Putting Things Back Where They Belong: Tracing Cosmic-Ray Injection with H2

TL;DR

This study proposes using H2 gas distribution as a tracer for cosmic-ray injection in models of diffuse Galactic gamma-ray emission, leading to improved fits and altered interpretations of the Galactic center excess.

Contribution

It introduces a physically motivated H2-based tracer for cosmic-ray sources, improving gamma-ray emission models and challenging dark matter explanations for the Galactic center excess.

Findings

H2-based tracer improves global gamma-ray fit

Reduces residual emission in the Galactic center

Alters the spectrum and morphology of the gamma-ray excess

Abstract

At present, all physical models of diffuse Galactic gamma-ray emission assume that the distribution of cosmic-ray sources traces the observed populations of either OB stars, pulsars, or supernova remnants. However, since H2-rich regions host significant star formation and numerous supernova remnants, the morphology of observed H2 gas should also provide a physically motivated, high-resolution tracer for cosmic-ray injection. We assess the impact of utilizing H2 as a tracer for cosmic-ray injection on models of diffuse Galactic gamma-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_H2, of cosmic-ray sources to trace the observed H2 density, we find that a theoretically well-motivated value f_H2 ~ 0.20 -- 0.25 (i) provides a significantly better global fit to the diffuse Galactic gamma-ray sky and (ii) highly suppresses the intensity of the residual gamma-ray emission from the Galactic center region. Specifically, in models utilizing our best global fit values of f_H2 ~ 0.20 -- 0.25, the spectrum of the galactic center gamma-ray excess is drastically affected, and the morphology of the excess becomes inconsistent with predictions for dark matter annihilation.