Spin Temperature and Density of Cold and Warm HI in the Galactic Disk -
  Hidden HI -
  We present a method to determine the spin temperature $T_{\rm S}$ and volume
density $n$ of HI gas simultaneously along the tangent-point circle of galactic
rotation in the Milky Way by using the least-$\chi^2$ method. The best-fit
$T_{\rm S}$ is shown to range either in $T_{\rm S}=100-120$ K or in $1000-3000$
K, indicating that the gas is either in cold HI phase with high density and
large optical depth, or in warm HI with low density and small optical depth.
Averaged values at $3\le R \le 8$ kpc are obtained to be $T_{\rm S}=106.7 \pm
16.0$ K and $n=1.53\pm 0.86$ H cm$^{-3}$ for cold HI, and $1720 \pm 1060$ K and
$0.38 \pm 0.10$ H cm$^{-3}$ for warm HI, where $R=8\ |\sin \ l|$ kpc is the
galacto-centric distance along the tangent-point circle. The cold HI appears in
spiral arms and rings, whereas warm HI in the inter-arm regions. The cold HI is
denser by a factor of $\sim 4$ than warm HI. The present analysis has revealed
the hidden HI mass in cold and optically thick phase in the galactic disk. The
total HI mass inside the solar circle is shown to be greater by a factor of $2
- 2.5$ than the current estimation by optically thin assumption.
