Letter to the Editor

The distribution of cosmic-ray sources in the Galaxy, γ-rays and the gradient in the CO-to-H₂ relation

¹ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany e-mail: aws@mpe.mpg.de
² NASA/Goddard Space Flight Center, Code 661, Greenbelt, MD 20771, USA
³ Joint Center for Astrophysics, University of Maryland, Baltimore County, Baltimore, MD 21250, USA
⁴ Ruhr-Universität Bochum, 44780 Bochum, Germany
⁵ W.W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305, USA

Received: 5 April 2004
Accepted: 12 May 2004

Abstract

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 W_CO-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.