Number of photons emitted per unit velocity bin. The black histogram shows the frequency distribution in the sources frame, as sampled by all photons used in our G2 experiment. This line profile is the one we use as a model for Hii regions (Sect. 3.4). The dotted red (resp. dashed blue) histograms show how these photons are distributed in the observer frame when the galaxy is seen face-on (resp. edge-on).