Three-dimensional spatial modeling of nuclear de-excitation lines

Galactic cosmic rays (GCRs) are commonly thought to originate from shock acceleration in supernova remnants. Observations of ionization rates in the interstellar medium (ISM) have indicated an excess of low-energy CRs (LECRs) beyond that expected from standard propagation models. The LECRs excite nuclei in the ISM, producing MeV γ-ray line emission through subsequent de-excitation. Observations of this emission can provide direct constraints on the LECR flux, its morphology, and its acceleration mechanisms. We aim to construct all-sky maps of de-excitation γ-ray emission from the Milky Way based on 3D CR distribution models. By comparing the predicted de-excitation line emission with the Galactic diffuse continuum, we assess the detectability of these lines. We calculated the de-excitation emission by combining 3D CR distributions obtained with the CR propagation code GALPROP and γ-ray production cross sections calculated with the state-of-the-art nuclear reaction code TALYS. Beyond the standard Galactic CR component, we constructed models including an additional LECR component motivated by the high ionization rates measured in the ISM. The predicted de-excitation emission is relatively weak when integrated over the broad regions commonly adopted in MeV γ-ray analyses (|l|łeq30^̧irc and |b|łeq15^̧irc), contributing only łesssim 1% above the leptonic continuum, even when considering the additional LECR component. The all-sky maps revealed that in the standard scenario, the de-excitation-to-leptonic flux ratio remains low over the entire sky, reaching at most ∼20% toward the Galactic anticenter. In contrast, models including an additional LECR component exhibited a pronounced enhancement toward the GC, where the ratio exceeds 100%, indicating favorable prospects for detection through spatially resolved analyses. We also find that the 6.1-to-4.4 MeV intensity ratio is particularly sensitive to the maximum energy of the injected LECR spectrum, varying by up to ∼ 30%. Our results demonstrate that the next generation of MeV observatories can already establish the presence of a distinct LECR component and constrain its origin, acceleration mechanism, and transport in the Galaxy.