Diffuse Infrared Excess Features In The Canadian Galactic Plane Survey

As part of a multiwavelength study of the main constituents of the interstellar medium of the Milky Way, we investigate the possibility of tracing diffuse molecular hydrogen (H-2) through an excess in dust opacity. The dust column density inferred from infrared (IRAS) observations is found in some regions to be greater than predicted by correlations with neutral atomic hydrogen (HI) and carbon monoxide (CO). The Five College Radio Astronomy Observatory's entire CO J = 1 - 0 Outer Galaxy Survey (OGS) region is studied in the context of the Canadian Galactic Plane Survey (CGPS) datasets, to provide arcminute-scale intercomparisons of the HI, CO and dust features. After converting dust emission into optical depth, we compare the relative contributions of these species and identify areas where the dust opacity is greater than expected if a constant dust-to-gas ratio is assumed. An excess of dust that is not accounted for by either HI or CO may provide evidence of diffuse H-2 that is not detectable by the standard CO proxy. Alternatively, the excess may show evidence of variations in the assumed correlation between gas and dust.