Infrared Excess and Molecular Gas in Galactic Supershells.pdf

a r X i v : a s t r o - p h / 9 9 0 5 0 6 2 v 1 6 M a y 1 9 9 9 Journal of The Korean Astronomical Society 32: 41 ～ 53, 1999 INFRARED EXCESS AND MOLECULAR GAS IN GALACTIC SUPERSHELLS JEONG?EUN LEE, KEE?TAE KIM, AND BON?CHUL KOO Department of Astromony, Seoul National University, Seoul 151-742, Korea E-mail: jelee@astro.snu.ac.kr (Received Mar. 19, 1999; Accepted Apr. 9, 1999) ABSTRACT We have carried out high-resolution observations along one-dimensional cuts through the three Galactic super- shells 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 τ100 excesses, which are, respec- tively, the 100 μm intensity and 100 μm optical depth in excess of what would be expected from HI emission. We have found that both the I100 and τ100 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 τ100 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× 10 20 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 μm 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. Key Words : ISM: individual (GS 064?01?97, GS 090?28?17, GS 174+02?64) – infrared: ISM: continuum – ISM: molecules I. INTRODUCTION Since molecular hydrogen (H2), which