Monte Carlo simulation of the Galactic Al-26 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 s imulated the map using a Monte Carlo technique together with simple assumpt ions about the spatial distributions and yields of Al-26 sources (clustered core-collapse supernovae and Wolf-Rayet stars, low- and high-mass AGE star s, and novae). Although observed structures (e.g., tangents to spiral arms, bars, and known star-forming regions) are not included in the model, our s imulated gamma-ray distribution bears resemblance to the observed distribut ion. The major difference is that the model distribution has a strong smoot h background along the Galactic plane from distant sources in the disk of t he Galaxy. We suggest that the smooth background is to be expected and prob ably 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 lo w-yield, smoothly distributed sources (low-mass AGE stars and novae).