Infrared Excess and Molecular Gas in the Galactic Worm GW46.4+5.5

We have carried out high-resolution (∼3′) HI and CO line observations along onedimensional cuts through the Galactic worm GW46.4+5.5. By comparing the HI data with IRAS data, we have derived the distributions of I100 excess and τ100 excess, which are respectively the 100 μm intensity and 100 μm optical depth in excess of what would be expected from HI emission. In two observed regions, we were able to make a detailed comparison of the infrared excess and the CO emission. We have found that τ100 excess has a very good correlation with the integrated intensity of CO emission, WCO, but I100 excess does not. There are two reasons for the poor correlation between I100 excess and WCO: firstly, there are regions with enhanced infrared emissivity without CO, and secondly, dust grains associated with molecular gas have a low infrared emissivity. In one region, these two factors completely hide the presence of molecular gas in the infrared. In the second region, we could identify the area with molecular gas, but I100 excess significantly underestimates the column density of molecular hydrogen because of the second factor mentioned above. We therefore conclude that τ100 excess, rather than I100 excess, is an accurate indicator of molecular content along the line of sight. We derive τ100/N(H)=(1.00±0.02)×10−5 (10 cm−2)−1, and X ≡N(H2)/WCO'0.7×10 cm−2 (K km s−1)−1. Our results suggest that I100 excess could still be used to estimate the molecular content if the result is multiplied by a correction factor ξc ≡ < I100/N(H) >HI/< I100/N(H) >H2 (' 2 in the second region), which accounts for the different infrared emissivities of atomic and molecular gas. We also discuss some limitations of this work, which could stem from using single-temperature model and the IRAS 60 μm intensity in estimating the dust optical depth along the line