  We present a long-term, multi-wavelength project to understand the epoch of
fastest growth of the most massive black holes by using a sample of 40 luminous
quasars at z~4.8. These quasars have rather uniform properties, with typical
accretion rates and black hole masses of L/L_Edd~0.7 and M_BH~10^9 M_sol. The
sample consists of "FIR-bright" sources with a previous Herschel/SPIRE
detection, suggesting SFR > 1000 M_sol/yr, as well as of "FIR-faint" sources
for which Herschel stacking analysis implies a typical SFR of ~400 M_sol/yr.
Six of the quasars have been observed by ALMA in [CII]{\lambda}157.74 micron
line emission and adjacent rest-frame 150 micron continuum, to study the dusty
cold ISM. ALMA detected companion, spectroscopically confirmed sub-mm galaxies
(SMGs) for three sources - one FIR-bright and two FIR-faint. The companions are
separated by ~14-45 kpc from the quasar hosts, and we interpret them as major
galaxy interactions. Our ALMA data therefore clearly support the idea that
major mergers may be important drivers for rapid, early SMBH growth. However,
the fact that not all high-SFR quasar hosts are accompanied by interacting
SMGs, and their ordered gas kinematics observed by ALMA, suggest that other
processes may be fueling these systems. Our analysis thus demonstrates the
diversity of host galaxy properties and gas accretion mechanisms associated
with early and rapid SMBH growth.
