The Cosmic Ray Population of the Galactic Central Molecular Zone

TL;DR

This study models cosmic ray interactions in the Galactic Center to compare its gamma-ray and radio emissions with starburst galaxies, revealing limitations of single-zone models and suggesting an additional cosmic-ray electron population.

Contribution

It introduces an improved cosmic ray interaction model for the Galactic Center, highlighting the need for an extra cosmic-ray population to explain observations.

Findings

Model agrees with TeV gamma-ray data but not GeV energies.

Single-zone model insufficient to explain all emissions.

Cosmic-ray electron spectrum energetically favored.

Abstract

The conditions in the Galactic Center are often compared with those in starburst systems, which contain higher supernova rates, stronger magnetic fields, more intense radiation fields, and larger amounts of dense molecular gas than in our own Galactic disk. Interactions between such an augmented interstellar medium and cosmic rays result in brighter radio and gamma-ray emission. Here, we test how well the comparisons between the Galactic Center and starburst galaxies hold by applying a model for cosmic ray interactions to the Galactic Center to predict the resulting gamma-ray emission. The model only partially explains the observed gamma-ray and radio emission. The model for the gamma-ray spectrum agrees with the data at TeV energies but not at GeV energies. Additionally, as the fits of the model to the radio and gamma-ray spectra require significant differences in the optimal wind speed and magnetic field strength, we find that the single-zone model alone cannot account for the observed emission from the Galactic Center. Our model is improved by including a soft, additional cosmic-ray population. We assess such a cosmic ray population and its potential sources and find that a cosmic-ray electron spectrum is energetically favored over a cosmic-ray proton spectrum.