  We have produced sensitive, high-resolution radio maps of 12 SMGs in the
Lockman Hole using combined MERLIN and VLA data at a frequency of 1.4 GHz.
Integrating for 350hr yielded an r.m.s. noise of 6.0 uJy/beam and a resolution
of 0.2-0.5". For the first time, wide-field data from the two arrays have been
combined in the (u,v) plane and the bandwidth smearing response of the VLA data
has been removed. All of the SMGs are detected in our maps as well as sources
comprising a non-submm luminous control sample. We find evidence that SMGs are
more extended than the general uJy radio population and that therefore, unlike
in local ULIRGs, the starburst component of the radio emission is extended and
not confined to the galactic nucleus. For the eight sources with redshifts we
measure linear sizes between 1 and 8 kpc with a median of 5 kpc. Therefore,
they are in general larger than local ULIRGs which may support an early-stage
merger scenario for the starburst trigger. X-rays betray AGN in six of the 33
sources in the combined sample. All but one of these are in the control sample,
suggesting a lower incidence of AGN amongst the submm-luminous galaxies which
is, in turn, consistent with increased X-ray absorption in these dust-obscured
starbursts. Only one of our sources is resolved into multiple, distinct
components with our high-resolution data. Finally, compared to a previous study
of faint radio sources in the GOODS-N field we find systematically smaller
source sizes and no evidence for a tail extending to ~4". Possible reasons for
this are discussed.
