Diffuse High-Energy Gamma-Ray Emission beyond the Solar Circle: The Cepheus and Polaris Flares and the Perseus Arm

We present a study of the diffuse, high-energy gamma-ray emission detected in the outer Galaxy toward the Cepheus flare by the EGRET instrument on the Compton Gamma Ray Observatory. The region studied (l = 100° to 130°, b = -5° to +32°) contains the prominent local clouds of the Cepheus and Polaris flares and the molecular cloud complex associated with Cas A and NGC 7538, the largest in the Perseus arm. Gas along the line of sight is partitioned into three distance ranges. The gamma-ray emissivity and N(H₂)/WCO ratio in the local range are found to be (1.84±0.10) x 10⁻²⁶ s⁻¹sr⁻¹ and (0.92±0.14) x 10²⁰ cm⁻² (K km s⁻¹)⁻¹, respectively, both derived for the energy range E > 100 MeV. In the Perseus arm, these quantities are (1.11±0.09) x 10⁻²⁶ s⁻¹ sr⁻¹ and (2.5±0.9) x 10²⁰ (K km s⁻¹)⁻¹. The decrease of emissivity from the solar circle to the Perseus arm is significant, although the variation of N(H₂)/WCO is not. No significant variations of these quantities within each distance range are found. The gamma-ray emissivity in the Perseus arm is much less than predicted by models that assume a simple coupling between cosmic-ray density and surface density of interstellar gas. The derived emissivity in the Perseus arm constrains the amount of molecular gas not detected in CO to be at most one-tenth the amount of atomic hydrogen. A bright, soft spectrum gamma-ray source in this field reported from analysis of COS B data is not seen by EGRET. The spectrum of the extragalactic isotropic emission toward Cepheus is consistent with findings from other regions that have been studied with EGRET data.