Galactic Population Synthesis of Radioactive Nucleosynthesis Ejecta

Abstract

Diffuse gamma-ray line emission traces freshly produced radioisotopes in the interstellar gas, providing a unique perspective on the entire Galactic cycle of matter from nucleosynthesis in massive stars to their ejection and mixing in the interstellar medium. We aim at constructing a model of nucleosynthesis ejecta on galactic scale which is specifically tailored to complement the physically most important and empirically accessible features of gamma-ray measurements in the MeV range, in particular for decay gamma-rays such as ${}^{26}$Al, ${}^{60}$Fe or ${}^{44}$Ti. Based on properties of massive star groups, we developed a Population Synthesis Code which can instantiate galaxy models quickly and based on many different parameter configurations, such as the star formation rate, density profiles, or stellar evolution models. As a result, we obtain model maps of nucleosynthesis ejecta in the Galaxy which incorporate the population synthesis calculations of individual massive star groups. Based on a variety of stellar evolution models, supernova explodabilities, and density distributions, we find that the measured ${}^{26}$Al distribution from INTEGRAL/SPI can be explained by a Galaxy-wide population synthesis model with a star formation rate of $4$-$8{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ and a spiral-arm dominated density profile with a scale height of at least 700 pc. Our model requires that most massive stars indeed undergo a supernova explosion. This corresponds to a supernova rate in the Milky Way of $1.8$-$2.8$ per century, with quasi-persistent ${}^{26}$Al and ${}^{60}$Fe masses of $1.2$-$2.4{\mathrm{M}}_{\odot }$ and $1$-$6{\mathrm{M}}_{\odot }$, respectively. Comparing the simulated morphologies to SPI data suggests that a frequent merging of superbubbles may take place in the Galaxy, and that an unknown but strong foreground emission at 1.8 MeV could be present.

arXiv comment16 pages, 12 figures, 2 page appendix, accepted for publication in A&A

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• 2D chemical evolution models II. Effects of multiple spiral arm patterns on O, Eu, Fe and Ba abundance gradients
• ${}^{26}$Al gamma rays from the Galaxy with INTEGRAL/SPI
• Gamma-ray line emission from the Local Bubble
• Enhancing Compton telescope imaging with maximum a posteriori estimation: a modified Richardson-Lucy algorithm for the Compton Spectrometer and Imager