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tokens: the component CO lines in a merger model, measured in km s 1 . The HST images (Rix etal. 2004 ) show various clumps that indicate ALESS 122.1 and ALESS 67.1 could be mergers (Fig. 3 ). Taking the merger model and R to be 0.5arcsec (4.2kpc at z 2), which is the approximate separation of the clumps in HST optical counterparts (Fig. 3 ), the dynamical mass, M dyn sin 2 i , is 510 11 M for both ALESS 122.1 and ALESS 67.1. Figure 3. The CO(10) velocity structure of ALESS 122.1 (left) and ALESS 67.1 (right), overlaid on HST z band ( F 850 LP ) images from Rix etal. ( 2004 ) 3 . Both optical counterparts are clumpy or complex in the rest-frame UV, indicating either structured dust or a possible merging system. Blue contours indicate the CO(10) emission integrated from 300 to 0kms 1 at 3, 5, 7.... Red contours indicate the CO(10) emission integrated from 0 to 300kms 1 at 3, 5, 7.... Red and blue crosses mark the centroid of the blueshifted and redshifted emission, respectively. The images are 2020arcsec. However, the HST image does not reveal whether or not the systems contain a gaseous disc. In the presence of a gas disc, the dynamical mass can be estimated from the CO line kinematics, by comparing the spatial offset between the redshifted and blueshifted components of the CO line. This was determined by making 300kms 1 wide channel maps centred at 300 to 0kms 1 and 0 to 300kms 1 (Fig. 3 ). The centroid of the CO emission was determined for the maps, and we derive a spatial offset between the redshifted and blueshifted emissions of 1.20.6arcsec (10.25.1kpc) and 1.8 0.5arcsec (15.24.2kpc) for ALESS 122.1 and ALESS 67.1, respectively. This is comparable to CO(10) sizes observed in other SMGs by Ivison etal. ( 2011 ). Assuming this spatial offset represents rotating gas, then the dynamical mass M dyn sin 2 i = (2.11.1)10 11 M and (3.20.9)10 11 M for ALESS 122.1 and ALESS 67.1, respectively. These estimates are consistent with the dynamical masses derived from using the CO line FWHM and optical size from HST imaging. For a rotating model, Bothwell etal. ( 2013 ) found their sample of SMGs to have a median dynamical mass of (1.60.3)10 10 R M , where radius R is in kpc. Using the measured spatial offsets of 10 and 15kpc for R , the dynamical masses of ALESS 122.1 and ALESS 67.1 are consistent with the median dynamical mass of their SMG sample. The total baryonic mass can be calculated by combining the gas and stellar mass estimates for the SMGs. The stellar masses of ALESS 122.1 and ALESS 67.1 have been estimated by Simpson etal. ( 2014 ), who used absolute rest-frame H -band magnitudes from best-fitting SEDs to multiband photometry and a model mass-to-light ratio. They estimate ALESS 122.1 has a stellar mass of 5.210 10 M and ALESS 67.1 has a stellar mass of 4.410 10 M . These estimates are uncertain by a factor of at least a few, due to the difficulty in distinguishing between different model SF histories to predict an accurate mass-to-light ratio