THE DISCOVERY OF A MOLECULAR CAVITY IN THE NORMA NEAR ARM ASSOCIATED WITH H.E.S.S γ -RAY SOURCE LOCATED IN THE DIRECTION OF WESTERLUND 1

Abstract

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 ∼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 106 M , which is expanding at velocities of 6 to 8 km s−1. The amount of kinetic energy involved in the expanding shell is ∼1051 erg. Inside this cavity, the atomic Hi 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.