Synthetic 26Al emission from galactic-scale superbubble simulations

Abstract

Emission from the radioactive trace element 26Al has been observed throughout the Milky Way with the COMPTEL and INTEGRAL satellites. In particular, the Doppler shifts measured with INTEGRAL connect 26Al with superbubbles, which may guide 26Al flows off spiral arms in the direction of Galactic rotation. In order to test this paradigm, we have performed galaxy-scale simulations of superbubbles with 26Al injection in a Milky Way-type galaxy. We produce all-sky synthetic γ-ray emission maps of the simulated galaxies. We find that the 1809 keV emission from the radioactive decay of 26Al is highly variable with time and the observer’s position. This allows us to estimate an additional systematic variability of 0.2 dex for a star formation rate derived from 26Al for different times and measurement locations in Milky Way-type galaxies. High-latitude morphological features indicate nearby emission with correspondingly high-integrated γ-ray intensities. We demonstrate that the 26Al scale height from our simulated galaxies depends on the assumed halo gas density. We present the first synthetic 1809 keV longitude-velocity diagrams from 3D hydrodynamic simulations. The line-of-sight velocities for 26Al can be significantly different from the line-of-sight velocities associated with the cold gas. Over time, 26Al velocities consistent with the INTEGRAL observations, within uncertainties, appear at any given longitude, broadly supporting previous suggestions that 26Al injected into expanding superbubbles by massive stars may be responsible for the high velocities found in the INTEGRAL observations. We discuss the effect of systematically varying the location of the superbubbles relative to the spiral arms.

Steady-state nucleosynthesis throughout the Galaxy

“…A large-scale simulation had indicated that the 26 Al mass in the Galaxy could be even lower [49]. On the other hand, other Galactic-scale hydrodynamic simulations with 26 Al sources distributed along spiral arms reproduce the INTEGRAL observations with a steady-state mass of ∼4 M , values between 2.5 and 6 M being consistent with these simulations [51]. It remains to be seen how morphology improvements from imaging as well as simulation work will refine the role of nearby and intermediate-distance clusters.…”

“…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.…”

Galactic 26Al traces metal loss through hot chimneys

“…The simulation we analyse here have been presented in detail in Rodgers-Lee et al (2019). In short, it is a set of 3D hydrodynamic simulations of the interstellar medium of an isolated disc galaxy set in a gravitational potential that represents the static dark matter halo, stellar bulge, disc and rotating spiral arms.…”

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.…”