The physical scale of the far-infrared emission in the most luminous submillimetre galaxies - II. Evidence for merger-driven star formation

We present high-resolution 345-GHz interferometric observations of two extremely luminous (L-ir greater than or similar to 1013 L-circle dot), submillimetre-selected galaxies (SMGs) in the Cosmic Evolution Survey (COSMOS) field with the Submillimeter Array (SMA). Both targets were previously detected as unresolved point sources by the SMA in its compact configuration, also at 345 GHz. These new data, which provide a factor of greater than or similar to 3 improvement in resolution, allow us to measure the physical scale of the far-infrared in the submillimetre directly. The visibility functions of both targets show significant evidence for structure on similar to 0.5-1-arcsec scales, which at z greater than or similar to 1.5 translates into a physical scale of similar to 5-8 kpc. Our results are consistent with the angular and physical scales of two comparably luminous objects with high-resolution SMA follow-up, as well as radio continuum and CO sizes of other SMGs. These relatively compact sizes (less than or similar to 5-10 kpc) argue strongly for merger-driven starbursts, rather than extended gas-rich discs, as the preferred channel for forming SMGs.