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

A large fraction of Galactic very-high energy (VHE; E≳100 GeVE≳100 GeV) γ-ray sources is cataloged as unidentified. In this work we explore the possibility that these unidentified sources are located in environment 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 12CO(1→01→0) line to trace the molecular material, HI hyperfine transition at 21 cm 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 12CO(1→01→0) line intensity might underestimate the total mass component in some locations.