Diffuse gamma-ray emission is a key tracer of cosmic rays (CRs) in galaxies, encoding information about their transport, energetics, and interaction with the interstellar medium. Interpreting the Milky Way’s gamma-ray sky, however, remains challenging because the observed emission depends jointly on the CR distribution gas distribution, the position of the observer within the Galaxy. Using the Rhea suite of CR–magnetohydrodynamic (MHD) simulations of a Milky Way analog, we investigated how pion-decay gamma-ray emission varies with galactic environment, local conditions, and CR transport physics. The emission was computed in post-processing under steady-state CR cooling and interaction assumptions, thus enabling us to analyze luminosities, spectra, full-sky emission maps, and angular power spectra (APS) for many observer positions, including those located inside Local Bubble-like cavities. The simulated galaxy naturally reproduces Milky Way-like gamma-ray luminosities and spectral slopes without any parameter tuning. While the total luminosity remains comparatively stable across the galaxy, the detailed morphology of the gamma-ray sky varies strongly with observer location due to the complex distribution of gas in the nearby environment . Across all observers, the APS closely follows the structure of the gas column density rather than the more diffuse CR energy density . Comparisons with Fermi–LAT data show good agreement for both the all-sky spectrum and the APS. A diffusion coefficient energy-scaling with power-law index δ = 0.5 generally matches the observations best. three-dimensional and as well as , consistent with longstanding observational results , in agreement with previous gamma-ray analyses and CR propagation models Our results show that these well-established features of Galactic gamma-ray emission arise naturally in fully self-consistent CR–MHD galaxy simulations . Gas density fluctuations are the primary drivers of the morphology of the pion-decay emission, while CR transport parameters govern its spectral and structural details. The Rhea simulations thus provide a physically grounded framework for interpreting diffuse gamma-ray observations and highlight the importance of understanding the observer’s local surroundings when using gamma rays to trace Galactic CR physics.
Kjellgren et al. (Tue,) studied this question.