Frame a: Integrated map of N₂H⁺ (in K km s⁻¹) of the junction region. The contours show the emission of ¹²CO (J = 1–0) at large velocities with respect to the bulk of the cloud. Each contour represent a different velocity as indicated in the inserted legend, and the corresponding levels in T_mb in order of increasing v_lsr are 0.5, 1.0, 3.0, 3.0, 1.5, 1.1, 0.8 and 0.6 K. Frame b: Spectra of ¹²CO (J = 1–0) emission at different locations along the cuts, averaged over the Gaussian beams represented by the black circles in frame a, whose diameters show the beam FWHM (18′′). Frames c–d: Position-velocity diagrams of ¹²CO emission along the grey arrows shown in frame a, with offset increasing from east to west. The colour scales are cut to emphasise the fainter structures at large velocities. The contours show the emission of the central lines of N₂ H ⁺ J = 1–0 (unresolved blend of F₁F = 21–11, 23–12 and 22–11 lines) at T_mb = 0.15, 0.3, 0.5, 1.0 K.