Three-colour JHK_s-band image from 2MASS of W51 (20′ × 40′), overlaid by 1.1 mm dust continuum contour from the Bolocam survey (Aguirre et al. 2011) in green and the 1.4 GHz NRAO VLA Sky Survey (NVSS, Condon et al. 1998) contours in white. The four white boxes mark the areas where photometry and spectroscopy are obtained. From left to right: field I and II cover HII region G49.5–0.4, field III G49.2–0.3 and field IV covers G48.9–0.3. The yellow arcs mark the location and extent of the 3 subregions in W51: W51A, W51B and W51C used throughout the paper.