DETECTION OF HIGH-VELOCITY AL-26 TOWARDS THE GALACTIC-CENTER

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 asymptoti c giant branch phase. Observations of 1,809-keV gamma-ray emission lin e from Al-26 can therefore be used as a tracer of Galactic nucleosynth esis during the past million years(2,3). The irregularity of the emiss ion in the plane of the Galaxy(4-7) suggests that the dominant sources are likely to be massive stars and supernovae; the other predicted so urces are older, and therefore expected to be distributed more uniform ly. Here we report the detection of the 1,809-keV emission line from t he direction of the Galactic Centre, and we show that the line width i s approximately three times that expected(8,9) from the effect of Dopp ler broadening due to Galactic rotation. The high velocities inferred from the line width favour an origin of the Al-26 in supernovae or Wol f-Rayet stars. Moreover, the fact that the Al-26 has maintained such h igh velocities is difficult to reconcile with our current understandin g of the propagation of material in the interstellar medium.