Spectral distribution of the differential gamma-ray source function (top) and the energy-integrated gamma-ray emissivity, ﹩{{\rm{\Lambda }}}_{\gamma }﹩, vs. energy (middle) that results from hadronic CR interactions for different CR spectral indices, α (colored differently). We also present ﹩{{\rm{\Lambda }}}_{\gamma }﹩ vs. spectral index in the Fermi-energy band (bottom) for various low-momentum cutoffs q of the distribution function (colored differently). Shown are pion-decay-induced gamma-ray spectra (solid) and IC spectra from a steady-state secondary CR electron population (dashed–dotted) in the weak-field regime (B = 0). The model calculations assume a fixed CR energy density of ﹩{\varepsilon }_{\mathrm{cr}}={10}^{-11}\,\mathrm{erg}\,{\mathrm{cm}}^{-3}﹩ and target nucleon density ﹩{n}_{{\rm{n}}}=1\,{\mathrm{cm}}^{-3}﹩, which both impact the absolute normalization of the spectra in equal measures.