Comparing simulated ²⁶Al maps to gamma-ray measurements

Abstract

Context. The diffuse gamma-ray emission of 26 Al at 1.8 MeV reflects ongoing nucleosynthesis in the Milky Way, and traces massivestar feedback in the interstellar medium due to its 1 Myr radioactive lifetime. Interstellar-medium morphology and dynamics are investigated in astrophysics through 3D hydrodynamic simulations in fine detail, as only few suitable astronomical probes are available. Aims. We compare a galactic-scale hydrodynamic simulation of the Galaxy's interstellar medium, including feedback and nucleosynthesis, with gamma-ray data on 26 Al emission in the Milky Way extracting constraints that are only weakly dependent on the particular realisation of the simulation or Galaxy structure. Methods. Due to constraints and biases in both the simulations and the gamma-ray observations, such comparisons are not straightforward. For a direct comparison, we perform maximum likelihood fits of simulated sky maps as well as observation-based maximum entropy maps to measurements with INTEGRAL/SPI. To study general morphological properties, we compare the scale heights of 26 Al emission produced by the simulation to INTEGRAL/SPI measurements.Results. The direct comparison shows that the simulation describes the observed inner Galaxy well, but differs significantly from the observed full-sky emission morphology. Comparing the scale height distribution, we see similarities for small scale height features and a mismatch at larger scale heights. We attribute this to the prominent foreground emission sites that are not captured by the simulation.

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

“…The derived 26 Al flux for the full Galaxy is also consistent with the previous 26 Al map study with SPI data (Bouchet et al 2015). In addition, a recent SPI analysis reported the 26 Al flux values for both the inner Galaxy and the whole sky (Pleintinger et al 2019): ∼ 0.29 × 10 −3 ph cm −2 s −1 for the inner region, and ∼ (1.7 − 2.1) × 10 −3 ph cm −2 s −1 for the whole sky. We show the 60 Fe / 26 Al ratio distribution from all 512 exponential-disk configurations in Fig.…”

Steady-state nucleosynthesis throughout the Galaxy

“…Massive star ejecta as traced by 26 Al therefore may recycle to the next generation of stars on a long way over the halo of the Milky Way on timescales likely exceeding 10 Myrs. Recent galaxy-scale hydrodynamics simulations with 26 Al ejection show that the 26 Al scale height is sensitive to the assumed galaxy structure, clustering of star formation as well the gas density in the lower halo [49,50,51]. With realistic assumptions, scale heights of about 2 kpc near the solar radius are found [51], similar to values derived from the INTE-GRAL data [50].…”

“…Recent galaxy-scale hydrodynamics simulations with 26 Al ejection show that the 26 Al scale height is sensitive to the assumed galaxy structure, clustering of star formation as well the gas density in the lower halo [49,50,51]. With realistic assumptions, scale heights of about 2 kpc near the solar radius are found [51], similar to values derived from the INTE-GRAL data [50]. The simulations thus support the interpretation of the INTEGRAL data as tracing massive-star ejecta through superbubbles into the Galactic halo.…”

“…The INTEGRAL/SPI project has been completed under the responsibility and leadership of CNES; we are grateful to ASI, CEA, CNES, DLR (No. 50…”

Study of {^{26}Al}$ in the COSI 2016 Superpressure Balloon Flight

“…The background model is inferred from the data in the background region of the 2016 flight (Figure 1). Although high-latitude emission at 1.8 MeV is recently discussed in the literature [16,17], COSI observations of these regions are dominated by instrumental background radiation and were thus deemed adequate for estimating the background contribution.…”