OPTICALLY THICK H I DOMINANT IN THE LOCAL INTERSTELLAR MEDIUM: AN ALTERNATIVE INTERPRETATION TO “DARK GAS”

Dark gas in the interstellar medium (ISM) is believed to not be detectable either in CO or H I radio emission, but it is detectable by other means including gamma rays, dust emission, and extinction traced outside the Galactic plane at b > 5[degrees]. In these analyses, the 21 cm H I emission is...

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Bibliographic Details
Published in:The Astrophysical journal 2015-01, Vol.798 (1), p.1-15
Main Authors: Fukui, Y, Toru, K, Onishi, T, Yamamoto, H, Okamoto, R, Hayakawa, T, Tachihara, K, Sano, H
Format: Article
Language:English
Subjects:
ABSORPTION
Aircraft detection
ASTROPHYSICS, COSMOLOGY AND ASTRONOMY
ATOMS
CARBON MONOXIDE
CLOUDS
COSMIC DUST
DENSITY
Dust
Emission
Emission analysis
GALAXIES
HYDROGEN 1
Interstellar matter
INTERSTELLAR SPACE
Local interstellar medium
MILKY WAY
Radio emission
SPIN
Spin temperature
STRATIFICATION
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Summary:Dark gas in the interstellar medium (ISM) is believed to not be detectable either in CO or H I radio emission, but it is detectable by other means including gamma rays, dust emission, and extinction traced outside the Galactic plane at b > 5[degrees]. In these analyses, the 21 cm H I emission is usually assumed to be completely optically thin. We have reanalyzed the H I emission from the whole sky at b > 15[degrees] by considering temperature stratification in the ISM inferred from the Planck/IRAS analysis of the dust properties. The results indicate that the H I emission is saturated with an optical depth ranging from 0.5 to 3 for 85% of the local H I gas. This optically thick H I is characterized by spin temperature in the range 10K-60K, significantly lower than previously postulated in the literature, whereas such low temperature is consistent with emission/absorption measurements of the cool H I toward radio continuum sources. The distribution and the column density of the H I are consistent with those of the dark gas suggested by gamma rays, and it is possible that the dark gas in the Galaxy is dominated by optically thick cold H I gas. This result implies that the average density of H I is 2-2.5 times higher than that derived on the optically thin assumption in the local ISM.
ISSN:1538-4357
0004-637X
1538-4357
DOI:10.1088/0004-637X/798/1/6