Gamma-Ray Emission of ⁶⁰Fe and ²⁶Al Radioactivity in Our Galaxy

The isotopes ⁶⁰Fe and ²⁶Al originate from massive stars and their supernovae, reflecting ongoing nucleosynthesis in the Galaxy. We studied the gamma-ray emission from these isotopes at characteristic energies 1173, 1332, and 1809 keV with over 15 yr of SPI data, finding a line flux in ⁶⁰Fe combined lines of (0.31\pm 0.06)\× 10-3\,\&ph;\,cm-2\,s-1 and the Al line flux of (16.8\pm 0.7)\× 10-4\,\&ph;\,-2\,s-1 above the background and continuum emission for the whole sky. Based on the exponential disk grid maps, we characterize the emission extent of ²⁶Al to find scale parameters R0=7.0-1.0+1.5 and z 0=0.8-0.2+0.3 kpc; however, the ⁶⁰Fe lines are too weak to spatially constrain the emission. Based on a point-source model test across the Galactic plane, the ⁶⁰Fe emission would not be consistent with a single strong point source in the Galactic center or somewhere else, providing a hint of a diffuse nature. We carried out comparisons of emission morphology maps using different candidate source tracers for both ²⁶Al and ⁶⁰Fe emissions and suggest that the ⁶⁰Fe emission is more likely to be concentrated toward the Galactic plane. We determine the ⁶⁰Fe/ ²⁶Al γ-ray flux ratio at 18.4% ± 4.2% when using a parameterized spatial morphology model. Across the range of plausible morphologies, it appears possible that ²⁶Al and ⁶⁰Fe are distributed differently in the Galaxy. Using the best-fitting maps for each of the elements, we constrain flux ratios in the range 0.2-0.4. We discuss the implications for massive star models and their nucleosynthesis.