2010ApJ...713L..45L

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, which differs by as much as ∼40 km/s 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 10⁶ M_☉, which is expanding at velocities of 6 to 8 km/s. The amount of kinetic energy involved in the expanding shell is ∼10⁵¹ erg. Inside this cavity, the atomic H I gas surface density is also the lowest. Structure of the extended Very High Energetic γ-ray emission, recently reported by the H.E.S.S. collaboration, coincides with the cavity. The observed morphology suggests that the inner wall of the molecular shell is the zone of the γ-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 γ-ray emission is the pulsar PSR J1648-4611.