  Deep surveys at 850microns from Mauna Kea using the SCUBA camera on the JCMT
appear to have discovered a substantial population of ultraluminous infrared
galaxies (ULIGs: L_ir > 10^{12} L_sun). The cumulative space density of these
sources (~10,000 per sq.deg with S_850 > 1mJy) is sufficient to account for
nearly all of the extragalactic background light at submillimeter wavelengths.
Current estimates of the redshift distribution suggest a peak in the comoving
space density of SCUBA sources at z = 1-3, similar to what is observed for QSOs
and radio galaxies. The luminosity density in the far-infrared/submillimeter
exceeds that in the UV by factors of 3-10 over this redshift range, implying
that as much as 80-90% of the "activity" in galaxies at z < 4 is hidden by
dust. The SCUBA sources plausibly represent the primary epoch in the formation
of spheroids and massive black holes triggered by major mergers of large
gas-rich disks.
