Estimating Galactic gas content using different tracers: Compatibility of results, dark gas, and unidentified TeV sources

TL;DR

This study compares different tracers of galactic gas to evaluate their consistency and explores the possibility of unaccounted material contributing to very-high energy gamma-ray sources, finding no evidence for significant hidden gas components.

Contribution

It systematically assesses the compatibility of molecular, atomic, and dust tracers for estimating galactic gas content in VHE gamma-ray regions, highlighting uncertainties in conversion factors.

Findings

The three tracers yield compatible gas mass estimates within uncertainties.

No additional untraced gas component is detected in the studied regions.

CO-based mass estimates may underestimate total mass locally.

Abstract

A large fraction of Galactic very-high energy (VHE; E$\gtrsim$100 GeV) $\gamma$-ray sources is cataloged as unidentified. In this work we explore the possibility that these unidentified sources are located in ambients particularly rich in material content unaccounted by traditional tracers. In a scenario where the VHE emission is due to the interaction of the accelerated particles with a target mass, a large mass of untraced material could be substantially contributing to the VHE emission from these regions. Here, we use three tracers for the commonly explored components: intensity of the $^\textrm{12}$CO(1$\rightarrow$0) line to trace the molecular material, HI hyperfine transition at 21cm to trace atomic hydrogen, and dust emission to trace the total hydrogen content. We show that the estimates of material content from these three tracers are compatible if the uncertainty on the respective conversion factors is taken into account. No additional gas component is found in these regions. However, a simple mass estimation from the $^\textrm{12}$CO(1$\rightarrow$0) line intensity might underestimate the total mass component in some locations.