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

Abstract

We present a new reconstruction of the distribution of atomic hydrogen in the inner Galaxy that is based on explicit radiation transport modeling 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 HI4PI 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 of a dark-matter signal, be it cuspy or cored. But an additional so-called boxy bulge is still favored 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.

Increased Support for a New Millisecond Pulsar Population in the Galactic Bulge

“…In comparison to previous studies, our new GDE model2 for the Galactic center region contains numerous substantial improvements. First, our atomic hydrogen model is based on explicit radiationtransport modeling of line, absorption, and continuum emission [23], allowing a more realistic representation of hydrogen distribution in the GC. Second, our inverse Compton (IC) templates reproduce the state-of-the-art templates recently constructed by the GALPROP team [24].…”

“…Increased Support for a New Millisecond Pulsar Population in the Galactic Bulge Oscar Macias H𝐼 hydrodynamic refers to the new hydrodynamic gas maps introduced in Ref. [23], and H𝐼 interpolated to the standard gas maps widely used in the community (e.g., Ref. [28]).…”

“…in Ref. [23] for further details. These were obtained using a bin-by-bin analysis technique [15,19] with which we agnostically reconstruct the spectra of each template based solely on their spatial morphology.…”

“…As part of this contribution (see Ref. [23] for in-depth discussions), we present a much improved model for the GDE in the inner Galaxy and evaluate how this impacts the GCE's characteristics. Our results confirm previous findings that stellar mass in the Galactic bulge (e.g., Ref [15][16][17]) better matches the GCE's spatial morphology than dark matter templates.…”

Probing the pulsar explanation of the Galactic-Center GeV excess using continuous gravitational-wave searches

“…While early studies suggested that the almost spherically symmetric Galactic-Center excess spatial morphology was well-fit by simple dark-matter models [1][2][3][4][5][6][7], an astrophysical explanation of unresolved millisecond pulsars also appears consistent with the observed excess [8][9][10][11]. The debate between these two explanations is intense: some studies claim that the spatial morphology of the Galactic-Center excess matches better with the mass distribution of the galactic bulge [12][13][14][15], while other studies indicate a preference for a spherically symmetric distribution [16,17]. There have also been potential detections of gamma rays from point sources in the inner Galaxy [18][19][20], but it was shown recently that systematic biases favoring individual sources may exist in these works [21][22][23].…”