De-projection of radial Hα surface brightness profiles. To demonstrate how strongly potential projection effects of galaxy inclination could impact our results, all disk-dominated galaxies of the sample were subjected to a maximal geometrical de-projection. For this test, it was assumed that these galaxies were infinitely thin disks and that observed ellipticities were fully due to an inclination of the disk with respect to the observer’s line of sight. Both radius and gas surface density projection were considered. The comparison to Fig. 3 shows a stretch of the radius axis for some objects, but neither qualitative nor quantitative difference in surface brightness excess for the galaxies over the point-source line is shown.