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Young massive stellar clusters are extreme environments and potentiallyprovide the means for efficient particle acceleration. Indeed, they areincreasingly considered as being responsible for a significant fraction ofcosmic rays (CRs) accelerated within the Milky Way. Westerlund 1, the mostmassive known young stellar cluster in our Galaxy is a prime candidate forstudying this hypothesis. While the very-high-energy $\gamma$-ray source HESSJ1646-458 has been detected in the vicinity of Westerlund 1 in the past, itsassociation could not be firmly identified. We aim to identify the physicalprocesses responsible for the $\gamma$-ray emission around Westerlund 1 andthus to better understand the role of massive stellar clusters in theacceleration of Galactic CRs. Using 164 hours of data recorded with the HighEnergy Stereoscopic System (H.E.S.S.), we carried out a deepspectromorphological study of the $\gamma$-ray emission of HESS J1646-458. Wefurthermore employed H I and CO observations of the region to infer thepresence of gas that could serve as target material for interactions ofaccelerated CRs. We detected large-scale ($\sim 2^\circ$ diameter) $\gamma$-rayemission with a complex morphology, exhibiting a shell-like structure andshowing no significant variation with $\gamma$-ray energy. The combined energyspectrum of the emission extends to several tens of TeV, and is uniform acrossthe entire source region. We did not find a clear correlation of the$\gamma$-ray emission with gas clouds as identified through H I and COobservations. We conclude that, of the known objects within the region, onlyWesterlund 1 can explain the bulk of the $\gamma$-ray emission. Several CRacceleration sites and mechanisms are conceivable, and discussed in detail.(abridged)

A deep spectromorphological study of the $\gamma$-ray emission surrounding the young massive stellar cluster Westerlund 1