High-resolution radio observations of submillimetre galaxies

We have produced sensitive, high-resolution radio maps of 12 submillimetre (submm) galaxies (SMGs) in the Lockman Hole using combined Multi-Element Radio Linked Interferometer Network (MERLIN) and Very Large Array (VLA) data at a frequency of 1.4 GHz. Integrating for 350 h yielded an rms noise of 6.0 mu Jy beam(-1) and a resolution of 0.2-0.5 arcsec. 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 mu Jy radio population and that therefore, unlike in local ultraluminous infrared galaxies (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 active galactic nuclei (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 similar to 4 arcsec. Possible reasons for this are discussed.