High-energy emission from the starburst galaxy NGC 253

Measurement sensitivity in the energetic γ-ray region has improved considerably and is about to increase further in the near future, motivating a detailed calculation of high-energy (HE; ≥100 MeV) and very high-energy (VHE; ≥100 GeV) γ-ray emission from the nearby starburst galaxy NGC 253. Adopting the convection-diffusion model for energetic electron and proton propagation, and accounting for all the relevant hadronic and leptonic processes, we determine the steady-state energy distributions of these particles by a detailed numerical treatment. The electron distribution is directly normalized by the measured synchrotron radio emission from the central starburst region; a commonly expected theoretical relation is then used to normalize the proton spectrum in this region. Doing so fully specifies the electron spectrum throughout the galactic disc and, with an assumed spatial profile of the magnetic field, the predicted radio emission from the full disc matches well the observed spectrum, confirming the validity of our treatment. The resulting radiative yields of both particles are calculated; the integrated HE and VHE fluxes from the entire disc are predicted to be f(≥100 MeV) ≃ (1.8 ^+1.5_-0.8) × 10^-8 cm^-2 s ^-1 and f(≥100 GeV) ≃ (3.6^+3.4_-1.7) × 10^-12 cm^-2 s^-1, with a central magnetic field value B₀ ≃ 190 ± 10 μG. We discuss the feasibility of measuring emission at these levels with the space-borne Fermi and ground-based Cherenkov telescopes.