Central panel: continuum image of the central region of 3C 236 shown in Fig. 1. The contour lines start at 3σ_VLBI,cont and increase logarithmically by a factor of 2. Solid and dashed contour lines correspond to positive and negative flux density, respectively. The black crosses mark areas where the spectrum was extracted between 29 000–30 000 km s⁻¹. Outer panels: optical depth spectrum shown as the ratio of the absorbed flux density to the continuum flux density. The dashed line represents the optical depth detection limit at the 3σ_VLBI,cube noise level. The solid vertical line marks the systemic velocity.