[C II] ²P_3/2 → ²P_ı/2 spectrum (2015–2021 average) towards the central line of sight. The average off-centre spectrum (at 32″ from the centre) is overlaid in green. The systemic velocity (V_sys) is marked with a red dotted line, and V_sys+V_exp and V_sys−V_exp values are marked with an orange and blue dotted line, respectively, where V_exp is the gas terminal expansion velocity. The emission feature at 15 km s⁻¹ could be from the HNC J = 21−20 transition, and is marked as such.