We report on the discovery of a molecular cavity in the Norma near arm in the general direction of Westerlund 1 ( Wd1 ) , but not associated with it .
The cavity has a mean radial velocity of -91.5 km s ^ { -1 } , which differs by as much as \sim 40 km s ^ { -1 } from the mean radial velocity of the Wd1 stars .
The cavity is surrounded by a fragmented molecular shell of an outer diameter of about 100 pc and 10 ^ { 6 } M _ { \odot } , which is expanding at velocities of 6 to 8 km s ^ { -1 } .
The amount of kinetic energy involved in the expanding shell is \sim 10 ^ { 51 } erg .
Inside this cavity the atomic HI gas surface density is also the lowest .
Structure of the extended Very High Energetic ( VHE ) \gamma -ray emission , recently reported by the H.E.S.S .
collaboration ( 14 ) , coincides with the cavity .
The observed morphology suggests that the inner wall of the molecular shell is the zone of the \gamma -ray emission , and not the dense gas surrounding massive stars of Wd1 as had been speculated by the H.E.S.S .
collaboration .
A likely candidate responsible for creating the observed cavity and the \gamma -ray emission is the pulsar PSR J1648-4611 .