Monte Carlo simulation of the Galactic 26Al gamma-ray map

The observed map of 1.809 MeV gamma rays from radioactive Al-26 shows clear evidence of a Galactic plane origin with an uneven distribution. We have simulated the map using a Monte Carlo technique together with simple assumptions about the spatial distributions and yields of Al-26 sources (clustered core-collapse supernovae and Wolf-Rayet stars, low- and high-mass AGE stars, and novae). Although observed structures (e.g., tangents to spiral arms, bars, and known star-forming regions) are not included in the model, our simulated gamma-ray distribution bears resemblance to the observed distribution. The major difference is that the model distribution has a strong smooth background along the Galactic plane from distant sources in the disk of the Galaxy. We suggest that the smooth background is to be expected and probably has been suppressed by background subtraction in the observed map. We have also found an upper limit of 1 M-. to the contribution of flux from low-yield, smoothly distributed sources (low-mass AGE stars and novae).