Zoom on the northernmost pixels 6, 15 and 16 of the HIFI map. The SL9 model is displayed in blue, while the IDP model with a flux of 3.6 × 10⁶  cm s is plotted in red. The IDP model overestimates both the l/c and the line width in each pixel. Even if the IDP flux is lowered to 2.0 × 10⁶  cm s (green line) to roughly fit the l/c, it still fails to fit the wings. A model based on the philosophy of the “hybrid” model of Lellouch et al. (2002) with the SL9 source and a background IDP source with a flux of 8 × 10⁴  cm s, corresponding to the upper limit placed on the IDP source by the authors, is shown in orange. This model can barely be distinguished from the pure SL9 model, which means that an IDP background source is compatible with our observations.