Left panel: Ratio of the number density of objects in the three protoclusters to that in the field, for AGN (stars) and SMGs (circles). Errors are computed in the Poissonian assumption. The environment of AGN with respect to field in our targets is 1–2 dex larger than overdensity of the parent population of galaxies, impliying a direct environmental effect on the SMBH growth in these structures. We also show the ratios for other protoclusters at various redshifts (Tozzi et al. 2022a; Vito et al. 2020, 2024; Travascio et al. 2025). Right panel: Ratio of AGN to SMG overdensity, as a function of SMG overdensity (top) and redshift (bottom), for the same protoclusters.