Hubble/ACS I-band images of the 3 GHz detected SMGs whose morphological classification in the rest-frame UV is uncertain as discussed in Sect. 4.4. The greyscale is inverted, and displayed using a power-law stretch to better show the low and high surface brightness features. The black contours show the 3 GHz radio emission as in Fig. A.1. Each image is centred on the ALMA 1.3 mm peak position, is 2″ × 2″ in size, and displayed with north up and east left. The red plus signs mark the positions of the optical-NIR counterparts for which the photometric redshifts were derived by Brisbin et al. (2017), except towards C66 where the plus sign marks the ALMA position from which the optical-NIR photometry was manually extracted owing to blending issues.