²⁶Al kinematics: superbubbles following the spiral arms?

Abstract

Context. High-energy resolution spectroscopy of the 1.8 MeV radioactive decay line of 26 Al with the SPI instrument onboard the INTEGRAL satellite has recently revealed that diffuse 26 Al has higher velocities than other components of the interstellar medium in the Milky Way. 26 Al shows Galactic rotation in the same sense as the stars and other gas tracers, but reaches excess velocities of up to 300 km s −1 . Aims. We investigate whether 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. Methods. We derived energy output and 26 Al mass of star clusters as a function of the cluster mass by population synthesis from stellar evolutionary tracks of massive stars. Using the limiting cases of weakly and strongly dissipative superbubble expansion, we linked this to the size distribution of H I supershells and assessed the properties of possible 26 Al-carrying superbubbles. Results. 26 Al is produced by star clusters of all masses above ≈200 M , is 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 straightforward manner. To avoid the added volume of all superbubbles exceeding the volume of the Milky Way, individual superbubbles have to merge frequently. If any two superbubbles merge, or if 26 Al is injected off-centre into a larger HI supershell, we expect the hot 26 Al-carrying gas to obtain velocities of the order of the typical sound speed in superbubbles, ≈300 km s −1 before decay. For star formation coordinated by the spiral arm pattern which, inside co-rotation, is overtaken by the faster moving stars and gas, outflows from spiral arm star clusters would preferentially flow into the cavities that are inflated by previous star formation associated with this arm. These cavities would preferentially be located towards the leading edge of a given arm. Conclusions. This scenario might explain the 26 Al kinematics. The massive-star ejecta are expected to survive ≥10 6 yr before being recycled into next-generation stars.

The26Al Gamma-ray Line from Massive-Star Regions

“…The Eridanus cavity presents a viewing geometry similar to what we propose for massive star groups in the Galaxy's spiral arms to explain the large excess above Galactic rotation velocities [5,6]. Fitting a Gaussian line shape to the Orion 26 Al γ-ray signal, the line width is found to be compatible with the instrumental resolution, and it is indicated (hatched region in Fig.…”

“…Measurements of systematic Doppler shifts of the line with Galactic longitude [4,5] had shown that the observed 26 Al γ rays originate from sources throughout the Galaxy, including its distant and otherwise occulted regions at and beyond the inner spiral arms and bulge. Moreover, the Doppler shifts of the 26 Al-line centroid energy were found larger than expected from large-scale Galactic rotation, and suggested that large cavities around massive star groups play a major role in guiding ejecta flows from massive-star and supernova nucleosynthesis [6,7].…”

Finding the Remnants of the Milky Way's Last Neutron Star Mergers

“…High energy resolution γ-ray spectroscopy reveals that 26 Al rotates in the same general sense as the Galaxy but has large velocities in comparison to other components of the ISM. Krause et al (2015) explore the hypothesis that this behavior can be explained from the kinematics of "superbubbles" created by the correlated SN explosions mixing the ejecta into the hot phase of the ISM. By contrast, the delay time of NSM should result in uncorrelated activity and therefore the kinematics could be substantially different.…”

The e-ASTROGAM mission

“…INTEGRAL/SPI data for the 1809 keV 26 Al line suggest that on the global, Galactic scale, superbubbles are key structures in the transport of fresh ejecta towards new star forming regions [86,87]. With its huge increase in sensitivity, e-ASTROGAM will provide a detailed view of the morphology of this emission, with high precision measurements of the line flux from many regions of the Galaxy.…”