Norma tangency spectra. The [C ii], H i, and ¹²CO emission spectra are plotted against velocity (V_LSR). Each panel shows the spectra for the indicated longitude ranges. Note that the intensity scale is normalized to the peak emission within the velocity range. The corresponding 1σ error bars are indicated on the [C ii] and ¹²CO spectra. The tangent velocity is marked on each panel by a vertical arrow. Panel a): Off-tangent. Panel b): On-tangent.The dashed lines indicate the V_LSR shift between the [C ii] and ¹²CO peaks.