Cosmic rays (CRs) are charged, relativistic particles, that constitute a significant energy component of the interstellar medium. They move relative to the thermal gas, which, combined with their inefficient cooling, allows them to establish long-lived pressure gradients that can have significant dynamical impacts on their host galaxies, for example by launching galactic outflows. They also affect the chemistry of gas through ionization and regulate star formation. The precise impact, however, is highly sensitive to the details of CR transport. This is a complex theoretical problem to untangle and currently largely unconstrained by observations.
Except in our local solar neighborhood, we observe CRs only indirectly through the radiation they produce, for example gamma-rays. The diffuse gamma-ray sky of our Milky Way is dominated by emission originating from neutral pions, which have decayed into gamma-ray photons. The pions themselves are created in collisions between CR protons and gas particles. Observations of the diffuse gamma-ray sky of the Milky Way provide an indirect probe of the underlying CR distribution, and traces the most dynamically important CRs.
To progress our understanding of CRs and constrain their impact from the simulation side, we need to produce synthetic observations and see how well they agree with real ones. Simulations also allow us to investigate the role of the local environment on the observed gamma-ray sky in a quantitative way, something that has not been attempted before in large-scale, self-consistent galaxy simulations.
For this project we ran a full simulation of an isolated Milky Way-like galaxy from the Rhea suite, including both magnetic fields and a relativistic CR fluid that is advected with and diffuses relative to the gas, where diffusion is anisotropic and directed along the magnetic field lines. The gamma-ray emission from CR protons is calculated in a post-processing step and assumes that the CRs are in a steady-state.
We pick locations in our galaxy that mimic our own location in the Local Bubble, and compute the full diffuse gamma-ray sky as seen by a simulated observer at these points. The result is an incredibly diverse set of gamma-ray skies with unique filaments and features, especially at higher latitudes, see above (also Fig. 5 in the paper). This shows how important the local environment is for setting the features we see, which we quantify in detail in the paper.
One of our most surprising results is how well our simulation reproduces key observational properties of the Milky Way gamma-ray sky, both in terms of the total luminosity and flux and the shape of the gamma-ray spectrum, see above (and Fig. 10 in the paper). This is despite of the simplifying assumptions involved in our CR and gamma-ray modeling. For example, the CR transport parameters have been picked to match local CR observations, even though the underlying CR microphysics is almost certainly more complex. Our results suggest that our large-scale treatment captures a lot of the essential physics and provides an important benchmark for future studies with more sophisticated CR models. This is currently work in progress.
The diffuse gamma-ray sky of a Milky Way analog: Local diversity and global constraints. , , , , , , , , , and
© The Authors (2026). CC-BY 4.0