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 be not detectable either in CO or HI radio emission, but it is detected in the other means including gamma-rays, dust emission and extinction. 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 degrees 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 5 for 80 % of the local HI gas. This optically thick HI is characterized by spin temperature in the range 15 K – 70 K, significantly lower than previously postulated in the literature, whereas such low temperature is consistent with emission/absorption measurements of HI toward radio continuum sources. The distribution and the column density of the HI are consistent with those of the dark gas suggested by gamma-rays, and we infer 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 two times higher than that derived on the optically-thin assumption in the local interstellar space.