26Al kinematics: superbubbles following the spiral arms?

Abstract

High energy resolution spectroscopy of the 1.8 MeV radioactive decay line of 26Al with the SPI instrument on board the INTEGRAL satellite has recently revealed that diffuse 26Al has large velocities in comparison to other components of the interstellar medium in the Milky Way. 26Al shows Galactic rotation in the same sense as the stars and other gas tracers, but reaches excess velocities up to 300 km/s. We investigate if this result can be understood in the context of superbubbles, taking into account the statistics of young star clusters and H I supershells, as well as the association of young star clusters with spiral arms. We derive energy output and 26Al mass of star clusters as a function of the cluster mass via population synthesis from stellar evolutionary tracks of massive stars. [...] We link this to the size distribution of HI supershells and assess the properties of likely 26Al-carrying superbubbles. 26Al is produced by star clusters of all masses above about 200 solar masses, roughly equally contributed over a logarithmic star cluster mass scale, and strongly linked to the injection of feedback energy. The observed superbubble size distribution cannot be related to the star cluster mass function in a straight forward manner. In order to avoid that the added volume of all superbubbles exceeds the volume of the Milky Way, individual superbubbles have to merge frequently. If any two superbubbles merge, or if 26Al is injected off-centre in a bigger HI supershell we expect the hot 26Al-carrying gas to obtain velocities of the order of the typical sound speed in superbubbles, about 300 km/s before decay. [...]