Assessing the Impact of Hydrogen Absorption on the Characteristics of the Galactic Center Excess

TL;DR

This study introduces a new atomic hydrogen distribution model for the inner Galaxy, improving gamma-ray emission fits and challenging the need for dark matter explanations for the Galactic Center excess.

Contribution

The paper presents a novel radiation-transport based hydrogen gas model that enhances the understanding of gamma-ray emissions in the Galactic Center region.

Findings

Improved fit to Fermi-LAT gamma-ray data using the new hydrogen model.

No evidence for dark matter signals after including the nuclear bulge.

The boxy bulge remains favored by the data.

Abstract

We present a new reconstruction of the distribution of atomic hydrogen in the inner Galaxy that is based on explicit radiation-transport modelling of line and continuum emission and a gas-flow model in the barred Galaxy that provides distance resolution for lines of sight toward the Galactic Center. The main benefits of the new gas model are, a), the ability to reproduce the negative line signals seen with the H$I$4PI survey and, b), the accounting for gas that primarily manifests itself through absorption. We apply the new model of Galactic atomic hydrogen to an analysis of the diffuse gamma-ray emission from the inner Galaxy, for which an excess at a few GeV was reported that may be related to dark matter. We find with high significance an improved fit to the diffuse gamma-ray emission observed with the Fermi-LAT, if our new H$I$ model is used to estimate the cosmic-ray induced diffuse gamma-ray emission. The fit still requires a nuclear bulge at high significance. Once this is included there is no evidence for a dark-matter signal, be it cuspy or cored. But an additional so-called boxy bulge is still favoured by the data. This finding is robust under the variation of various parameters, for example the excitation temperature of atomic hydrogen, and a number of tests for systematic issues.