Total N₂H⁺ (1–0) emission integrated between 80 and 110 km s⁻¹ overlaid on the wide-field IR Spitzer 8 μm image of the G31 star-forming region (middle panel). The coordinates are J2000 right ascension and declination. The white contours show emission levels from 0.5 to 3 K km s⁻¹ (in steps of 0.5 K km s⁻¹), and from 4 to 24 K km s⁻¹ (in steps of 2 K km s⁻¹). The red dots and the red dashed areas denote selected positions and regions, respectively, whose spectra are shown in the left and right panels. Left and right panels: N₂H⁺ (1–0) spectra (gray histograms) toward selected positions and regions. The red curves show the best LTE fit obtained with MADCUBA. Blue and green curves in P1 show the two velocity components that better fit the data. The vertical dashed line corresponds to v − v₀ = 0, where v₀ is the systemic velocity of 96.5 km s⁻¹ (Beltrán et al. 2018). The y-axis scale shows the main beam temperature, T_B, in K, at the peak positions (P1 and P2) or averaged within the regions (F1, F2, F2, and F4).