The Distribution of Cosmic-Ray Sources in the Galaxy, Gamma-Rays and the Gradient in the CO-to-H2 Relation

We present a solution to the apparent discrepancy between the radial gradient in the diffuse Galactic γ-ray emissivity and the distribution of supernova remnants, believed to be the sources of cosmic rays. Recent determinations of the pulsar distribution have made the discrepancy even more apparent. The problem is shown to be plausibly solved by a variation in the WCO-to-N(H2) scaling factor. If this factor increases by a factor of 5-10 from the inner to the outer Galaxy, as expected from the Galactic metallicity gradient, we show that the source distribution required to match the radial gradient of γ-rays can be reconciled with the distribution of supernova remnants as traced by current studies of pulsars. The resulting model fits the EGRET γ-ray profiles extremely well in longitude, and reproduces the mid-latitude inner Galaxy intensities better than previous models.