The ionisation rate in the Central Molecular Zone has been measured using various methods along many lines of sight. The estimated value is approximately $2\times10^{-14}\,\mathrm{s^{-1}}$, which is 3-4 orders of magnitude higher than the local ionisation rate. Due to the high gas column densities, cosmic rays are assumed to be the main ionising agents in the Galactic Centre. Does this unusually high ionisation rate then reveal an equally high cosmic-ray density in this region? This excess is, however, not reflected in the gamma-ray emissions.
In this work, we explore the Galactic Centre ionisation scenario in which cosmic rays are the exclusive ionising agents and infer the particle injection conditions that need to be satisfied.
In the case of both protons and electrons, the injection in low energies needs to be very steep (spectral index in momentum above 5.0). Moreover a huge part of the power in cosmic rays available in the entire galaxy needs to be injected in the central 100 parsecs of the galaxy. We conclude that cosmic rays can not be the only ionising agents in the Galactic Centre.