We report new Herschel Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA . observations of 25 z \simeq 4.8 extremely luminous optically selected active galactic nuclei ( AGNs ) . Five of the sources have extremely large star forming ( SF ) luminosities , L _ { SF } , corresponding to SF rates ( SFRs ) of 2800–5600 M _ { \odot } yr ^ { -1 } assuming a Salpeter IMF . The remaining sources have only upper limits on their SFRs but stacking their Herschel images results in a mean SFR of 700 \pm 150 M _ { \odot } yr ^ { -1 } . The higher SFRs in our sample are comparable to the highest observed values so far , at any redshift . Our sample does not contain obscured AGNs , which enables us to investigate several evolutionary scenarios connecting super-massive black holes and SF activity in the early universe . The most probable scenario is that we are witnessing the peak of SF activity in some sources and the beginning of the post-starburst decline in others . We suggest that all 25 sources , which are at their peak AGN activity , are in large mergers . AGN feedback may be responsible for diminishing the SF activity in 20 of them but is not operating efficiently in 5 others .