Ratio between the integrated flux of the spiral models, and the integrated flux of its 1D equivalent, in which all material is located in a homogeneous outflow. In principle this figure should exhibit identical amounts of red and blue markers, but a major overlap of both has resulted in an artificial bias favouring the red-labelled models. The green zone represents the instrumental calibration errors. The blue vertical line respresents the Σ value found in the hydrodynamical simulations of Kim & Taam (2012).