  We present two-dimensional, integral field spectroscopy covering the
rest-frame wavelengths of strong optical emission lines in nine sub-mm-luminous
galaxies (SMGs) at 2.0<z<2.7. The GEMINI-NIFS and VLT-SINFONI imaging
spectroscopy allows the mapping of the gas morphologies and dynamics within the
sources, and we measure an average Halpha velocity dispersion of
sigma=220+-80km/s and an average half light radius of r=3.7+-0.8kpc. The
average dynamical measure, V_obs/2sigma=0.9+-0.1 for the SMGs, is higher than
in more quiescent star-forming galaxies at the same redshift, highlighting a
difference in the dynamics of the two populations. The SMGs' far-infrared SFRs,
measured using Herschel-SPIRE far-infrared photometry, are on average
370+-90Mo/yr which is ~2 times higher than the extinction corrected SFRs of the
more quiescent star-forming galaxies. Six of the SMGs in our sample show strong
evidence for kinematically distinct multiple components with average velocity
offsets of 200+-100km/s and average projected spatial offsets of 8+-2kpc, which
we attribute to systems in the early stages of major mergers. Indeed all SMGs
are classified as mergers from a kinemetry analysis of the velocity and
dispersion field asymmetry. We bring together our sample with seven other SMGs
with IFU observations to describe the ionized gas morphologies and kinematics
in a sample of 16 SMGs. By comparing the velocity and spatial offsets of the
SMG Halpha components with sub-halo offsets in the Millennium simulation
database we infer an average halo mass for SMGs of 13<log(M[h^-1Mo])<14.
Finally we explore the relationship between the velocity dispersion and star
formation intensity within the SMGs, finding the gas motions are consistent
with the Kennicutt-Schmidt law and a range of extinction corrections, although
might also be driven by the tidal torques from merging or even the star
formation itself.
