Astrophysical contributions to the Fermi Galactic Centre GeV excess. (2020)
Type of ContentElectronic Thesis or Dissertation
Degree NameDoctor of Philosophy
PublisherUniversity of Canterbury
AuthorsPaterson, Dylanshow all
An excess of GeV gamma rays above the expected astrophysical background has been detected in the region of the sky towards the centre of the Milky Way using observations from the Fermi Large Area Telescope. Early studies of the gamma-ray excess found that the spatial morphology and spectrum was consistent with annihilation of dark matter particles in the core of the galaxy. This thesis considers the astrophysical contributions to the gamma-ray landscape in the inner Milky Way. Spatial models are presented for the diffuse background of gamma rays from cosmic ray interactions with interstellar gas and extended emission from an unresolved population of millisecond pulsars.
Cosmic rays diffusing through the Milky Way produce gamma rays via the produc- tion and subsequent decay of π mesons, or bremsstrahlung, when interacting with the interstellar medium. The expected spatial distribution of the resulting gamma rays can then be modelled by the 3-D distribution of gas in the galaxy. Assuming the gas clouds move on circular orbits in the galaxy with a fixed rotation curve, the Doppler shift of the 21cm spectral line is used to place the atomic hydrogen into annular regions. A similar process is used for 2.6mm line emission of carbon monoxide to construct the 3-D distribution of molecular hydrogen, assuming the two species are well mixed. The gas column density is linearly interpolated along galactic longitude in the inner galaxy (|l| < 10◦) to account for the lack of kinematic resolution. An alternative model that uses hydrodynamic simulations to predict the orbital velocity field is also considered. In an analysis of 7 years of Fermi Large Area Telescope observations, the gas maps generated using the hydrodynamic method provides a statistically better fit to the gamma-ray background than the interpolated method recommended by the Fermi Collaboration. Including a contribution to the gamma-ray emission model from an unresolved population of millisecond pulsars, using the nuclear bulge and stellar boxy-bulge as a proxy, further improves the fit. Once the stellar contribution is accounted for, a spherically symmetric component, consistent with the annihilation of dark matter, only improves the fit by 2.7 σ.
To improve the previously used description of the boxy-bulge, a non parametric model is developed to reconstruct the stellar density in the Galactic bulge using the red clump giant stars in the Vista Variables in Via Lactea survey. The non parametric model uses maximum entropy regularisation and algorithmic smoothing regularisation to penalise the likelihood, allowing the stellar density to be reconstructed on a high resolution grid with nearly two million free parameters. A parametric model is also fitted to the VVV catalogues to use as a prior estimate of the density for the non parametric method. A number of systematic tests are performed on both the VVV data and a realistic synthetic population to test the robustness of the non parametric model. The reconstructed stellar density of VVV stars exhibits the boxy/peanut/X-shaped morphology as found with previous studies. Both the parametric and non parametric model densities provide an improved fit to the Fermi data, further supporting the millisecond pulsar explanation of the Galactic centre excess.