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Title: 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 γ rays, dust emission, and extinction traced outside the Galactic plane at |b| > 5°. 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° 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 10 K-60 K, 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 γ rays, and it is possible that the dark gas in the Galaxy is dominated by optically thick coldmore » 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.« less
Authors:
; ; ; ; ; ;  [1] ;  [2]
  1. Department of Physics, Nagoya University, Chikusa-ku, Nagoya, Aichi 464-8601 (Japan)
  2. Department of Astrophysics, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Nakaku, Sakai, Osaka 599-8531 (Japan)
Publication Date:
OSTI Identifier:
22364776
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 798; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ABSORPTION; ATOMS; CARBON MONOXIDE; CLOUDS; COSMIC DUST; DENSITY; GALAXIES; HYDROGEN 1; INTERSTELLAR SPACE; MILKY WAY; SPIN; STRATIFICATION