Detection of high-velocity Al-26 towards the Galactic Centre

THEORY predicts(1) that radioactive Al-26 (which has a half-life of 0.72 Myr) is released into the interstellar medium by nova and supernova explosions, from the winds of massive stars in the Wolf-Rayet phase, and from less-massive giant stars in very late stages of the asymptotic giant branch phase. Observations of 1,809-keV gamma-ray emission line from Al-26 can therefore be used as a tracer of Galactic nucleosynthesis during the past million years(2,3). The irregularity of the emission in the plane of the Galaxy(4-7) suggests that the dominant sources are likely to be massive stars and supernovae; the other predicted sources are older, and therefore expected to be distributed more uniformly. Here we report the detection of the 1,809-keV emission line from the direction of the Galactic Centre, and we show that the line width is approximately three times that expected(8,9) from the effect of Doppler broadening due to Galactic rotation. The high velocities inferred from the line width favour an origin of the Al-26 in supernovae or Wolf-Rayet stars. Moreover, the fact that the Al-26 has maintained such high velocities is difficult to reconcile with our current understanding of the propagation of material in the interstellar medium.