W43 surface density images of H i and H₂ gases in a) and b), respectively. They were calculated using H i and ¹²CO 1–0 lines integrated over 60–120 km s^-1 and Eqs. (1) and (3). Contours are 50 M_⊙pc^-2 in a) and 250, 500, and 750 M_⊙pc^-2 in b). The white boxes outline the extent of the W43 molecular complex (continuous lines) and its H i envelope (dashed lines) as defined in Nguyen Luong et al. (2011b). The black box locates the largest ring used in the R_H2 diagram (see Fig. 7) and the white cross its center.