The extended TeV gamma-ray source HESS J1023-575 is coincident with the massive , young stellar cluster Westerlund 2 ( Wd2 ) and its surrounding HII region RCW 49 .
On the basis of an analysis of the CO emission and 21Â cm absorption along the line of sight to Wd2 , it is argued that this cluster , and by assumption the TeV source as well , must be associated with a giant molecular cloud in the far side of the Carina arm with a mass of 7.5 x 10 ^ { 5 } M _ { \odot } .
Analysis of the spatial and velocity structure of the cloud reveals clear evidence of interaction with Wd2 .
The cloud ’ s kinematic distance of 6.0 \pm 1.0 kpc is shown to be consistent with distances inferred both from the radius-linewidth relation of molecular clouds and from the foreground gas column derived from 230 X-ray sources in Wd2 .