Ratio of integrated τ(OH 1667 MHz) absorption and C¹⁸O(2–1) emission in W43. The top-left panel is the same as the rightmost panel in Fig. 10 and is shown for orientation. The other panels show the ratio of τ(OH 1667 MHz) and C¹⁸ O(2–1) at the indicated velocities after binning three channels of 1.5 km s−1 width. Overlayed on all panels are contours of 18 cm continuum emission (black, in levels of 0.1, 0.2, 0.4, 0.6, 0.8, 1.0, 1.25, 1.5 and 1.75 Jy beam−1). The 1667 MHz optical depth has been masked at 3σ detection levels in the original OH absorption data. For pixels with no C¹⁸O emission counterpart, 3σ detection limits have been used, and are indicated by blue contours. The ratio is quoted in brackets for these locations.