Comparison of different tracers in the Serpens filament. Integrated-intensity maps (on a color scale) of (a) H¹³CN (1−0), (b) HN¹³C (1−0), (c) H¹³CO⁺ (1−0), and (d) N₂H⁺ (1−0). The blue contours represent the respective integrated intensities, which are 2ⁿ × 0.15 K km s⁻¹ and n = 0, 1, 2,… The black contours of HCNH⁺ (2−1) integrated intensities are the same as in Fig. 1a. The intensity was integrated over 7–9.5 km s⁻¹ for HN¹³C (1−0) and H¹³CO⁺ (1−0), while a broader velocity range (5–9.5 km s⁻¹) was chosen to cover the three main HFS components of N₂H⁺ (1−0). The beam size is shown in the lower-right corner of each panel. In all panels, the (0, 0) offset corresponds to α_J2000 = 18^h28^m50^s.4, δ_J2000 = 00°49′58″.72. The markers are the same as in Fig. 1.