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Cosmic-Ray Injection from Star-Forming Regions.

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Utilizing molecular hydrogen (H2) as a tracer for cosmic-ray sources improves models of diffuse Galactic gamma-ray emission. This approach better fits observations and reduces excess gamma-ray signals from the Galactic center.

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Area of Science:

  • Astrophysics
  • Cosmic Ray Physics
  • Gamma-ray Astronomy

Background:

  • Current models for diffuse Galactic gamma-ray emission rely on cosmic-ray sources tracing OB stars, pulsars, or supernova remnants.
  • Molecular hydrogen (H2)-rich regions are sites of active star formation and host numerous supernova remnants, suggesting their potential as cosmic-ray injection tracers.

Purpose of the Study:

  • To investigate the impact of using molecular hydrogen (H2) distribution as a tracer for cosmic-ray injection in diffuse Galactic gamma-ray emission models.
  • To assess if H2 tracing improves model fits to observed gamma-ray data and influences the interpretation of Galactic center gamma-ray excess.

Main Methods:

  • Employed advanced 3D particle diffusion and gas density models.
  • Utilized a physical star-formation rate model based on global Schmidt laws.
  • Incorporated a fraction (fH2) of cosmic-ray sources tracing the observed H2 density.

Main Results:

  • A theoretically motivated fraction of H2 tracing (fH2 ~ 0.20-0.25) significantly enhances the global fit to the diffuse Galactic gamma-ray sky.
  • This H2 tracing model substantially suppresses the intensity of residual gamma-ray emission from the Galactic center region.
  • The spectrum of the Galactic center gamma-ray excess is drastically altered, and its morphology becomes inconsistent with dark matter annihilation predictions.

Conclusions:

  • Molecular hydrogen (H2) distribution offers a physically motivated, high-resolution tracer for cosmic-ray injection, improving diffuse gamma-ray emission models.
  • The improved modeling challenges interpretations of the Galactic center gamma-ray excess as solely due to dark matter annihilation.
  • Future studies should consider H2 distribution for more accurate modeling of cosmic-ray propagation and gamma-ray production in the Milky Way.