Systematic Study of Cosmic Rays in Star-Forming and Starburst Galaxies using Fermi-LAT and Multiwavelength Data
The majority of $γ$ rays from star-forming galaxies (SFGs) and starburst galaxies (SBGs) are expected to originate from interactions between high-energy cosmic rays (CRs) and the interstellar medium, making $γ$-ray observations a powerful tool for probing CR populations and interstellar environments. Using 15 years of Fermi-LAT data, we conducted a systematic study of seven sources. The $γ$-ray spectra of the bright SBGs M82 and NGC~253 exhibit a decrease in energy flux below a few hundred MeV, suggesting a substantially smaller contribution from CR electrons than previously inferred. Detailed fits of the spectral energy distribution, which combine our GeV $γ$-ray measurements with reported radio and TeV $γ$-ray results, indicate that the electron-to-proton ratios in M82 and NGC~253 are comparable to that of the Milky Way. In contrast, energy densities of CRs in these two SBGs are 2.5--8 times lower than that of the magnetic field, unlike the Milky Way. For the nearby Local Group galaxy M31, we decomposed the $γ$-ray emission into two components, revealing a central source with a soft spectrum and a compact size of less than 0.33 degree, which indicates a considerable contribution from unresolved sources. Our new analysis also reduces the $γ$-ray fluxes of several SBGs, bringing all SBG sample members to levels comparable to or below the calorimetric limit, with calorimetric efficiencies ranging from 0.33 to 1.20. In contrast, all SFG sample members exhibit low efficiencies at the 10\% level. We developed a theoretical model of interstellar $γ$-ray emission that successfully reproduces the observed calorimetric efficiencies, providing a robust framework for interpreting and predicting $γ$-ray emission from SFGs and SBGs.