  We describe the optical, near-infrared, and radio properties of a sample of
hard (2-7 keV) X-ray sources detected in a deep Chandra observation of the
field surrounding the Abell 370 cluster. We combine these data with similar
observations of the Chandra Deep Field-North and the Hawaii Survey Field SSA13
to obtain a sample of 69 hard X-ray sources (45 are spectroscopically
identified) with extremely deep 20 cm observations. We find that about 4% of
the >Lstar galaxy population is X-ray luminous at any time and hence that black
hole accretion has a duration of about half a Gyr. We find that about 30% of
the summed 2-7 keV flux from our total sample is from sources at z<1. We
estimate the bolometric luminosities of accretion onto supermassive black holes
and the mass inflow rates. The time history of the accretion rate density is
evaluated, and its integrated value is reasonably consistent with the value
inferred from the local black hole mass to bulge mass ratio.
