GeV -ray emission in the field of young massive star cluster RCW 38

We report the detection of gamma-ray emission by the Fermi Large Area Telescope towards the young massive star cluster RCW 38 in the 1-500 GeV photon energy range. We found spatially extended GeV emission towards the direction of RCW 38, which is best modelled by a Gaussian disc of 0.23 degrees radius with a significance of the extension of similar to 11.4 sigma. Furthermore, the spatial correlation with the ionized and molecular gas content favours the hadronic origin of the gamma-ray emission. The gamma-ray spectrum of RCW 38 has a relatively hard photon index of 2.44 +/- 0.03, which is similar to other young massive star clusters. We argue that the diffuse GeV gamma-ray emission in this region likely originates from the interaction of accelerated protons in the stellar cluster with the ambient gas.