Observed [C ii] 158 μm spectrum obtained by integrating the data cube corrected for the primary beam in a box encompassing the southern part of the arc. The dashed vertical lines show the window within which the channels are summed to obtain the line flux. The Doppler velocities (lower axis) are computed with respect to the [C ii] 158 μm frequency at z = 2.013, namely 630.78 GHz.