Geometry, Not Calorimetry, Drives the Radio/Infrared/Gamma-Ray Correlation

Abstract

We investigate whether the observed radio-infrared-$\gamma$-ray correlation in star-forming galaxies is a geometric effect rather than a signature of local cosmic-ray (CR) calorimetry. Using the GALPROP framework, we generate synthetic observations for external viewers from a grid of 3D Milky Way models with varied CR source, gas, interstellar radiation, and magnetic field distributions, all normalised to reproduce local CR data. We find that a tight, quasi-linear correlation arises naturally from line-of-sight integration through the extended, radially-structured disc, even when local calorimetry is absent. The correlation's properties depend strongly on viewing geometry, preserving its form under moderate inclination but breaking down in edge-on views where galactic components are stratified. We conclude that the correlation is primarily an emergent property of geometric projection and viewing angle, not a direct tracer of local calorimetry. While these findings directly apply to systems with similar structure and star formation properties, their extension to the diverse population of galaxies that form the global radio-infrared-$\gamma$-ray correlation requires further investigation. This geometric perspective implies that the scatter in these relations can be a powerful diagnostic of galactic structure and CR escape.