Galactic very high energy sources and enhancements of material content

The association of very-high energy (VHE; E > 100GeV) sources with regions of the sky rich in dust and gas has been noticed in the study of individual VHE sources. However, the statistical significance of this correlation for the whole population of TeV detections has not been assessed yet. To trace the material content, we make use of the publicly released all-sky maps of astrophysical foregrounds of the Planck Collaboration, and of an extensive existing CO mapping of the Galactic sky. To test the correlation, we construct randomized samples of VHE source positions starting from the inner Galactic plane survey. A positive correlation cannot be firmly established yet. Additionally we explore the possibility that unidentified VHE sources are located in environment particularly rich in material content unaccounted by traditional tracers, including HI hyperfine transition at 21 cm to trace atomic hydrogen in regions where the gas is diffuse. No additional gas component is found in the direction of unidentified VHE sources.