Galactic 26Al traces metal loss through hot chimneys

Radioactive Al-26 is an excellent tracer for metal ejection in the Milky Way, and can provide a direct constraint on the modelling of supernova feedback in galaxy evolution. Gamma-ray observations of the Al-26 decay line have found high velocities and hence require a significant fraction of the Galactic Al-26 in the hot component. At the same time, meteoritic data combined with simulation results suggest that a significant amount of Al-26 makes its way into stars before decay. We investigated the distribution into hot and cold channels with a simulation of a Milky-Way-like galaxy with massive-star feedback in superbubbles and with ejecta traced by Al-26. About 30-40 per cent of the ejecta remain hot, with typical cooling times of the order Gyr. Al-26 traces the footpoints of a chimney-fed outflow that mixes metals turbulently into the halo of the model galaxy on a scale of at least 50 kpc. The rest diffuses into cold gas less than or similar to 10(4) K, and may therefore be quickly available for star formation. We discuss the robustness of the result by comparison to a simulation with a different global flow pattern. The branching ratio into hot and cold components is comparable to that of longer term average results from chemical evolution modelling of galaxies, clusters, and the intracluster medium.