Optically thick HI 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 Hi radio emission, but it is detected by other means including
gamma-rays, dust emission and extinction traced outside the Galactic plane at
|b|>5 degree. In these analyses, the 21-cm HI emission is usually assumed to be
completely optically thin. We have reanalyzed the HI emission from the whole
sky at |b|>15 degree by considering temperature stratification in the ISM
inferred from the Planck/IRAS analysis of the dust properties. The results
indicate that the HI emission is saturated with an optical depth ranging from
0.5 to 3 for 85% of the local HI gas. This optically thick HI 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 HI toward radio continuum sources.
The distribution and the column density of the HI are consistent with those of
the dark gas sug- gested by gamma-rays, and it is possible that the dark gas in
the Galaxy is dominated by optically thick cold HI gas. This result implies
that the average density of HI is 2-2.5 times higher than that derived on the
optically-thin assumption in the local ISM.
