Extreme star formation in the host galaxies of the fastest growing
  super-massive black holes at z=4.8
  We report new Herschel observations of 25 z=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_sol/yr 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 +/- 150 M_sol/yr. 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.
