Two-dimensional Hα maps of our sample SMGs (Table 1) with a size of 25 × 25 kpc and a color stretch from zero to 99.5% of the peak. The solid and the dashed contours starting from 3σ are overlaid on top to show two versions of the ALMA 870 μm continuum, respectively; one was produced using natural weighting, resulting in ∼02 resolution, and the other is tapered to ∼05 resolution, matching to the resolution of the Hα images. The resolution beams in sizes of FWHM are plotted in the bottom corners and the exact sizes are given in Table 1. White and green crosses mark the positions adopted as the centers of the curve-of-growth analyses, for 870 μm continuum and Hα, respectively. These positions are centroids of the best-fit ellipses to the isophotes of the corresponding 2D images, with the details described in Sect. 3. Sources that are identified to host an AGN are marked with an asterisk after the ID. The nearby foreground quasar next to ALESS66.1 is marked with a red dot. We find that the rest-frame FIR emissions appear to be smaller in scale than Hα in most sources.