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

TL;DR

The paper demonstrates that the observed radio-infrared-$\gamma$-ray correlation in star-forming galaxies is mainly due to geometric projection effects rather than local cosmic-ray calorimetry, highlighting the importance of viewing angle.

Contribution

It shows that the correlation arises naturally from line-of-sight integration in galaxy models, emphasizing geometry over local physics as the primary driver.

Findings

The correlation persists under moderate inclination but breaks down in edge-on views.

Line-of-sight integration through structured discs explains the correlation without local calorimetry.

Viewing geometry significantly influences the observed correlation properties.

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, not local physics. This implies that its scatter is likely not random noise but a diagnostic of underlying galactic structure and viewing angle.