  Sub-mm detected galaxies located at redshifts $z > 1$ host a major fraction
of the bolometric luminosity at high redshifts due to thermal emission from
heated dust grains, yet the nature of these objects remains a mystery. The
major problem in understanding their origin is whether the dust heating
mechanism is predominantly caused by star formation or active galactic nuclei,
and what triggered this activity. We address this issue by examining the
structures of a sample of 11 sub-mm galaxies imaged with STIS on the Hubble
Space Telescope. We argue that ~61pm21% of these sub-mm sources are undergoing
an active major merger using the CAS quantitative morphological system. We rule
out at ~5sigma confidence that these sub-mm galaxies are normal Hubble types at
high redshift. This merger fraction appears to be higher than for Lyman-break
galaxies undergoing mergers at similar redshifts. Using reasonable constraints
on the stellar masses of Lyman-Break galaxies and these sub-mm sources, we
further argue that at redshifts z~2-3 systems with high stellar masses are more
likely than lower mass galaxies to be involved in major mergers.
