Debris from stellar explosions in the Galaxy's fast lane

A team led by Karsten Kretschmer from the Astroparticules et Cosmologie (APC) laboratory, supported by CNES, analysed the observations made by the SPI spectrometer on  ESA's Integral gamma-ray observatory to measure the velocity of the hot gas ejected by massive stars into the inner regions of the Milky Way. The velocity is suprisingly high, about 200 km/s faster than that of stars and the gas induced by the Milky Way rotation. The velocity was  measured using the Doppler effect of the gamma-rays emitted by ²⁶Al, a radioactive isotope of aluminium which is produced in massive stars and expelled into the interstellar medium where it decreases after a lifetime of 1 million years. This means that a big portion of this gas can travel fast velocity over a long period and thus over long distances before stopping when it meets other gases. The scientists deduced that the massive stars ejecting ²⁶Al are probably located at the edge of the spiral arms, and expel most of their ejecta over great distances in the regions located between the spiral arms. This observation tells us how stars and supernovae drive the long-term evolution of interstellar gas in our Milky Way. The gas enrichment, initially mainly composed of hydrogen, the most simple and abundant element, with atomic nuclei freshly produced and recycled in new-generation stars, and the impacts on large-scale gas fluxin the Galaxy, are fundamental questions in astrophysics.

Longitude-velocity diagram of the Milky Way showing the high speed of hot gas containing aluminium-26 (blue shading) compared to measurements of cold gas and theoretical expectations.

Reference:

• "Kinematics of massive star ejecta in the Milky Way as traced by Al", Karsten Kretschmer, Roland Diehl, Martin Krause, Andreas Burkert, Katharina Fierlinger, Ortwin Gerhard, Jochen Greiner and Wei Wang, Astronomy & Astrophysics, Volume 559, November 2013 doi:10.1051/0004-6361/201322563.