Expected line peak temperatures T_mb (upper panel) and line opacity τ (lower panel) values for all N₂H⁺ transitions (J ≤ 10–9) predicted by our RADEX models assumed as single-line components (aka without hyperfine structure). For simplicity, we represent only the models with densities n(H₂) = 5 × 10⁵ cm⁻³ (grey) and 5 × 10⁷ cm⁻³ (colours), and temperatures of T_K = 15, 25, and35 K (see legend). We notice how density significantly changes (aka skew) both T_mb and τ distributionstowards high J-transitions. In this context, the detection of bright N₂H⁺ (7–6) emission above T_mb > 2 K can be used as direct probe of gas at densities above n(H₂) > 10⁷ cm⁻³.