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Phys Rev Lett. 2016 Sep 9;117(11):111101. doi: 10.1103/PhysRevLett.117.111101. Epub 2016 Sep 9.

Cosmic-Ray Injection from Star-Forming Regions.

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Department of Physics and Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, California 95064, USA.
Kavli Institute for Cosmological Physics, University of Chicago, Illinois 60637, USA and Center for Cosmology and AstroParticle Physics (CCAPP) and Department of Physics, The Ohio State University, Columbus, Ohio 43210, USA.


At present, all physical models of diffuse Galactic γ-ray emission assume that the distribution of cosmic-ray sources traces the observed populations of either OB stars, pulsars, or supernova remnants. However, since H_{2}-rich regions host significant star formation and numerous supernova remnants, the morphology of observed H_{2} gas (as traced by CO line surveys) should also provide a physically motivated, high-resolution tracer for cosmic-ray injection. We assess the impact of utilizing H_{2} as a tracer for cosmic-ray injection on models of diffuse Galactic γ-ray emission. We employ state-of-the-art 3D particle diffusion and gas density models, along with a physical model for the star-formation rate based on global Schmidt laws. Allowing a fraction, f_{H_{2}}, of cosmic-ray sources to trace the observed H_{2} density, we find that a theoretically well-motivated value f_{H_{2}}∼0.20-0.25 (i) provides a significantly better global fit to the diffuse Galactic γ-ray sky and (ii) highly suppresses the intensity of the residual γ-ray emission from the Galactic center region. Specifically, in models utilizing our best global fit values of f_{H_{2}}∼0.20-0.25, the spectrum of the galactic center γ-ray excess is drastically affected, and the morphology of the excess becomes inconsistent with predictions for dark matter annihilation.

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