Probing Galaxy structure with VHE $\gamma$ rays

TL;DR

This paper estimates the Sun's position relative to the Galactic plane using VHE gamma-ray source data, highlighting its influence on observed source distribution and the importance of accounting for this offset in Galactic models.

Contribution

It introduces a method to determine the Sun's height above the Galactic plane based on gamma-ray source distribution, considering observational biases and different spatial models.

Findings

Sun's position varies between -6 pc and 94 pc depending on the model.

Sun's offset significantly affects the observed gamma-ray source distribution.

The asymmetry may not be solely due to the Sun's position.

Abstract

As an observer from within the Milky Way, it is difficult to determine its global structure. Despite extensive observational data from surveys at different wavelengths, we have no conclusive description of the structure of our own Galaxy. For very-high-energy (VHE) $\gamma$ rays, the most comprehensive catalogue of Galactic sources resulting from the H.E.S.S. Galactic Plane Survey (HGPS) shows a striking asymmetry in the distribution of the sources in the latitudinal direction. This could be the result of a local feature in the spatial distribution of the sources or it could be due to the position of the Sun above the Galactic plane. In this contribution, we estimate the position of the Sun based on the latitudinal flux profile of VHE $\gamma$-ray sources, assuming three mirror-symmetric models for the spatial distribution of the sources in three-dimensional space and taking into account the observational bias of the HGPS. We verify our method using simulations and find values for $z_{\odot}$ between $-6\,\mathrm{pc}$ and $94\,\mathrm{pc}$ depending on the considered model. Our results show that the position of the Sun has a significant impact on the observed source distribution and must therefore be taken into account when modelling the population of Galactic VHE $\gamma$ sources. However, it is not conclusive whether the Sun's offset from the Galactic plane is the only factor leading to the asymmetry in the latitudinal profile.