  Deep SCUBA surveys have uncovered a large population of massive submm
galaxies (SMGs) at z>~1. Although it is generally believed that these galaxies
host intense star-formation activity, there is growing evidence that a
substantial fraction also harbor an AGN. We present here possibly the strongest
evidence for this viewpoint to date: the combination of ultra-deep X-ray
observations (the 2 Ms CDF-N) and deep Keck spectroscopic data. We find that
the majority (~75%) of the spectroscopically identified SMGs host AGN activity;
the other ~25% have X-ray properties consistent with star formation (X-ray
derived SFRs of ~1300-2700 solars/yr). The AGNs have properties generally
consistent with those of nearby luminous AGNs (L_X~1E43-3E44) and the majority
(~80%) are heavily obscured. We construct composite rest-frame 2-20 keV spectra
for three different obscuration classes. An ~1 keV equivalent width FeK-alpha
emission line is seen in the composite X-ray spectrum of the most heavily
obscured AGNs (N_H>5x10^{23}), suggesting Compton-thick or near Compton-thick
absorption. Even taking into account the effects of absorption, we find that
the average X-ray-FIR ratio of the AGN-classified SMGs is approximately one
order of magnitude below that found for typical quasars. This result suggests
that intense star-formation activity dominates the bolometric output; however,
we also explore whether the X-ray-FIR ratio is intrinsically less than that
found for typical quasars and postulate that some SMGs may be AGN dominated. We
investigate the growth of massive black holes, discuss prospects for deeper
observations, and explore the potential of the next generation of X-ray
observatories. (ABRIDGED).
