Two-component diffuse Galactic gamma-ray emission revealed with Fermi-LAT

The enigma of cosmic ray origin and propagation stands as a key question in particle astrophysics. The precise spatial and spectral measurements of diffuse Galactic gamma-ray emission provide new avenues for unraveling this mystery. Based on 16 years of Fermi-LAT observations, we find that the diffuse gamma-ray spectral shapes are nearly identical for low energies (below a few GeV) but show significant dispersion at high energies (above a few GeV) across the Galactic disk. We further show that the diffuse emission can be decomposed into two components, a universal spectral component dominating at low energies which is consistent with the expectation from interactions of background cosmic rays and the interstellar matter, and a spatially variant component dominating at high energies which is likely due to local accelerators. These findings suggest that there is dual-origin of the Galactic diffuse emission, including the ``cosmic ray sea'' from efficient propagation of particles and the ``cosmic ray islands'' from inefficient propagation of particles, and thus shed new light on the understanding of the propagation models of Galactic cosmic rays.