Integrated optical depth of the OH 1667 MHz line in the W43 star-forming region. In the top-left panel, τ is integrated over the same velocity range as in Fig. 10. The other panels show τ around the indicated velocities after integrating over three channels of 1.5 km s−1 width. For each pixel, only channels that are detected at a 3σ level or higher contribute to the integrated τ-map. The optical depth map is overlayed with contours of the 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). Symbols as in Fig. 10.