Contribution of young massive star clusters to Galactic diffuse γ-ray emission

Context: Young massive stellar clusters (YMSCs) have emerged as potential γ-ray sources, after the recent association of a dozen YMSCs with extended γ-ray emission. The large size of the detected halos, comparable to that of the wind-blown bubble expected around YMSCs, makes the γ-ray detection of individual YMSCs rather challenging. As a result, the emission from most of the Galactic YMSCs could be unresolved, thus contributing to the diffuse γ-ray radiation observed along the Galactic Plane. Aims: In this study, we estimate the possible contribution to the Galactic diffuse γ-ray emission from a synthetic population of YMSCs, and we compare it with observations obtained with different experiments, from 1 GeV to hundreds of TeV, in two regions of the Galactic Plane. Methods: As the population of galactic YMSCs is only known locally, we evaluate the contribution of γ-ray emission relying on the simulation of synthetic populations of YMSCs based on the observed properties of local clusters. We compute the γ-ray emission from each cluster assuming that the radiation is purely hadronic in nature and produced by cosmic rays accelerated at the cluster's collective wind termination shock. Results: We find that the γ-ray emission from unresolved YMSCs can significantly contribute to the observed Galactic diffuse flux, especially in the inner part of the Galaxy. The result is independent of the assumed particle transport, but an important role is played by Wolf-Rayet stars. The predicted γ-ray flux should be considered as a lower limit, given that our calculation neglects the contribution of supernovae exploding in YMSCs.