26Al kinematics: superbubbles following the spiral arms? Constraints
  from the statistics of star clusters and HI supershells
  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. [...]
