Line-of-sight velocity profile for all the bins intersecting the midplane of IC 1553, measured on the Hα emission line (gray circles). The dashed lines mark fits of Eq. (C.1) to the data in this work (dashed blue line) and from Comerón et al. (2019; dashed red line). The solid yellow curve marks the rotation curve from the best-fitting model of the gravitational potential obtained, as described in Sect. 3.3.