HI 21 cm line emission map at a radial velocity of about  − 55 km s^-1 (McClure-Griffiths et al. 2005) between 20 and 80 K. Bright regions are HI-depleted whereas dark regions are HI-dense. Overlaid are the CO contours (red) at 12.5, 22.5 and 32.5 K (Dame et al. 2001) along with the smoothed HESS J1646–458 contours in blue. The estimated position of the HI void B3 is marked by the small green dashed circle (Kothes & Dougherty 2007). The large green dashed circle indicates the region used for the spectral reconstruction in the TeV regime and to compute the ambient matter density. The position of Wd 1 is marked by a white star in the centre. The circles in the lower left corner indicate the beam sizes of the respective radio observations.