ar X iv : a st ro - p h / 99 04 09 9 v 1 8 A pr 1 99 9 Infrared Excess and Molecular Gas in the Galactic Worm GW 46 . 4 + 5 . 5

We have carried out high-resolution (∼3 ′) HI and CO line observations along one-dimensional cuts through the Galactic worm GW46.4+5.5. By comparing the HI data with IRAS data, we have derived the distributions of I 100 excess and τ 100 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 τ 100 excess has a very good correlation with the integrated intensity of CO emission, W CO , but I 100 excess does not. There are two reasons for the poor correlation between I 100 excess and W CO : firstly, there are regions with enhanced infrared emissiv-ity 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 I 100 excess significantly underestimates the column density of molecular hydrogen because of the second factor mentioned above. We therefore conclude that τ 100