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

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.

Figures (7)

• Figure 1: Comparison of the different components and models for the source distribution in the Galactic bulge, described in the text. The figure illustrates the source density as a function of radial distance for each model, with the normalizations detailed in the main text.
• Figure 2: Top panel: Positron injection spectra of the signals described by Eq. \ref{['eq:LogInj']} for different mean energies (indicated by the colors in the legend). The width of each spectrum is fixed to one-fiftieth of its mean energy. Bottom panel: Injection spectra for different $\beta^+$ radionuclides commonly associated to massive stars, SNe and novae, as indicated in the legend.
• Figure 3: H$_2$ ionization rate expected at the CMZ from positron sources following different source density distributions and assuming an log-normal injection spectrum. Top-left panel: H$_2$ ionization rate expected from positron sources following the boxy-bulge distribution. Top-right: Ionization rate expected from positron sources following the nuclear stellar disk (NSD). Bottom-left: Expected ionization rate in the CMZ from a combination of the nuclear stellar disk and boxy-bulge distributions. Bottom-right: Ionization rate at the CMZ from a combination of the nuclear stellar disk, nuclear stellar cluster and the boxy-bulge distributions.
• Figure 4: H$_2$ ionization rate expected at the CMZ from positron sources following a combination of the nuclear stellar disk, nuclear stellar cluster and the boxy-bulge distributions and assuming radionuclides injecting the positrons (left panel) and a power-law positron injection (right panel). In the left panel, the different colors indicate different radionuclides. In the right panel, the different colors indicate different cut-off energies. In this panel solid lines correspond to a power-law where the spectral index is $\gamma = 2.2$, while the dashed lines correspond to a power-law of $\gamma = 1.2$.
• Figure 5: Same as shown in Fig. \ref{['fig:Distribs']}, but including the dark matter density distribution expected from a Navarro-Frenk-White with inner slopes of 1 (classical NFW) and 1.26 (referred to as cNFW in the legend). The left panel focuses on the inner Galaxy, using a logarithmic x-axis, while the right panel uses a linear scale to highlight the large-scale behavior.