Do the sources of the 511 keV excess explain the anomalous CMZ ionization?

TL;DR

This paper investigates whether sources of the 511 keV excess, such as positron injection from radionuclides or pulsars, can explain the anomalously high and uniform ionization rates observed in the Galactic Centre's Central Molecular zone, concluding they are insufficient alone.

Contribution

The study evaluates the potential of 511 keV source populations to account for CMZ ionization, providing new constraints on their role in Galactic Centre ionization anomalies.

Findings

Positron injection alone cannot fully explain the ionization anomaly.

Expected ionization from these sources exceeds previous candidate estimates.

Positron sources contribute but are insufficient to match observed ionization levels.

Abstract

The anomalous rate of molecules ionization observed at the Central Molecular zone (CMZ) challenges known mechanisms of ionization observed in molecular clouds across the Galaxy, due to the exceptionally high levels of ionization measured (orders of magnitude above what cosmic rays can explain) and its uniform spatial distribution within the CMZ. Recent studies suggest that the source of the $511$~keV excess can be correlated with this anomalous ionization rate or contribute significantly to the ionization in the Galactic Centre (GC). One of the leading hypotheses attributes the $511$~keV signal to positron injection from radionuclides or pulsars distributed following the stellar bulge, which is rather flat around the GC and, hence, could help explaining the uniform ionization profile. In this work, we investigate whether such a population of sources, injecting MeV positrons at rates consistent with the $511$~keV observations, can account for the ionization levels and distribution observed in the CMZ. Our results indicate that positron injection alone falls short at explaining the anomaly, although their expected ionization is larger than expected from any previously studied candidates.