Upper panel: pixel-to-pixel comparison between the N₂H⁺ (1–0) and NH₃ (1, 1) integrated emission within NGC 1333, both convolved to 60 arcsec. The blue line denotes the linear fit described by Eq. (3). The results obtained in Perseus by Johnstone et al. (2010) for both prestellar (solid triangles) and protostellar cores (open circles) are superposed in black. Lower subpanels: histograms of the differences in velocity (left) and non-thermal velocity dispersion (right) between N₂H⁺ and NH₃ for all the positions fitted with a S/N ≥ 3. For positions with multiple lines, only the comparison to the nearest component in velocity is displayed (see Sect. 5 for a discussion of the gas kinematics).