On the correlation between young massive star clusters and gamma-ray unassociated sources

TL;DR

This study investigates the link between young massive star clusters and gamma-ray sources, finding a strong correlation with early-phase clusters and gamma-ray emissions at GeV energies, but less so at TeV energies.

Contribution

It provides the first systematic analysis comparing star cluster catalogs with gamma-ray source catalogs, revealing correlations with early-stage clusters and gamma-ray emissions.

Findings

Strong correlation between Fermi-LAT unidentified sources and HII regions.

Gamma-ray emission can originate from winds of massive stars in early clusters.

No significant association found between Gaia star clusters and gamma-ray sources.

Abstract

Star clusters (SCs) are potential cosmic-ray (CR) accelerators and therefore are expected to emit high-energy radiation. However, a clear detection of gamma-ray emission from this source class has only been possible for a handful of cases. This could in principle result from two different reasons: either detectable SCs are limited to a small fraction of the total number of Galactic SCs, or gamma-ray-emitting SCs are not recognized as such and therefore are listed in the ensemble of unidentified sources. In this Letter we investigate this latter scenario, by comparing available catalogs of SCs and HII regions, obtained from Gaia and WISE observations, to the gamma-ray GeV and TeV catalogs built from Fermi-LAT, H.E.S.S. and LHAASO data. The significance of the correlation between catalogs is evaluated by comparing the results with simulations of synthetic populations. A strong correlation emerges between Fermi-LAT unidentified sources and HII regions which trace massive SCs in the earliest (< 1-2 Myr) phase of their life, where no supernova explosions have happened yet, confirming that winds of massive stars can alone accelerate particles and produce gamma-ray emission at least up to GeV energies. The association with TeV-energies sources is less evident. Similarly, no significant association is found between Gaia SCs and GeV nor TeV sources. We ascribe this fact to the larger extension of these objects, but also to an intrinsic bias in the Gaia selection towards SCs surrounded by a lower target gas density, that would otherwise hinder the detection in the optical waveband.