Contribution of young massive star clusters to Galactic diffuse $\gamma$-ray emission

Abstract

Context: Young massive stellar clusters (YMSCs) have emerged as potential $\gamma$-ray sources, after the recent association of a dozen YMSCs with extended $\gamma$-ray emission. The large size of the detected halos, comparable to that of the wind-blown bubble expected around YMSCs, makes the $\gamma$-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 $\gamma$-ray radiation observed along the Galactic Plane. Aims: In this study, we estimate the possible contribution to the Galactic diffuse $\gamma$-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 $\gamma$-ray emission relying on the simulation of synthetic populations of YMSCs based on the observed properties of local clusters. We compute the $\gamma$-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 $\gamma$-ray emission from unresolved YMSCs can significantly contribute to the observed Galactic diffuse flux, especially in the inner part of the Galaxy, and that an important role is played by the power injected by Wolf-Rayet stellar winds. The predicted $\gamma$-ray flux should be considered as a lower limit, given that our calculation neglects the contribution of supernovae exploding in YMSCs.

Accepted for publication on A&A (20/01/2025)

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• The Origin of the Very-High-Energy Diffuse $\gamma$-Ray Emission: The Case for Galactic Source Cocoons
• Hadronic acceleration in the young star cluster NGC 6611 inside the M16 region unveiled by Fermi-LAT: constraints on the acceleration efficiency
• Massive Star Clusters as sources of high-energy gamma radiation
• Exploring non-thermal emission from the star-forming region NGC 3603 through a realistic modelling of its environment
• How interacting winds shape the mechanical feedback of massive star clusters over millions of years
• Low-frequency spectra of neutron star + OB supergiant binaries: Does wind density drive persistent and flaring modes of accretion?
• Interpreting the LHAASO Galactic diffuse emission data