters in the proto-cluster are on average bluer than Lyman Break Galaxies (LBGs). Also, the galaxies are faint (sub L_*) and small (half light radii < 1.7 kpc, which is smaller than the average size of LBGs). This might indicate that, at least a fraction of, Ly-alpha emitters could be young (~ 10^6 yr), nearly dust-free, forming galaxies.
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