The isotopes $^{ 60}$Fe and $^{ 26}$Al 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 years of SPI data, finding a line flux in $^{ 60}$Fe combined lines of $(0.31\pm 0.06) \times 10^{ -3}$ ph cm$^{ -2}$ s$^{ -1}$ and the $^{ 26}$Al line flux of $(16.8\pm 0.7) \times 10^{ -4}$ ph cm$^{ -2}$ s$^{ -1}$ above the background and continuum emission for the whole sky. Based on the exponential-disk grid maps, we characterise the emission extent of $^{ 26}$Al to find scale parameters $R_0 =7.0^{ +1.5}_{ -1.0}$ kpc and $z_0=0.8^{ +0.3}_{ -0.2}$ kpc, however the $^{ 60}$Fe lines are too weak to spatially constrain the emission. Based on a point source model test across the Galactic plane, the $^{ 60}$Fe emission would not be consistent with a single strong point source in the Galactic center or somewhere else, providing a hint for a diffuse nature. We carried out comparisons of emission morphology maps using different candidate-source tracers for both $^{ 26}$Al and $^{ 60}$Fe emissions, and suggests that the $^{ 60}$Fe emission is more likely to be concentrated towards the Galactic plane. We determine the $^{ 60}$Fe/$^{ 26}$Al $\gamma$-ray flux ratio at $(18.4\pm4.2)\,\%$ , when using a parameterized spatial morphology model. Across the range of plausible morphologies, it appears possible that $^{ 26}$Al and $^{ 60}$Fe 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 its implications for massive star models and their nucleosynthesis.
Categories: astro-ph.HE astro-ph.GA astro-ph.SR