Top panel: gas column densities derived from CO data, integrated in the velocity range of [−11, 21] km   s ⁻¹. Middle panel: H ii column density derived from the Planck free–free maps. Bottom panel: Total gas column density by summing up the molecular gas and H ii gas. The γ-ray emission is overlaid with white contours together with the positions of the energetic pulsars in the field and the position of Westerlund 2 (green circle). The white disk indicates the γ-ray template, in which the total gas mass is calculated.