  We present a quantitative morphological analysis using HST NICMOS H160- and
ACS I775- band imaging of 25 spectroscopically confirmed submillimetre galaxies
(SMGs) which have redshifts between z=0.7-3.4. Our analysis also employs a
comparison sample of more typical star-forming galaxies at similar redshifts
(such as LBGs) which have lower far-infrared luminosities. This is the first
large-scale study of the morphologies of SMGs in the near-infrared at ~0.1"
resolution (<1kpc). We find that the half light radii of the SMGs
(r_h=2.3+/-0.3 and 2.8+/-0.4kpc in the observed I- and H-bands respectively)
and asymmetries are not statistically distinct from the comparison sample of
star-forming galaxies. However, we demonstrate that the SMG morphologies differ
more between the rest-frame UV and optical-bands than typical star-forming
galaxies and interpret this as evidence for structured dust obscuration. We
show that the composite observed H-band light profile of SMGs is better fit
with a Sersic index with n~2, suggesting the stellar structure of SMGs is best
described by a spheroid/elliptical galaxy light distribution. We also compare
the sizes and stellar masses of SMGs to local and high-redshift populations,
and find that the SMGs have stellar densities which are comparable to local
early-type galaxies, as well as luminous, red and dense galaxies at z~1.5 which
have been proposed as direct SMG descendants, although the SMG stellar masses
and sizes are systematically larger. Overall, our results suggest that the
physical processes occuring within the galaxies are too complex to be simply
characterised by the rest-frame UV/optical morphologies which appear to be
essentially decoupled from all other observables, such as bolometric
luminosity, stellar or dynamical mass.
