Galactic population synthesis of radioactive nucleosynthesis ejecta

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 (ISM). We aim to construct a model of nucleosynthesis ejecta on a galactic scale that 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 Al-26, Fe-60, or Ti-44. Based on properties of massive star groups, we developed a Population SYnthesis COde (PSYCO), which can instantiate galaxy models quickly and based on many different parameter configurations, such as the star formation rate (SFR), 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 (SN) explodabilities, and density distributions, we find that the measured Al-26 distribution from INTEGRAL/SPI can be explained by a Galaxy-wide population synthesis model with a SFR of 4-8 M-circle dot yr(-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 SN explosion. This corresponds to a SN rate in the Milky Way of 1.8-2.8 per century, with quasi-persistent Al-26 and Fe-60 masses of 1.2-2.4 M-circle dot and 1-6 M-circle dot, 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 yet strong foreground emission at 1.8 MeV could be present.

Automated summary

Diffuse gamma-ray line emission allows us to study the entire cycle of matter in the Milky Way, from nucleosynthesis in massive stars to their ejection and mixing in the interstellar medium. We developed a model called Population SYnthesis COde (PSYCO) that incorporates various parameters to simulate the distribution of nucleosynthesis ejecta. Our model suggests that a high star formation rate and a spiral-arm-dominated density profile are needed to explain the observed distribution of Al-26. The comparison of simulated morphologies with observational data implies frequent merging of superbubbles in the Milky Way and the presence of unknown foreground emission at 1.8 MeV.