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 H i 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 ∼ −32 km s⁻¹ coincide with the arc-like structures seen at 8 μm, likely illuminated by strong far-UV radiation from Wd1. ¹²CO(J = 3–2) emission at the same velocity reveals a cavity-like structure with an expansion velocity of ∼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 ∼ −55 and ∼−32 km s⁻¹ clouds, connected by an intermediate-velocity component at V_LSR ∼  −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 ∼ −36 to −23 km s⁻¹ shows a moderate spatial correspondence with the TeV gamma-ray shell. Together with this correlation, a substantial gas mass of ∼1.6 × 10⁶ M_⊙, and the absence of bright synchrotron X-rays, the TeV gamma-ray emission surrounding Wd1 is consistent with a hadronic origin. The present finding allows us to calculate the total energy of accelerated cosmic-ray protons to be ∼6 × 10⁴⁹ erg.