[Paper Review] Discovery of Gamma-Ray Emission from the X-shaped Bulge of the Milky Way

Oscar Macías, Chris Gordon+5 more|arXiv (Cornell University)|Nov 21, 2016

Dark Matter and Cosmic Phenomena5 citations

TL;DR

This paper presents evidence that the Galactic Centre Excess in gamma-ray emission is better explained by the non-spherical stellar over-density of the Milky Way's bulge and nuclear bulge than by a spherical dark matter distribution. Using hydrodynamical modelling of interstellar gas and gamma-ray emission, the authors find that the spatial morphology of the excess aligns with the stellar bulge, rejecting dark matter as the primary source and implicating the stellar population instead.

ABSTRACT

An anomalous gamma-ray excess emission has been found in Fermi Large Area Telescope data covering the centre of the Galaxy. Several theories have been proposed for this `Galactic Centre Excess'. They include self-annihilation of dark matter particles, an unresolved population of millisecond pulsars, an unresolved population of young pulsars, or a series of burst events. Here we report on a new analysis that exploits hydrodynamical modelling to register the position of interstellar gas associated with diffuse Galactic gamma-ray emission. We find evidence that the Galactic Centre Excess gamma rays are statistically better described by the stellar over-density in the Galactic bulge and the nuclear stellar bulge, rather than a spherical excess. Given its non-spherical nature, we argue that the Galactic Centre Excess is not a dark matter phenomenon but rather associated with the stellar population of the Galactic bulge and nuclear bulge.

Motivation & Objective

• To determine whether the Galactic Centre Excess in gamma-ray emission is consistent with a dark matter origin or better explained by astrophysical sources.
• To investigate the spatial morphology of the excess and assess whether it aligns with the non-spherical distribution of the Galactic bulge and nuclear stellar bulge.
• To use hydrodynamical modelling of interstellar gas to trace diffuse gamma-ray emission and improve spatial correlation with stellar structures.
• To test the hypothesis that the excess is not spherically symmetric, thereby ruling out a dominant dark matter contribution.
• To provide a physical explanation for the excess by linking it to the stellar population in the Galactic bulge.

Proposed method

• Hydrodynamical modelling of interstellar gas to map the spatial distribution of diffuse Galactic gamma-ray emission.
• Cross-correlation of gamma-ray emission data from the Fermi Large Area Telescope with the stellar over-density in the Galactic bulge and nuclear bulge.
• Statistical comparison of the observed gamma-ray excess against spherical and non-spherical emission models.
• Use of spatial morphology analysis to assess whether the excess aligns better with the stellar bulge than with a spherically symmetric dark matter distribution.
• Application of emission models based on interstellar gas density and cosmic-ray interactions to predict diffuse gamma-ray emission.
• Evaluation of competing explanations—dark matter self-annihilation, millisecond pulsars, young pulsars, and burst events—by comparing their predicted spatial distributions to observations.

Experimental results

Research questions

• RQ1 Is the Galactic Centre Excess in gamma-ray emission consistent with a spherically symmetric dark matter distribution or better described by a non-spherical structure?
• RQ2 Does the spatial morphology of the excess align with the stellar over-density of the Galactic bulge and nuclear bulge?
• RQ3 Can hydrodynamical modelling of interstellar gas improve the accuracy of diffuse gamma-ray emission mapping in the Galactic centre?
• RQ4 Is the observed excess more likely due to astrophysical sources such as the stellar population than to dark matter self-annihilation?
• RQ5 What is the relative contribution of the stellar bulge to the diffuse gamma-ray emission in the Galactic centre compared to other potential sources?

Key findings

• The Galactic Centre Excess gamma-ray emission shows a significantly better statistical fit to the non-spherical distribution of the Galactic bulge and nuclear stellar bulge than to a spherical model.
• The spatial morphology of the excess is inconsistent with a dominant dark matter origin, as dark matter would produce a spherically symmetric signal.
• The analysis confirms that the excess emission is better correlated with the stellar over-density in the Galactic centre than with any spherically symmetric component.
• Hydrodynamical modelling of interstellar gas provides a robust framework for linking diffuse gamma-ray emission to underlying astrophysical structures.
• The results challenge the hypothesis that the excess is due to dark matter self-annihilation, favoring instead a connection to the stellar population in the bulge.
• The findings suggest that the excess is more plausibly explained by unresolved high-energy processes associated with the dense stellar environment of the Galactic bulge.