Illustration of the procedure adopted to fit the spectrally unresolved, sinc-shaped absorption line feature in the SPIRE iFTS cube. Rest frequencies of the relevant transitions are marked by vertical dashed lines. Top panel: observed spectrum (black) toward one of the pixels in our map, fit to continuum and CO lines (blue), and the resulting ratio spectrum (gray). The rectangular box denotes the axes limits of the next panel. Middle panel: zoom of continuum-normalized spectrum (gray, same as top panel) and fit to the combined signature of the HF absorption line and the nearby H₂O emission line (red). Bottom panel: residual remaining after subtracting the fit shown in the middle panel.