Top: 3 GHz angular major axis FWHM size plotted against the radio spectral index between the observed frequencies of 1.4 and 3 GHz. The arrows pointing right indicate lower limits to , while the down-pointing arrows show the upper size limits. The data points highlighted with red filled circles are for the additional 3 GHz components seen towards AzTEC5 (upper data point) and AzTEC8 (lower data point); as discussed in Appendix A, those are probably not physically related to the SMGs. The horizontal dashed line marks the median major axis FWHM size of our SMGs (). For reference, the red shaded region shows the radio spectral index range of −0.8...–0.7, which is typical of the non-thermal synchrotron radio emission from star-forming galaxies. Within the errors, 11 out of 20 sources (55% ± 17%) shown here have a value consistent with this range, and 6 additional sources have a lower limit to less than −0.8. Bottom: 3 GHz brightness temperature as a function of . The symbols are as in the top panel, except that the arrows pointing up indicate the lower T_B limits, and the lower and upper red filled circles are for AzTEC5-N and AzTEC8-E, respectively.