N₂H⁺ (J:4–3), ortho-H₂D⁺ (${\mathrm{J}}_{K_a{K}_c}$: 1₁₀−1₁₁) and para-D₂H⁺ (${\mathrm{J}}_{K_a{K}_c}$: 1₁₀−1₀₁) spectra (in black, from top to bottom), extracted along the cut displayed in Fig. 2, with best-fit model of ortho-H₂D⁺ and para-D₂H⁺ (overlaid in red). The three weakest para-D₂H⁺ spectra have been smoothed to 0.21 km s⁻¹. The velocity axis in each box runs from 1 to 6 km s⁻¹. The green dashed lines mark the 3σ individual noise levels and the systemic velocity (3.6 km s⁻¹).