The Milky Way Heart: Investigating molecular gas and γ-ray morphologies in the Central Molecular Zone

Abstract

Since the discovery of a broad distribution of very high energy (VHE; > 0:1 TeV) gamma-rays in the Central Molecular Zone (CMZ) of the Galaxy in 2006 by the HESS collaboration, the correlation of this emission with the integrated intensity of the CS(1-0) molecular line emission has inferred a hadronic origin for the gamma-rays. Here we describe the beginning of our investigation into the strength of this correlation utilising new multi-line millimeter data from the Mopra CMZ and HOP surveys and multi-wavelength GBT radio continuum observations towards the CMZ and compare these in detail with the diffuse TeV g-ray emission from HESS. The benefit of these new data is that they allow us to simultaneously observe and analyse correlations using a large number (> 10) of molecular species, some of which contain their isotopologue pairs. The use of isotopologue pairs is especially powerful, since it allows one to analyse the optical depth of a number of different molecular species, thus investigating the nature of the correlation over a range of different physical conditions. Here we begin by comparing the integrated line emission and continuum radio emission with the diffuse g-ray emission, and, by using isotopologue pairs such as HCN/H13CN, obtain optical depths throughout the CMZ corresponding to regions of both strong and weak g-ray emission. We find that the radio continuum better matches the peak of the g-ray emission, which corresponds to the more compact – compared to the relatively coarse resolution of the g-ray images – sources in the CMZ. Using the isotopologue pairs, we find that the optical depth at all positions and velocities within the CMZ are about t 2 4. This is similar to that found for the CS(1–0) line and would underestimate the mass of the CMZ, potentially explaining why molecular line emission peaks appear offset from the g-ray peaks.