Discovery of Diffuse γ -Ray Emission in the Vicinity of G172.8+1.5: An Old Supernova Remnant with Different Turbulence Properties

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We report the detection of high-energy γ -ray emission in the vicinity of G172.8+1.5, which has long been debated as to whether it is a supernova remnant (SNR) or an ionized hydrogen (H II ) region. Using 16 yr Pass 8 data from the Fermi Large Area Telescope, we found that the GeV emission can be described by two extended sources with different photon spectra. Among them, the much more extended γ -ray source SrcA with a power-law spectrum is spatially coincident with a giant neutral hydrogen shell structure and several OB stars inside a huge H II region. The softer log-parabola spectra γ -ray source SrcB is spatially coincident with a star-forming region with several OB stars, maser sources, and IR sources. Gas observation results display a dense molecular cloud (MC) located within SrcB’s γ -ray emission region in the velocity range of [−25, −5] km s −1 . The possible origins of γ -ray emission are discussed, including CRs that escaped from the SNR shock surface and illuminated a nearby MC, the young massive star clusters associated with the star-forming region, and a pulsar halo generated by an invisible energetic pulsar inside the SNR. Furthermore, under the hadronic scenario, the measured diffusion coefficient in the compact SrcB region is significantly lower than that of the more extended SrcA. This suppression is likely attributed to cosmic-ray-driven instabilities, which reduce the diffusion coefficient in the vicinity of the source relative to the standard conditions in the interstellar medium. Future advanced analysis from LHAASO observation results would help distinguish the origins of γ -ray emission in this region and clarify the nature of this source.