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

Abstract

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_{\mathrm{LSR}}\sim$$-32$ km s${}^{-1}$ coincide with arc-like structures seen at 8 $\hat{I}¼$m, likely illuminated by strong FUV radiation from Wd1. ${}^{12}$CO($J$ = 3-2) emission at the same velocity reveals a cavity-like structure with an expansion velocity of $\sim$$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_{\mathrm{LSR}}\sim$$-55$ and $\sim$$-32$ km s${}^{-1}$ clouds, connected by an intermediate-velocity component at $V_{\mathrm{LSR}}\sim$$-44$ km s${}^{-1}$. These characteristics are consistent with signatures of triggered star formation through a cloud-cloud collision and imply that both clouds are physically associated with Wd1. On larger scales, the total interstellar proton column density at $V_{\mathrm{LSR}}$ $\sim$$-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 $\sim$$1.6\times 10^6$ $M_{\odot }$, and the absence of bright synchrotron X-rays, the TeV gamma-ray emission surrounding Wd1 is consistent with the hadronic origin. The present finding allows us to calculate the total energy of accelerated cosmic-ray protons to be $\sim$$6\times 10^{49}$ erg.

18 pages, 9 figures, accepted for publication in The Astrophysical Journal (ApJ)