Kinematics of massive star ejecta in the Milky Way as traced by 26Al

Abstract

Massive stars form in groups and their winds and supernova explosions create superbubbles up to kpc in size. Their ejecta are important for the dynamics of the interstellar medium and chemical evolution models. However, ejecta kinematics and the characteristic scales in space and time are rather unexplored beyond ~10 ka. Through measurement of radioactive 26Al with its decay time constant of ~10^6 years, we aim to trace the kinematics of massive-star and supernova ejecta over million-year time scales. Our goal is to identify the mixing time scale and the spatio-kinematics of such ejecta from the pc to kpc scale. We use the SPI spectrometer on INTEGRAL and its observations along the Galactic ridge to trace the line shape of the 1808.63 keV gamma-ray line from 26Al decay. We determine line centroids and compare these to Doppler shift expectations from large-scale systematic rotation around the Galaxy's center, as observed in other Galactic objects. We measure the radial velocities of gas traced by 26Al, averaged over the line of sight, as a function of Galactic longitude. We find substantially higher velocities than expected from Galactic rotation, the average bulk velocity being ~200 km s^-1 larger than the Galactic-rotation prediction. The observed radial velocity spread implies a Doppler-broadening of the gamma-ray line that is consistent with our measurements of the overall line width. We can reproduce the observed characteristics with 26Al sources located along the inner spiral arms, when we add a global blow-out into the forward direction away from arms, such as expected from massive stars offset towards the spiral-arm leading edge. The superbubbles-halo connection implies angular-momentum transfer in the disk-halo system and also radial gas flows.