Left: modelled integrated intensities (i.e. intensity integrated along a line-of-sight considering a face-on PDR) of H₂ lines as a function of q_AF. Right: modelled integrated intensities of H₂ lines for q_AF = 2% (red dots) and q_AF = 17% (blue dots). Observed integrated intensity (the spectra are averaged in an area centred at the emission peak (see black boxes in Fig. 3 in Habart et al. 2011)) in the Horsehead with uncertainties (1σ) is shown by the green squares. Models have been scaled on the 1–0 S(1) H₂ observed line.