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

The isotopes Fe-60 and Al-26 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 Fe-60 combined lines of (0.31 +/- 0.06) x 10(-3) ph cm(-2) s(-1) and the Al line flux of (16.8 +/- 0.7) x 10(-4) ph cm(-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 Al-26 to find scale parameters R-0 = 7.0(-1.5)(+1.0) and z = 0.8(-0.3)(+0.2) kpc; however, the Fe-60 lines are too weak to spatially constrain the emission. Based on a point-source model test across the Galactic plane, the Fe-60 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 Al-26 and Fe-60 emissions and suggest that the Fe-60 emission is more likely to be concentrated toward the Galactic plane. We determine the Fe-60/Al-26 gamma-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 Al-26 and Fe-60 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.