Constraining the low-energy cosmic ray flux in the central molecular zone from MeV nuclear deexcitation line observations

TL;DR

This paper investigates the high ionization rate in the Galactic Center's molecular zone by modeling low-energy cosmic rays and predicting MeV nuclear deexcitation line emissions, proposing future observations to identify their origin.

Contribution

It introduces a model of soft anomalous cosmic rays explaining high ionization and predicts MeV line emissions, guiding future observational efforts.

Findings

Soft anomalous CRs can explain high ionization rates.

MeV line emissions are detectable by next-generation instruments.

Future MeV observations can determine the origin of ionization.

Abstract

Low-energy cosmic rays (LECRs) dominate the ionization in dense regions of molecular clouds in which other ionizers such as UV or X-ray photons are effectively shielded. Thus it was argued that the high ionization rate at the central molecular zone (CMZ) of our Galaxy is mainly caused by LECRs. However, the required LECR flux is orders of magnitude higher than the extrapolation of GeV cosmic ray (CR) flux derived from GeV gamma-ray observations. In this paper, we considered two types of additional LECR components and found that only very soft anomalous CR components can explain such a high ionization rate. This LECR component will inevitably produce MeV nuclear deexcitation lines due to their inelastic scattering with the ambient gas. We calculated the MeV line emission and discussed the detectability of next-generation MeV instruments. We found that future MeV observations can be used to pin down the origin of the high ionization rate in the CMZ.