Galactic diffuse neutrino emission from sources beyond the discovery horizon

The IceCube Neutrino Observatory has recently reported strong evidence for neutrino emission from the Galactic plane. The signal is consistent with model predictions of diffuse emission from cosmic ray propagation in the interstellar medium. However, owing to IceCube's limited potential of identifying individual neutrino sources, it is also feasible that unresolved Galactic sources could contribute to the signal. We investigate the contribution of this quasidiffuse emission and show that the observed Galactic diffuse flux at 100 TeV could be dominated by the hard emission of unresolved sources. Particularly interesting candidate sources are young massive stellar clusters that have been considered as cosmic-ray PeVatrons. We examine whether this hypothesis can be tested by the upcoming KM3NeT detector or the planned future facility IceCube-Gen2 with about 5 times the sensitivity of IceCube.