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Fukui_2014_apj_796_1_59.pdf (3.32 MB)

H ɪ, CO, and Planck/IRAS dust properties in the high latitude cloud complex, MBM 53, 54, 55 and HLCG 92-35. Possible evidence for an optically thick HI envelope around the CO clouds

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journal contribution
posted on 2023-05-18, 05:56 authored by Fukui, Y, Okamoto, R, Kaji, R, Yamamoto, H, Torii, K, Hayakawa, T, Tachihara, K, John DickeyJohn Dickey, Okuda, T, Ohama, A, Kuroda, Y, Kuwahara, T
We present an analysis of the Hɪ and CO gas in conjunction with the Planck/IRAS submillimeter/far-infrared dust properties toward the most outstanding high latitude clouds MBM53, 54, 55 and HLCG 92 − 35 at b = −30° to−45°. The CO emission, dust opacity at 353 GHz (τ353), and dust temperature (Td) show generally good spatial correspondence. On the other hand, the correspondence between the Hɪ emission and the dust properties is less clear than in CO. The integrated Hɪ intensity W and τ353 show a large scatter with a correlation coefficient of ∼0.6 for a Td range from 16K to 22K. We find, however, that W and τ353 show better correlation for smaller ranges of Td every 0.5K, generally with a correlation coefficient of 0.7–0.9.We set up a hypothesis that the Hɪ gas associated with the highest Td ≥ 21.5K is optically thin, whereas the Hɪ emission is generally optically thick for Td lower than 21.5K. We have determined a relationship for the optically thin Hɪ gas between atomic hydrogen column density and τ353, N (cm−2) = (1.5 × 1026)· τ353, under the assumption that the dust properties are uniform and we have applied this to estimate N from τ353 for the whole cloud. N was then used to solve for τs and N over the region. The result shows that the Hɪ is dominated by optically thick gas having a low spin temperature of 20–40K and a density of 40–160 cm−3. The Hɪ envelope has a total mass of ∼1.2 × 104 M, an order of magnitude larger than that of the CO clouds. The Hɪ envelope properties derived by this method do not rule out a mixture of Hɪ and H2 in the dark gas, but we present indirect evidence that most of the gas mass is in the atomic state.


Australian Research Council


Publication title

The Astrophysical Journal



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School of Natural Sciences


Univ Chicago Press

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1427 E 60Th St, Chicago, USA, Il, 60637-2954

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Copyright 2014 The American Astronomical Society

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