  Deep SCUBA surveys have uncovered a population of dust-enshrouded
star-forming galaxies at z~2. Using the ultra-deep 2 Ms Chandra Deep
Field-North survey we recently showed that a large fraction of these systems
are also undergoing intense black-hole growth. Here we provide further
constraints on the properties of the black holes in SCUBA galaxies using the
virial black-hole mass estimator. We show that typical SCUBA galaxies are
likely to host black holes with M_BH~10^7-10^8 M_solar which are accreting at,
or close to, the Eddington limit. These results provide qualitative support for
our earlier conclusion that the growth of the black hole lags that of the host
galaxy in these massive ultraluminous galaxies.
