Discovery of Molecular and Atomic Gas associated with HESS J1646-458 (Westerlund 1): Spatial TeV Gamma-Ray and Interstellar Proton Correspondence

We report CO and HI studies of molecular and atomic gas toward the TeV gamma-ray source HESS J1646 - 458, widely considered to be associated with the young massive cluster Westerlund 1 (Wd1). We found that molecular clouds at V LSR about-32 km s-1 coincide with arc-like structures seen at 8 μ m, likely illuminated by strong FUV radiation from Wd1. 12 CO(J = 3-2) emission at the same velocity reveals a cavity-like structure with an expansion velocity of about 5 km s-1 toward the central region of Wd1, suggesting a recently formed wind-blown bubble driven by the cluster. We also identify a complementary spatial distribution between the V LSR about-55 and about-32 km s-1 clouds, connected by an intermediate-velocity component at V LSR about-44 km s-1. These characteristics are consistent with signatures of triggered star formation through a cloud-cloud collision and imply that both clouds are physically associated with Wd1. On larger scales, the total interstellar proton column density at V LSR about-36 - -23 km s-1 shows a moderate spatial correspondence with the TeV gamma-ray shell. Together with this correlation, a substantial gas mass of about 1.6 times 10 6 M, and the absence of bright synchrotron X-rays, the TeV gamma-ray emission surrounding Wd1 is consistent with the hadronic origin. The present finding allows us to calculate the total energy of accelerated cosmic-ray protons to be about 6 times 10 49 erg.