Speaker
Description
Cosmic rays provide non-thermal pressure in the interstellar medium (ISM), affecting star formation and galactic winds, which in turn alter phase structure and pressure balance in the circumgalactic medium (CGM). Recent numerical studies have shown that these effects depend strongly on the diffusion of cosmic rays in the ISM, typically quantified by an effective diffusion coefficient. Methods for constraining the diffusion coefficient observationally often rely on one-dimensional correlations between the resolved radio and IR emission of galaxies. The radio emission traces cosmic ray electrons diffusing in the ISM, while the thermal IR emission is expected to trace the injection sites of cosmic rays.
We develop a new method to constrain the diffusion coefficient of cosmic rays based on the morphological similarity of radio and IR images. To test our method, we use numerical simulations where the diffusion coefficient is known. Specifically, we use cosmological zoom-in simulations of Milky-Way-type galaxies run as part of the FIRE suite of simulations. The simulations self-consistently evolve the transport of cosmic ray electron spectra from supernova injection through the ISM and CGM. We generate synthetic observations of radio and IR emission for simulations with varied treatments of cosmic ray transport and quantify the morphological similarity of images using spatial gradients of the intensity.
The method is tested for different spatial scales and wavelengths, and shows that models run with diffusion coefficient ~10^28 cm²/s show higher morphological similarity between radio and IR compared to cases with higher diffusion coefficients.
Our results show that comparing radio and IR morphology of Milky-Way-type galaxies can be used as a reliable probe of the cosmic ray diffusion coefficient.