Infrared Excess and Molecular Gas in Galactic Supershells

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We have carried out high-resolution observations along one-dimensional cuts through the three Galactic supershells GS 064-01-97, GS 090-28-17, and GS 174+02-64 in the HI 21 cm and CO J=1-0 lines. By comparing the HI data with IRAS data, we have derived the distributions of the I₁00 and tau₁00 excesses, which are, respectively, the 100 mum intensity and 100 mum optical depth in excess of what would be expected from HI emission. We have found that both the I₁00 and tau₁00 excesses have good correlations with the CO integrated intensity WCO in all three supershells. But the I₁00 excess appears to underestimate H₂ column density N(H₂) by factors of 1.5-3.8. This factor is the ratio of atomic to molecular infrared emissivities, and we show that it can be roughly determined from the HI and IRAS data. By comparing the tau₁00 excess with WCO, we derive the conversion factor X = N(H₂)/WCO = 0.26-0.66 in the three supershells. In GS 090-28-17, which is a very diffuse shell, our result suggests that the region with N(H₂) < 3*10²0 cm-2 does not have observable CO emission, which appears to be consistent with previous results indicating that diffuse molecular gas is not observable in CO. Our results show that the molecular gas has a 60/100 mum color temperature Td lower than the atomic gas. The low value of Td might be due either to the low equilibrium temperature or to the lower abundance of small grains, or a combination of both.