Gamma-ray Emission of 60Fe and 26Al Radioactivities in our Galaxy
  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.
