Can rotation curves reveal the opacity of spiral galaxies?

The observed anti-correlation between the inclination and the slope of the innermost part of the rotation curve can be attributed to dust extinction. However, the implied central face-on opacity of tau = 2-4 for the most luminous galaxies is embarrassingly high compared with studies of edge-on galaxies, which yield central face-on opacities equal to at most t = 1 in the optical band. I show that the controversy can be resolved by adopting a model that includes the velocity dispersion of the emitters. Through the use of such a model, I reproduce the correlation between galaxy inclination and the slope of its inner rotation curve without having to resort to excessively high optical depth.