Tracing galactic molecular gas using infrared excess sources

The optical depth of dust at 100 mum, tau100, is used to trace the column density of dust in a 330 square degree area of the Galactic Plane, and a majority of the dust correlates with observations of either the 21 cm atomic neutral hydrogen line, or the lowest rotational transition of carbon monoxide. However, analysis of datasets comprising the Canadian Galactic Plane Survey has shown that there exist regions in the Outer Galaxy where the dust content is greater than what is expected from measurements of the most common gaseous tracers. While some of these infrared excess sources are well-explained through an association with dust heated by energetic processes, a second population of infrared excess is devoid of warm dust. This class of infrared excess is related to the coldest phase of the interstellar medium, and appears to trace galactic molecular hydrogen in regions where carbon monoxide is an inefficient surrogate tracer of the galactic distribution of molecular gas. In these seventeen infrared excess regions, approximately half of the molecular hydrogen is not successfully detected through the spectral line observations. The adsorption of larger molecules onto dust grain mantles is a favourable mechanism leading to these cold infrared excess sources.