From 10 Kelvin to 10 TeraKelvin: Insights on the Interaction Between Cosmic Rays and Gas in Starbursts

TL;DR

This paper explores cosmic ray interactions with gas in starburst galaxies, examining gamma-ray production, ionization, radio emission origins, and the role of turbulent magnetic fields in explaining observed correlations.

Contribution

It introduces a comprehensive model of cosmic ray interactions in starbursts, emphasizing the roles of molecular clouds, H II regions, and turbulent magnetic fields.

Findings

Gamma rays and Al-26 decay can ionize dense molecular clouds at Milky Way levels.

Radio absorption is mainly due to small H II regions, not a uniform ionized slab.

Superwind gas with turbulent magnetic fields likely produces most synchrotron emission.

Abstract

Recent work has both illuminated and mystified our attempts to understand cosmic rays (CRs) in starburst galaxies. I discuss my new research exploring how CRs interact with the ISM in starbursts. Molecular clouds provide targets for CR protons to produce pionic gamma rays and ionization, but those same losses may shield the cloud interiors. In the densest molecular clouds, gamma rays and Al-26 decay can provide ionization, at rates up to those in Milky Way molecular clouds. I then consider the free-free absorption of low frequency radio emission from starbursts, which I argue arises from many small, discrete H II regions rather than from a "uniform slab" of ionized gas, whereas synchrotron emission arises outside them. Finally, noting that the hot superwind gas phase fills most of the volume of starbursts, I suggest that it has turbulent-driven magnetic fields powered by supernovae, and that this phase is where most synchrotron emission arises. I show how such a scenario could explain the far-infrared radio correlation, in context of my previous work. A big issue is that radio and gamma-ray observations imply CRs also must interact with dense gas. Understanding how this happens requires a more advanced understanding of turbulence and CR propagation.