Components of the Diffuse Gamma Ray Emission at High Galactic Latitudes

Observations by the COS-B and SAS-2 satellites have shown that the diffuse high energy emission is composed of a galactic component, correlated with the interstellar gas column density, and an isotropic, presumably extragalactic, component. The galactic component, originating largely from the interactions of cosmic rays with hydrogen gas, dominates the emission at low galactic latitudes, where the gas column density is largest. At high galactic latitudes ( | b | > ~ 30deg ), most of the gas is located within a few hundred parsecs of the solar system. Because the total gas column density is lower, the contribution of the extragalactic component rises to ~ 30% for E > 100 MeV, allowing separate measurement of the two components. In addition, the systematic uncertainty in the separation is reduced because the emissivity is determined only by the local cosmic ray density, no longer subject to large-scale variations along the line of sight. Using archival EGRET data and velocity-integrated 21 cm HI observations, we determine the relative contributions of the galactic and extragalactic components of the emission. We present measurements of the extragalactic energy spectrum and the local production function.