On the Radial Distribution of Gamma Rays in the Outer Galaxy

Abstract

We describe a new method which has the potential for determining the radial distribution of the diffuse component of galactic gamma rays outside the solar circle. We use the observation that a good correlation exists between gamma-ray intensities and total column densities of the local interstellar gas and that the fractional column density of H2≲0.1 HI outside the solar circle. Thus the gamma-ray intensities are shown to be proportional to N(HI). We use the kinematics of the HI to determine the distances from which various fractions of the emission originate in the second and third galactic quadrants. Preliminary results of our analysis show that a significant flux of gamma rays originates from distances as large as 18 kpc from the galactic centre.