Top: assumed density profile across the Horsehead (black line, see Sect. 2.3). Normalised dust-observed emission (blue line) in IRAC_3.6 (see Fig. 1). Normalised dust observed emission (green line) in SPIRE₅₀₀. The grey part corresponds to the inner part of the Horsehead, defined in Sect. 4.4. Bottom: density profile in 2D-space defined by the distance to the star, d_⋆, and the length of the Horsehead along the line of sight, l_PDR.