Comparison between the N₂H⁺ (1–0) integrated intensity (i.e., I(N₂H⁺)) and H₂ column density (i.e., N(H₂)) in NGC 1333. The vertical dotted line indicates the intensity threshold defining the first contour of the emission map in Fig. 5. The linear fit describing Eq. (4) is displayed by a blue solid line. Symbols are similar to those in Fig. 4. RADEX radiative transfer calculations for N₂H⁺ (1–0) emission at densities n(H₂) = [ 0.1, 0.5, 1.0, 5, 10 ] × 10⁵ cm^-3 are overploted using black dashed lines (see text).